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Government of India & Government of The Netherlands

DHV CONSULTANTS &DELFT HYDRAULICS withHALCROW, TAHAL, CES,ORG & JPS

MANUAL

DESIGN AND CONSTRUCTIONOF

LITHOSPECIFIC PIEZOMETER

September 2002

September - 2002 TOC

Table of Contents

Preface i

Scope of the Manual ii

1 Introduction 11.1 General 11.2 Characteristics of geological formations 21.3 Significance of lithology in the construction of piezometers: 31.4 Groundwater monitoring in India- an historical perspective 41.5 Updating the existing network- based on current objectives 51.6 Macro-level planning 61.7 Micro-level planning 71.8 Desk studies 81.9 Field investigations 91.10 Finalisation of piezometer location 121.11 Reporting of field investigations 131.12 Approval for piezometer construction 141.13 Discussion and interaction with local community 14

2 Drilling preparation 152.1 Planning 15

3 Construction of piezometer 193.1 Selecting the appropriate drilling technique 193.2 Deciding the depth of piezometers 193.3 Diameter of piezometer 213.4 Actions to be taken prior to drilling 213.5 Piezometer construction in unconsolidated formations 213.6 Sampling procedures during drilling 233.7 Down hole inspection 243.8 Piezometer Completion 24

4 Piezometers construction in consolidated formations 294.1 DTH drilling characteristics 294.2 Sampling procedures for consolidated rocks 29

5 Measuring water levels 31

6 Groundwater sampling 33

7 Documentation of piezometer construction 35

8 Piezometer nest 37

Annexure – I Hydrogeological Frame Work of Peninsular India 38

Annexure – II Geo-physical bore hole logging 48

Annexure – III Aquifer parameters and well characteristics 53

September - 2002 Page i

Preface

With the commencement of World Bank Assisted Hydrology Project in nine PeninsularStates of the country, a sizeable programme for establishing groundwater monitoringnetwork has been taken up. The main objectives were to augment the existing network ofobservation wells by construction of dedicated piezometers for monitoring groundwaterlevels and quality. Many of these piezometers have been provided with Digital Water LevelRecorders (DWLRs) enabling recording of high frequency water level data. Thesepiezometers are intended to serve as primary stations for monitoring purposes. This hasnecessitated formulation of guidelines for location and siting of piezometers, theirconstruction and design so that the primary stations truly reflect the groundwater regimebehaviour of the aquifer under monitoring. Besides, there is a need to lay down the precisepractices for the design and collection of data during drilling. The hydrogeological setting ofthe Peninsular India represents a varied environment with differing lithological settings,especially in consolidated formations, which are predominant in the HP States. The differentlithological environment in conjunction with climatological and land forms call for a separateprocedure to follow. Hence the manual is considered essential for reference to the fieldworkers and practicing Hydrogeologists.

A Manual on ‘Guidelines for Implementation of piezometers’ has already been released bythe Hydrology Project, during July 1998. The present Manual seeks to present the practices,which should be followed during selecting the location of the piezometer, drilling,construction and design in the different geological formations commonly encountered in atypical hydrogeological environment. The Manual also deals with the methods of sensitizingthe piezometer to respond to the aquifer inputs and out puts; maintenance and rehabilitationof the piezometers.

One of the main aims of the Hydrology project is to install scientifically designed andcorrectly installed piezometers to monitor piezometric head of shallow unconfined anddeeper confined aquifers. Though the design criteria and field operations are well known toall the field practitioners, certain aspects need to be well understood and assimilated into thepractice of implementing a piezometer. The manual gives the guidelines, which are expectedto assist the professionals in realising the necessary reorientation of their drilling experienceand expertise.

September - 2002 Page ii

Scope of the Manual

An earlier Manual on “Guidelines for the Implementation of piezometers” was prepared andreleased by the Consultants in July 1998. The Manual dealt with the optimal network designand the details of piezometers in unconsolidated and consolidated formations. However, asmajority of the participating HP States fall in hard rock areas of the country, a need has beenfelt to deal with various aspects of piezometers in different lithologies along with methods ofdrilling, design and pumping test of piezometers.

The manual is intended to serve as a practical guide to the groundwater field workers and asa tool to visualise the hydrogeological situations and ground realities of the piezometer siteand what results could be expected. And remedial measures to be adopted to revitalise apiezometer.

This Manual seeks to highlight the concept of lithospecific Piezometers, criteria forprioritisation of areas for location and site selection using Remote Sensing and othermethods. The procedures of Drilling and Design of Piezometers along with an account ofmethods of analysis of pumping test data of the piezometers for different types of aquifershave been described. Also topics on Development of piezometers and their Maintenancehave been discussed using inputs from various sources. Finally selection and installation ofappropriate type of water level recorders in tune with the requirements for litho specificpiezometers has been discussed in the manual. It is hoped the manual will meet theguidelines for hydrogeologists engaged in planning, execution and field operations and dataretrieval from piezometers as per their requirements.

September - 2002 Page 1

1 Introduction

1.1 General

Groundwater monitoring is an essential tool to obtain the information on groundwaterquantity and quality data through representative sampling. This helps in recording theresponse of groundwater system to a natural or artificial input and out put. Any planning forgroundwater development should be guided by regime monitoring indicators such as waterlevels and quality changes over a period. Groundwater is a dynamic resource requiringcontinuous monitoring of its quantity and quality data for updating and assessment ofavailable resource potential. Such an updating can be made possible by using a soundobservational database from a scientifically well established network monitoring system.

Groundwater observation monitoring network stations or piezometers are constructed torecord the response of groundwater regime to the natural and artificial recharge anddischarge conditions. Keeping in view the regional and local requirements, the planning anddesign of such a network depends upon hydrogeologic, physiographic and climaticsituations, purpose of the study, stage of development, as well as political and socialdemands (UNESCO, 1977). The various types of observation networks can be setupdepending on the objectives e.g., hydrogeological, water management, baseline waterquality and for specific purposes. In the ongoing Hydrology Project, the objectives of theobservational network mainly include high frequency groundwater level and groundwaterquality monitoring.

A few of the salient features of the groundwater monitoring system are as under:

– Strengthening of the existing network through construction of purpose built observationwells (piezometers) through identifying gaps in the data.

– Ensuring integration of networks of Central and State Groundwater Organisationsavoiding any duplication.

– Achieving optimum observation network density in the given area.– Installation of high frequency water level measuring devices like Digital Water Level

Recorders (DWLR’s) on piezometers at key /nodal locations.– Establishment of Data Centres at Unit level, Regional level and National level to handle,

storage, validation, synthesis, retrieval and dissemination of data generated to useragencies

– Development of Hydrological Information System (HIS)– To ensure transparency in the availability of ‘demand driven’ groundwater data required

by the User community.– Ascertain the data needs, data type (historical, real time etc.), parameters of data

requirements of user community through Hydrology Data User Group (HDUG) meetings.

For implementation of the observation network monitoring, CGWB and the StateGroundwater Agencies of the participating States have constructed a sizeable number ofpurpose built observation wells (piezometers) with the provision to install DWLR’s at keylocations and suitable pumps for water sampling. Some of the piezometers constructed arereplacement to old, defunct existing open wells, due to de-saturation of aquifer, disuse, andaging, among other factors.


The observation wells/piezometers are usually of small diameter so as to accommodate thewater level measuring device and the water-sampling pump. In unconsolidated formations,piezometers are provided with screens tapping the zone of interest; where as in theconsolidated rocks, piezometers are left open ended (uncased) beneath the loose soil/looseover-burden where the hole has to be provided with a casing. Up-gradation/strengthening ofthe observation well network is a continuous process which would require replacement ofnon performing open wells with dedicated piezometers as well as construction of deeppiezometers to cover aquifers that have not been previously monitored. Improvement in thedensity of the network would also arise with time. All this would involve construction of manymore piezometers. The present manual aims to serve as a reference guide duringpiezometer construction. It is expected that this manual would also help the differentagencies to formulate ‘Protocols and Procedures’ for construction of piezometers. Thedetailed piezometer construction procedure must contain number of elements plus anyadditional site-specific elements, which may be required. This manual describes thesignificance of different elements in the piezometer construction

1.2 Characteristics of geological formations

The unconsolidated geological materials are generally composed of sand, gravel, and clay invarious proportion as alternate layers and are characterised by occurrence of primary(interstitial) pore spaces which provide the main loci for storage and movement ofgroundwater in the saturated zone. These materials are often assumed to behave ashomogeneous and isotropic media. Yet, the homogeneous aquifers seldom occur in nature,with most aquifers being stratified to some degree. Due to this, the hydraulic conductivity isfound to differ in horizontal and vertical directions.

Rockgroup

Rock types Mode of occurrence Main featuresimportant forgroundwateroccurrence

Crystallinerocks

Non-volcanic igneous andmetamorphic rocks, viz.Granites, gneisses, schists,slates and phyllites, etc.

Large size massifs andplutons; regional metamorphicbelts

Weathered horizon,fractures andlineaments withsecondary porosity

Volcanicrocks

Basalts, andesites andrhyolites

Lava flows at placesinterbedded with sedimentarybeds

Fractures, vesiclesand inter-flowsediments

Carbonaterocks

Limestones and dolomites Mostly as chemicalprecipitates with varyingadmixtures of clastics in alayered sedimentary sequence

Fractures andsolution cavities

Clasticrocks

Consolidated sandstonesand shales

Interbedded sedimentarysequence

Inter-granular porespaces andfractures

Table 1.1: Hydrogeological Classification of Consolidated Rocks(after Singhal & Gupta, 1999)


The consolidated geological formations dominate the Peninsular India. These formations aredevoid of primary porosity and permeability, but tend to acquire some hydraulic conductivitythrough joints, fracturing, weathering and other geological processes. From hydrogeologicalpoint of view, these are classified into Crystalline rocks, Volcanic rocks, Clastic rocks andCarbonate rocks. Groundwater occurrence in these rocks is mainly dependent upon thedegree of weathering and consolidation of fractures and fissures, which form the main flowconduits. Table 1.0 gives chief rock types and brief mode of occurrence of groundwateralong with main features of occurrence of groundwater in each such formations.

The table shows that in contrast to the dominant primary porosity as a main feature forgroundwater storage and movement in the unconsolidated formations, distinctly differenthydrogeological frame work and flow features characterise the consolidated formations. Thelocation and depth of the network observation wells and/or piezometers in such formationssolely depends on the factors like the thickness of weathered zone, occurrence andcharacteristics of fractures and related hydrological features. In the case of uniformly anddensely fractured rocks, the site selection and construction of such piezometers can be moreor less similar to that in unconsolidated aquifers. However, in case of non-uniform fracturing,or in weathered zones of crystalline rocks, in carbonate aquifers with solution cavities and inbasaltic aquifers with lava vesicles and tubes, the decision on the placement and depth ofpiezometers may require detailed studies of the hydrogeological situation. Thehydrogeological framework of peninsular India is described in details in Annexure-I.

1.3 Significance of lithology in the construction of piezometers:

Groundwater occurs in the aquifers either as an individual horizon or as multiple layers. Forproper accounting of resources and judicious planning of exploitation, it essential to monitorthe water levels which are indicators of its potential at different times. The quantity andquality of water occurring in the aquifers depend upon its mineralogical and geochemicalconditions at the particular level. For this independent observation wells are constructedknown as ‘piezometers’.

All water level monitoring programs depend on the design of piezometer. Decisions madeabout the design of the piezometer and its location are crucial to water data collectionprogram. Ideally, the piezometer constructed as part of the monitoring network need toprovide data representative of the different geology, lithology and groundwater developmentenvironments. Decisions about the real-areal distribution and depth of completion ofpiezometers should take into consideration the physical boundaries and geologicalcomplexity of the aquifers under study.

Water level monitoring in complex geological and lithological environment may requiremeasurements of water levels in multiple piezometers (nested) constructed at differentdepths tapping different aquifer units representing varied lithological and geological units inthe area. Large geological/lithological units that extend beyond the state boundaries requirea network of piezometers that have representation beyond the states distributed among oneor more states. One of the purpose of a network is to monitor ambient groundwaterconditions or the effects of natural, climatic-induced hydrologic stresses, the piezometernetwork will require monitoring structures that are representative of regional geological,lithological units that have lateral and vertical continuity and represent the horizontalgroundwater flow regime without any major gaps. The aim should be to ensure that there areno mixing up of information due to improper piezometer design. These and many othertechnical considerations pertinent to the design of a piezometer focussing on lithological andgeological units is discussed in detail.


Commonly overlooked is the need for study of geological map and available reports of thearea giving details on the mineralogical and lithological information before deciding about thelocation and design of the piezometer. Good understanding of the lithology of the area helpin designing the appropriate piezometers that enable collection of accurate, authentic andprecise water level data, which will reflect true conditions in the aquifer being monitored andprovide data that can be relied upon for many intended uses. Therefore field and officepractices that will provide the needed levels of quality assurance for water level data shouldbe carefully thought out and consistently employed. In the construction of piezometers theprincipal objectives should be:

– to monitor the water levels and water quality of independent aquifer– to understand the relationship between different aquifers– to understand the hydraulic characteristics of different aquifers.– to evaluate groundwater regime characteristics– to understand the regional flow characteristics– to refine groundwater resources assessment

The procedure and protocol for design and construction of piezometers shall be dictated bya number of factors including the geology, hydrogeology, lithology, aquifer geometry notforgetting the objectives of the monitoring network. Thus prioritisation of the piezometer siteas well as their design and construction should have a clear bearing and perception of thegeology, lithology and aquifer type. A geological map, lithological cross section, structuralmap, geomorphological map and geophysical survey reports are the important tools that willhelp in understanding the regional geological control on the groundwater system which is animportant consideration for the piezometer design. This brings to the fore the need toconsider different lithologies separately for hydrological studies necessary for identification of‘representative’ piezometer sites. This has lead to introduction of the concept of lithology-specific-piezometers, commonly referred to as ‘Lithospecific Piezometers’.

1.4 Groundwater monitoring in India- an historical perspective

The Central Groundwater Board in 1968 started groundwater monitoring as part of itsactivities with one observation well for each toposheet over the entire country. In all 68observation stations were established. Gradually with the need more number of stations withlithological representation were also added. Mostly existing open wells owned by farmers orutilized for drinking water were included in such monitoring systems. With the operation ofgroundwater exploration and resource evaluation projects under UNDP and other addedprojects many observation network stations were established tapping shallow as well asdeeper aquifers and amalgamated in the regular groundwater monitoring system.

These were mostly on the basis of availability of wells as a sort of compromise and not onthe basis of requirements at the specific locations. The water levels were measured initiallytwice, pre-monsoon and post monsoon period, which subsequently was converted to fivetimes in a year falling in the months of January, March, May, August and November months.From 1986 onwards 4 times in a year is measured in the months of January, May, Augustand November. The data collected is utilised in specific reports for reporting on fluctuationsand assessing water resources for the administrative divisions.

By 1972 the State Groundwater departments also came into establishment and graduallygroundwater monitoring was taken up. The density of network observation wells in alluviumwas about one well per 100 sq. km on an arbitrary basis, while in hard rock it was more than


that mostly in localized areas with groundwater development. Also in exploratory areas withpossible scope for groundwater development, monitoring was enhanced with addition ofpiezometers constructed for well field studies both in soft rock as well as hard rock areasunder various national /international added projects in the country. At best these monitoringnetwork stations served as indicators on baseline water level and water quality data. Theinformation emanating from such networks has generally permitted conceptualisation of thegroundwater system and its resource evaluation. These were in essence need-basedpiezometers rather than scientifically required for country monitoring system.

The data generated was mostly utilised for use in the internal report preparation by thedepartments and evaluation of groundwater resources for administrative units for the countryas a whole. The depth to water level maps were prepared and interpreted for response ofaquifers to various natural inputs from rainfall and canal/irrigation returns in terms of mapsboth for pre-monsoon and post-monsoon season. Also maps on water quality coveringelectrical conductivity and iso-chloride and total dissolved salts were prepared andinterpreted.

The data generated were with certain inaccuracies as the monitoring wells were one thoseused for drinking as well as irrigation, as a result exact water levels were not possible.Subsequently, with the advent of tube wells and bore wells in hard rock areas which werefitted with electrically operated pumps, the water levels started declining and many of thedug wells went dry during summer period. As a result, lowest water level data could not berecorded. Some of the old wells went into disuse or were dumped with garbage and as suchdata collection was not possible, leading to data gaps.

Topics include in this document are: network review, site investigations, piezometerconstruction, development, discharge measurement, performing aquifer tests, and waterquality sampling.

1.5 Updating the existing network- based on current objectives

The first task before construction of new piezometers is to review the existing monitoringnetwork at the micro level i.e; drainage basin, geological basin and in limited circumstances,considering only the administrative boundary as a unit. The review has to integrate themonitoring wells of all the agencies involved with water level and water quality monitoring.The review has to be necessarily be aquifer wise. The review should be based on allavailable data. The evaluation should lead to identification of the data gaps (spatial andvertical).

The review has to be based on the data available from the networks related to; aquifer wisedensity, depth of the aquifers and water level plus water quality. The review should alsoevaluate the areas where the data generated from the existing network has been used i.e.;

• groundwater resource assessment,

• understanding the groundwater flow dynamics,

• delineation of recharge/discharge areas,

• regional groundwater quality variations over space/time

Based on the review the adequacy of the network has to be evaluated; areas showing gapsin understanding have to be identified; areas showing more than adequate numbers ofobservation wells have also to be identified and duplicate observation wells, if any, have also


to be considered for elimination or reducing the monitoring frequency. In case an existingnetwork (with respect to a specific aquifer) is found to be inadequate, additional piezometers,tapping that specific aquifer, need to be provided.

The first step towards planning to the enhancement shall comprise of macro-level planning,i.e., estimating the required number of additional piezometers and their location at a macro-level (say on a map of scale 50,000). The subsequent step shall involve pinpointing the sitesfor the additional piezometers on the ground, i.e., micro-level allocation.

1.6 Macro-level planning

Depending upon the intended use of the data from the network, the macro-level planning ofthe network enhancement can be accomplished using the statistical tools in the dedicatedsoftware

1.6.1 Coefficient of variation method

The method requires the user to specify the maximum permissible error in the estimate ofthe mean water level. Subsequently, based upon an analysis of the data from the existingnetwork, the required number of the piezometers is computed, from which the additionalnumber of the piezometers are derived.

The following procedure is adopted for locating the additional piezometers within thespecified area.

• Employing the concurrent data from the existing network, draw contours of water level ata uniform interval.

• Divide the entire area into zones, each zone representing an area falling between twosuccessive contours.

• Divide the required number of piezometers equally among all the zones. This will ensurea greater density of the piezometers in the regions of steeply sloping piezometric headand vice versa.

• Count the number of existing piezometers in each zone and hence estimate zone-wise,the required number of additional piezometers.

• Locate the additional piezometers in each zone in such a way that the piezometers(existing and additional) are uniformly distributed within the zone.

1.6.2 Kriging

Kriging is a powerful tool for evaluating an existing network. It also assists in the macro-levellocation of additional piezometers, in case the existing network is found to be inadequate.The steps involved are as follows:

• Specify the level of permissible interpolation error.

• Conduct kriging on the concurrent piezometric data from the existing network. This shallyield contours of piezometric head and of the interpolation error.

• Study the error contours and hence identify the regions where the error is in excess ofthe specified permissible level. Additional piezometers are to be allocated to theseregions.


• Locate additional piezometers in the identified regions tentatively, generally ensuringthat the increase in the network density is consistent with the error excess.

• Conduct kriging on the tentatively enhanced network and plot contours of the error. Itmay be noted that kriging permits generation of such contours, even though the datafrom the newly introduced piezometers do not yet exist.

• Study the modified error contours and check whether the error everywhere falls belowthe specified limit and the enhancement has not been over-done. An over enhancednetwork shall display interpolation errors far less than the prescribed limit.

• Modify the network further, if necessary by repeating the relevant steps

1.7 Micro-level planning

After having decided the location of the piezometer sites on the map, it is essential topinpoint the site exactly on the ground. Certain micro-level deviations may be necessary toaccommodate various hydrogeological and logistical considerations.

1.7.1 Hydrogeological considerations

These considerations originate from the primary expectation out of a piezometer, i.e. itshould record harmonized natural behaviour of groundwater rather than local micro-trends.This can be ensured by keeping in mind the following:

• The site should show no impact of any external inputs such as from canal, tank,perennial river and irrigation return flows, except in special cases where interest is thestudy of the influence of these parameters on groundwater system.

• The site should not fall within the radius of influence of a well, which is under pumping;but it should be capable of recording the effects of the pumping as a regionalphenomenon.

• The piezometric head/water quality at the site should not be influenced by localrecharge/pollutant sources.

1.7.2 Logistical considerations

There could be many general as well as area-specific logistical considerations such as:

• No other agency is considering constructing a piezometer tapping the same aquifer, inthe vicinity.

• The site is approachable by the rig and support vehicles.

• Adequate space is available at the site for setting up drilling equipment, digging mud pitand draining the discharge, while the site should be clear of trees, overhead electriccables, under ground cables/ pipelines/ drainage lines etc.

• The ownership of the site is clear and agreements have been made for drilling thepiezometer and for continued monitoring.

• The site should be safe from vandalism, as a costly DWLR will be installed.

• The site should be neither too close nor too far off from the road.


1.8 Desk studies

Based on the network review and identification of the area where additional informations arerequired piezometer site selection has to be initiated. Desk studies need to be carried out inthe office through review of topographical maps, geological maps, geophysical surveys data,geological cross sections, drilling data, water table/piezometer maps, water quality maps etc.Data emerging from the desk studies should be systematically organised, location wise, forcarrying to the field for field review and investigations.

1.8.1 Remote sensing interpretation map

The Hydrology Project has been involved in the creation of GIS data sets in which thethematic maps are generated using satellite data. The thematic maps, should be used duringthe desk review for the locating the appropriate sites for the piezometers. The Remotesensing maps have to be the basis for delineating the faults, lineaments, study the geology,hydrogeology, land use etc. Using the GIS capabilities different themes should be overlaid tozero on the most appropriate location. Based on the GIS studies and remote sensinginterpretations inference on the subsurface soil moisture, recharge potentialities need to beestimated. The Remote sensing interpretations should be used to interpret features like karsttopography, dykes, reefs, unconfirmities, buried channels, salt encrustations, tide levels,alluvial fans and abandoned channels etc.

In the hard rock terrain's the remote sensing studies should help in understanding the spatialdistribution of rock out-crops, the catchment characteristics, the presence of structures anddrainage systems influencing the groundwater movement, the nature of the land form andthe slope based on which interpret is the likely thickness of regolith/overburden, the generalgroundwater potentiality and the most preferbale location for constructing the piezometer.For this purpose, the GIS datasets related to geology, its structures, geomorphology,drainage and soil should be integrated and interpreted.

The satellite imageries provide a good idea of drainage network for computing drainagedensity. Drainage density exhibits a very wide range of values in nature depending upon therelief, climate, and resistance to erosion and permeability of rock material. In general, lowdrainage density (1.9-2.5 km-1) is characteristic of region of highly resistant or highlypermeable surface and low relief. High drainage density (12.5 – 19.0 km-1) is found inregions of weak or impermeable subsurface materials, sparse vegetation and mountainousrelief. In areas of low relief, drainage density may be more indicative of permeability ofsurface material and therefore, could be used as a criterion for the selection of suitable sitesfor piezometers. The drainage analysis is utilized to differentiate the terrains into highlydissected plateau (HDP), moderately dissected plateau (MDP) and poorly dissected plateau(PDP) (see figure 1.1).


Figure 1.1: Map showing lineaments in the hardrock area in Salem area,of Tamil Nadu

Study of lineament is the most important aspect of satellite image interpretations forgroundwater studies in the hard rock terrains. It has been established that the groundwaterstructures constructed close to fractures of tensional origin, or close to their intersections,have proved extremely successful. Siting of piezometers near such favourable structuresshould be considered and such areas clearly marked on the toposheet of 50,000 scale andinspected in the field.

On the satellite imagery, the lineaments can be easily identified by digital image processingas well as visual interpretation, using tone, colour, texture, pattern, and association. Theautomatic techniques of digital edge (or line) detection can be applied for lineamentdetection (Singhal and Gupta, 1999). However, fracture traces having low dips, which havemore potential for groundwater may not be very easily deciphered. Staff with extensive fieldexperience would be able to make such interpretations easily.

1.9 Field investigations

The Field investigations consists of a number of elements including, geological,hydrogeological, geomorphological and hydrological investigations

1.9.1 Geological investigations

Geological map of the area on 1: 50,000 or 1:250,000 scale prepared by national agencieslike Geological Survey of India or State Mines and Geology departments which has beenconverted to digital format as part of the GIS data set preparation should be printed andcarried to the field these maps help to visualize the occurrence of rock formations, theirdisposition, sequences and structures, faults, dykes etc. Surfacial distribution of rocks and


their regional continuity should also be visualized. The different lithological and structuralfeatures like joints, lineaments, foliation, discontinuities, degree of susceptibility of rocks toweathering, from Dug well sections need to be studied.

Field investigations should also include information about the orientation and density offractures, although their subsurface distribution may be different which can be decipheredfrom subsurface investigation. A kinematic analysis of fracture pattern and lineaments isoften useful in delineation of their tectonic origin. Whereas, plotting of the dip and strike ofjoints on Schmidt's Stereo net and as rose diagrams can bring out synoptic, structurallyweak zones.

Data about the thickness and composition of the weathered zone (regolith) is particularlyimportant in crystalline rocks. The minerological composition of weathered products,particularly presence of interstial clay matrix or its absence is important. The texture ofquartz grains with respect to their roundness, sphericity, angularity and abundance of willindicate in-situ deposition or transported deposition. The abundance of orthoclase,anorthosite minerals give clue to the extent of weathering in the rock as these are mostdissoluble minerals. Similarly, mica is also unstable. The recharge, discharge zones withgeomorphic locations and drainage system help greatly in identifying the suitable location.

In volcanic rocks presence of vesicular basalts, its thickness and geomorphic locations areimportant from the view point of groundwater occurrence. The vesicular and amygdloidalbasalt is most susceptible to weathering. The vesicles with tubular structures form goodwater conduits in basalt. Added to this, fractures and lineaments enhance the potential of therock unit. The hard basalt with fractures underlying the vesicular basalt also forms potentialwater bearing zones in basalt. Attention should also be paid to the palaeodrainage,characters of individual flow units including their dips and inter-flow formations. The surfacedrainage plays an important role in basaltic rocks. The recharge-discharge zones shouldalso be identified. The above mentioned details will help greatly in identifying suitablelocation of a piezometer in basalts.

In carbonate rocks, mapping of various solutions (karst) features are of special importance.In carbonate rock areas, the geological map with occurrence of karstified and dolomitic typeof rock disposition, better groundwater potential can be visualized better. Presence ofsinkholes and valley depressions form main recharge zones. Presence of springs givesclues of solution channels. However, flaggy and bedded disposition of lime stone withmonotonous topography display low potential zones.

In unconsolidated and semi-consolidated formations nature of deposits are important. Valleyfill deposits tend to be of assorted nature, river/fresh water deposits are likely to be withfrequent variations in textures of grains, even though there may be continuity in sequence ofbed, but will be with variations in lateral porosity. Sudden truncation or swelling of aquifersare common. This needs to be properly visualized through lithological cross sections,prepared on the basis of existing drilling data.

Based on exploratory drilling and well log data, following subsurface maps and sections areprepared viz. fence diagrams, isopach maps, structural contour following maps etc. This isdone to able to project the subsurface distribution and configuration of aquifers, aquitardsand aquicludes. Water table/piezometric contour map if available should also be studied foridentifying gaps and location of suitable site for piezometer.


1.9.2 Hydrological Investigations:

Drainage pattern is the spatial arrangement of streams and is, in general, very characteristicof rock structure and lithology. These drainage patterns reflect the hydrogeologicalcharacteristics of the area and therefore can be useful in the location of piezometer sites.Figure 1.2 gives common drainage patterns in consolidated and unconsolidated formations.

Figure 1.2:Common drainage patterns (A.D. Howard)

The drainage maps have been created as part of the GIS data set perpetration. Thedrainages have been digitised from the toposheet of 50,000 scale and updated using thethematic maps. The drainage maps have also been used in delineating the differentdrainage order, from the major basins, down to watershed units.

During the field investigations the position of the piezometer location has to be ascertainedwith respect to recharge area/run off zone/discharge area.

1.9.3 Geomorphological investigations

Geomorphological map of the area on 1: 50,000 or 1:250,000, scale available as part of theGIS data sets, should be printed and taken to the field for visualizing the various landforms.Genetically, the landforms are divided into two groups: erosional, and depositionallandforms. Erosional landforms are typically associated with the resistant hard rock terrains.They comprise: (a) residual hills, (b) inselbergs, (c) pediments, (d) buried pediments withweathered basements, and (e) valley fills. Depositional landforms, developed by depositionalprocesses of various natural agencies, (e.g. river and wind) are typically made up ofunconsolidated sediments and may occur in the regional setting of hard rock terrains.Favourable landforms that contribute significantly to groundwater recharge should be


identified in the field. The possibility of locating the piezometer in such areas should beexamined.

1.9.4 Geophysical Surveys:

Geophysical surveys need to be carried out as a standard procedure for getting a clearunderstanding of the following depth to bed rock, thickness of weathered zone, extent ofsaturated zone, approximate quality of water in the saturated zone, thickness of differentlayers in layered formations and type of layered formations. Influence of structures like fault,unconfirmities and dykes can also will be evaluated. Occurrence of saline and fresh waterlayers with probable depth of occurrence also will be indicated.

Electrical resistivity survey is the most commonly used method to identify the verticallithological layering distribution in an area. New approaches using the VLF method, Electro-Magnetic methods, Gravity Methods have to be used wherever possible. The mainobjectives of geophysical surveys are to provide information on:

– Depth, thickness and extent of aquifers in stratified formations.– Depth, thickness and extent of weathered and fractured zones.– Depth to water table.– Selecting the site of a piezometer, out of the several target areas.– Gross Salinity distribution and contamination.

1.9.5 Hydrogeological investigations

Hydrogeological investigations should include detailed well inventory of 2-4 sq.kms aroundthe proposed area. All the groundwater abstraction structures need to be inventoried and theinformation to be collected should include the depth of the well, aquifer position, rate ofpumping, pumping duration, drawdown, rate of recuperation, area irrigated, lithologyencountered while construction, static water level, water quality details etc. Collect watersample and carry out field analysis for pH and EC. Collect two sets of representative samplefor detailed laboratory analysis.

The existing monitoring wells/piezometer around the proposed site needs to be visited andthe variations if any with the proposed site understood. Examine the water level hydrograph.Examine the water table elevation contour maps and depth to water table maps generatedusing two sets of data (pre-post monsoon).

Prepare lithological cross section/ fence diagram using the data from the inventoried wells,delineate the prominent aquifers in the area with their thickness and areal extent. Carry outpumping tests/ geophysical downhole logging where adequate information cannot begathered during well inventory.

1.10 Finalisation of piezometer location

Based on all the studies and keeping in mind the logistical and safety considerations thepotential site has to be identified. Where more than one site is considered then a joint teamof hydrogeologists should visit the area and identify the most favourable location. The siteselected should be verified for its true representation of the area specific lithology and


regime system. The interference from pumping wells, surface water sources, pollutingsources, seepage from return flows should be avoided at all costs.

1.11 Reporting of field investigations

Based on the field investigations a feasibility report has to be prepared. The followinginformation must be documented as a file giving details of the procedures followed indeciding the piezometer site. The report should include:-

• A sketch showing the identified site and important landmarks in the vicinity. The sketchshould incorporate the north direction and the distance of the site from the landmarks.

• Locate the site on the toposheet of 1:50,000 scale. Record its longitude, latitude and thereduced level as read from the toposheet. Use the hand held GPS wherever availablefor getting the geographical co-ordinate values.

• A narrative of the geographic setting of the piezometer site with administrative details.Details pertaining to sites adjacent to or in the vicinity of school, sub station, policestation, floodplains, wetlands should given.

• A narrative describing the regional lithologic, stratigraphic, structural, and hydrologicsettings of the area.

• A narrative must be provided which describes field procedures used to characterizegeologic and hydrologic conditions of the site. Standardized field procedures may bereferenced. Details of the site-specific geology and hydrology based on data collectedshould be explained. The narrative must describe the proposed piezometer design.Interpretations of results must be presented in a clear and concise manner. Allpublished information sources used in the compilation of the hydrogeologic investigationmust be listed.

• Appendices of the report must include:– Compiled logs of all borwells and piezometers.– The raw data for any and all tests (e.g., geophysical survey, bore hole logging,

water quality analysis, pumping tests).– Water level hydrographs of monitoring wells in the neighbourhood– Water table elevation contour maps– Hydrometerological data of the area– All additional information that may facilitate the clearance of the proposed site.

The exact location should be marked on the ground with paint. Lithologic cross-sectionsmust be constructed or inferred. At least one cross-section must be constructed parallel togroundwater flow. The subsurface conditions of the site must be illustrated in these cross-sections. Where more than one interpretation may be reasonably made, conservativeassumptions must be used. A clear picture has to be given of the thickness, depth andlateral extent of the aquifers in the area with a clear definition of the aquifer to be monitoredand the geo-hydrologic conditions. The type of monitoring required and the need if any for aDWLR and sampling pump should be brought out. The report should clearly bring out theneed for the Piezometer at the proposed site with a justification for the expenditure to bemade in establishing and running the network. The utility of the information emerging fromthe piezometer should be highlighted.

An estimate should also be prepared which should include site preparation, drilling, casing/screen installation, gravel pack, sealing, development, pump test, platform and well headconstruction.


1.12 Approval for piezometer construction

The site selection report from the field offices should be forwarded to the head quarters forapproval and clearance. It is expected that the justification for the construction of thepiezometer would be reviewed by a committee of senior officers at the head quarters, whowill look at the requirement from a national/state perspective as well as from a localperspective. The location of the piezometer should be superposed on the existing networkand its utility assessed. The aquifer to be monitored has to be verified on the cross section.The added value from the new piezometer should be verified from a technical, managementand financial perspective. On complete satisfaction of the utility of the piezometer thefinancial estimate has to be examined. While standard rates should be the norm, deviationsshould also be considered on case by case basis. The sanctions for depth of drilling, casingdepth, screen position should be based on the field report, which should come up forratification after completion of drilling. In the case where drilling contractors are to be hiredfor drilling the piezometers the procedure for hiring drilling contractors should follow theestablished norms. The tender document for inviting the drilling contractors should clearlymention that a qualified Hydrogeologist should be part of the drilling team and his/her CVshould be part of the enclosures. The utility of hiring more than one contractor when thepiezometer locations are far part should be examined seriously. Drilling Contractors whenused the terms of the contract should clearly specify the obligations of the contractor as wellas the department. Drilling being a seasonal task the procedures for selection of contractorsshould not be cumbersome. Acceptance of State Govt Approved Rates can reduce theprocess of selection. Since rain, water and mud are major hindrances, it is normallyrecommended that the most difficult holes be drilled first if they are accessible, saving themost convenient holes for last or to drill when the others can't be reached.

1.13 Discussion and interaction with local community

On obtaining the clearance for construction of the piezometer from head quarters, a meetingshall be convened in the village where the piezometer site is proposed. The invitees shouldinclude the village elected representatives, village officials, elders, farmers, women,schoolteachers and youth. The services of NGO groups active in the area should be used forconducting the meeting. The meeting should address the local groundwater issues and theneed for groundwater monitoring. The proposed plan for establishment of the piezometerand the most favourable site location identified need to be discussed. Any suggestions fromthe community should be considered and animated in detail. The agency should alsopromise the community to make available the interpretations of the data collected. As afollow up to the discussions, an agreement should be obtained from the community to makeavailable the required co-operation for safeguarding the piezometer as well as upkeep of thearea.


2 Drilling preparation

2.1 Planning

Successful piezometer construction requires careful advance planning to be conducted inthe most expedient manner. Proper drill site selection and preparation are essential to avoiddrilling at wrong site, minimise wastage of drill time and other associated costs. Landclearance is an essential item that cannot be taken lightly or ignored. Disputed lands canresult in a tremendous litigation and liability to the department. The following are somedetailed items to consider prior to commencing drilling of piezometer.

2.1.1 Site Preparation

Drilling sites need to be prepared prior to arrival of the drilling rig. The site has to be levelledin order to drill a vertical hole. Inclined bores considerably reduce the diameter and depthcalculations become enormous. Prior to extensive site work, the driller must visit the site andclearly place his requirements. Overhead area must be clear of obstructions. Sites with treesand overhead power line should be avoided. If it is necessary to work closer to power lines,the drill crew should inform the electrical authorities either to shut down the power supply orto make the working environment safe. Underground laid infrastructure such as water lines,sewer lines, electrical/telephone cables, if any, should be checked before commencing work.Roots are a major problem, they force their way into the piezometers,. In such areas properpreventive care should be taken by increasing the casing depth or identifying the root pathand treating them.

It has to be ensured that the drilling rig has access to the site upon arrival. Problems havearisen in the past from hostile villagers and uncooperative landowners, which can beavoided if the village meetings are conducted and local communities are taken intoconfidence. Bridges/culverts to be crossed must be inspected to check whether they havethe required width/ soil strength and have the capacity to take the weight of the rig, alongwith the spares.

2.1.2 Supervision of drilling

It is important to monitor the drilling and ensure that all procedures adopted should help inconstructing a quality piezometer. The piezometer on completion should be providing thetrue picture of the water level and water quality without any bias. The drilling of thepiezometer, geophysical down hole logging, development and pumping test need to becarried out under the supervision of an on site hydrogeologists. Where the work issubcontracted to a drilling contractor, then the drilling contractor should be responsible foremploying the site hydrogeologist who will be available at all times till the piezometerconstruction is complete. The site hydrogeologist shall be responsible to record the drillingdetails, examine and interpret the drill cuttings, describe and record the physical andlithological characteristics of the geological material, supervise the well design, welldevelopment, measure the discharge and collect the water samples.


2.1.3 Essential tools for field hydrogeologist

Field tools assist the field hydrogeologist in giving an accurate description of the drillcuttings. It is recommended the field hydrogeologist have these basic items (see Figure 2.1)which include:

• Pocket knife to cut the samples for testing hardness and exposing fresh surfacesMillimeter scale to determine the size of the particles

• Dilute hydrochloric acid to aid in recognizing calcium carbonate materials suchaslimestone, chalk, or dolomite

• Magnifying glass (a 10x) to make a better identification of materials by enabling closerinspection

Figure 2.1:Field tools for drill cuttingsexamination

2.1.4 Field notes

Field logs and notes on drilling should be prepared at the drill site itself.

The field description of drill cuttings should be simple and orderly so that the use of theterminology is uniform. A good field description of the drill cuttings is very important for thedesign and preparation of vertical sections. The site hydrogeologist and the drill crew are theonly people who witness the drilling and the material obtained. Therefore a reasonableamount of accurate information must be logged. At a minimum, the field hydrogeologistmust, in the field, note on a descriptive log the following: The field hydrogeologist must makesure to note the following on descriptive log:

• Start and stop times for drilling

• Names of field personnel

• Drill cuttings details-Colour, Texture , shape, mineral assemblage, rock type

• Diameter of drill bits

• Depth at which water encountered and discharge variations with depth

• Drilling rate


• Casing depth

• Drill completion depth

• Screen position

• Gravel pack position

• Well completion depth

• Water bearing zones

• Development time

• Discharge after development

• Water quality details pH, EC

• Depth to water upon completion

A Standard data collection format should be adopted. All field data should be computerisedsystematically as soon as the drilling is complete and the field data brought to theDistrict/Regional Data Centre.

2.1.5 Description of drill cuttings

The descriptions of the drill cuttings should be as simple as possible (see table 2.1). Everysmall variation does not necessarily warrant description on the log. The description shouldinclude:

Principal constituent: First determine the major constituent in the sample. If a significantportion (greater than five percent) of a secondary material is present then describe andidentify it.

Colour: Describe the primary color and restrict description to one colour. If one main colourdoes not exist in a sample, make a simple description of the multicolouration.

Texture: Mention the texture of the primary material under three to four main cateogoriessuch as Coarse-grained, medium grained, Fine-grained, Highly organic etc.

Shape: Cateogorise the most dominant shape of the drill cuttings under rounded, subrounded or angular.

Hardness: should be mentioned with respect to Mohs 'Hardness Scale

5.5 – 10: Rocks that will scratch the knife: Sandstone, Chert, Schist, Granite, Gneiss, someLimestone

3 - 5.5: Rocks that can be scratched with the knife blade: Siltstone, Shale, most Limestone

1 – 3: Rocks that can be scratched with fingernail: Gypsum, Calcite, Evaporites, Chalk,some Shale

Cementation: Identify the degree of cementation if any.


Descriptive adjectives: Use any descriptive adjectives that might further aid in theunderstanding.

Log form: To promote consistency, use the standard log form which is consistent with thedata entry system.

Depthto (m)

Lithoologicaldescription

Colour Texture Shape Remarks

0.2 Laterite red hard subangular-subrounded

6.5 Laterite verigated/wuggy

red medium subangular to angular

17.1 Lateritic clay red fine rounded

17.5 Basalt weathered black medium subangular-subrounded

29.5 Basalt weathered/fractured

black coarse subangular to angular

51 Basalt hard black fine subangular-subrounded

52 Clay black fine rounded

83 Basalt hard black fine subangular-subrounded

83.9 Basalt weathered/fractured

black coarse subangular to angular Water toucheddischarge0.2cum/hr

86 Clay Ash fine rounded

87 Sand White fine subangular-subrounded

Table 2.1: Sample description of a drill cuttings during the construction ofpiezometer


3 Construction of piezometer

The purpose of constructing a lithospecific piezometer is to obtain complete lithological dataand not just to drill a monitoring well. In order to obtain data of maximum accuracy, the fieldhydrogeologist must work closely with the driller and consult with him whenever changes arenoticed in penetration rate, slow returns, change in colour of samples, reduction in dischargeetc. The hydrogeologist must recognize the reasons for such changes. The difficulties indrilling, such as caving, boulders, caverns, etc. Whenever encountered, must be clearlyrecorded.

Construction of lithospecific piezometers must ensure that the piezometers meet the designcriteria for water level and water quality monitoring. Factors to be considered for piezometerconstruction shall include the following aquifer to be monitored, nature of materials thatmake up and overlie the aquifer (for example, unconsolidated or consolidated materials; ifconsolidated materials whether fractured or have cavities caused by dissolution); the depthto water, the type of drilling equipment required; access to the site; well casing and screenmaterials, length, and diameter, and cost. In unconsolidated deposits, the piezometerdesign, including the well screen, casing, annular space, back fill, gravel and surface seals.

Specific aspects of design however, can vary depending on specific requirements to meetlocal variations, site conditions encountered, and the drilling method used.

3.1 Selecting the appropriate drilling technique

Drilling technique for construction of piezometer will depend upon the type and nature offormations likely to be encountered below at the selected site. The technique to be adoptedfor soft and unconsolidated sediments shall be rotary, with bentonite mud or any otherbiodegradable mud as the drilling fluid. In the hard rocks, DTH drilling rigs are best suited.The DTH drilling technique uses air to bring the cuttings to the surface, as well as cleansesthe hole. Availability of high-pressure compressors makes drilling very fast. In suchsituations the fines get deposited in the fractures. The drilling in such cases should befollowed up systematic development. In the soft rocks, with poor accessibility and in riveralluvium, hand rotary drilling can be adopted as in parts of Orissa, Tamil Nadu and AndhraPradesh. In hard rocks, with heavy overburden having boulders the drilling has to be doneusing a combination of rotory and DTH rigs.

The drilling should ensure that it is capable of recording faithfully the harmonized arealbehaviour of groundwater of the targeted aquifer in the area, instead of a local micro trend.The piezometer should not be effected by wrong drilling techniques which can bring inexternal contaminants such as, poor quality water used in the mud pit, thick bentonite mud,drilling oil etc.

3.2 Deciding the depth of piezometers

The depth and diameter of piezometers are two important factors, which not only decidetheir best suited design, but may also affect the cost/economics of the piezometerinstallation.

In the unconsolidated formations, the aquifer horizon for construction of piezometer has tobe based on good understanding of the different vertically distributed aquifers, and the


specific aquifer of interest. In case all the aquifers need to be monitored, piezometer nestshave to be constructed. Systematic collection of drills cuttings and recording of drill time log,followed by electrical logging of the borehole, is very important in delineating the exactthickness of the aquifer. In the event of construction of nests, the deepest aquifer should bedrilled first. The identified zones should be correlated with the regional aquifer system,distributed in the sub-basin or basin, and accordingly the piezometer depth is then decided.

In crystalline rocks, the depth of the piezometer should be decided on the basis ofoccurrence of aquifer(s) to be monitored in a given hydrogeological environment. Threerypical situations are discussed

Case i: Weathered zone is made up of quartz and the fractured rock immediately underlyingit. The weathered zone acts as a good storage zone with its inter-granular connection, whilethe underlying fractured zone forms the main flow/conduit zone. In such a case the overlyingpermeable zone recharges the fractured zone and hence the two zones can be consideredas part of the same aquifer. The piezometer should be then drilled down to the fracturedzone.

Case ii: Fractured zone is overlain by clayey weathered zone. The weathered and fracturedzone exhibit different permeabilities. In such situations both the weathered and fracturedzone are to be considered as independent zones. The monitoring should be doneindependently for the weathered as well as the fractured zone. The shallow weathered zonecan be monitored using an existing open dug well while the fractured zone is monitored byconstructing a piezometer.

Case iii: Weathered zone is clayey and impermeable, the recharge to deeper fracture zonemay be from a distant recharge area. In such case the piezometer has to be installed againstthe fractured zone only. The extent and thickness of the fractures do not follow a systematicfashion, hence the need for greater care in identifying the fractured zone by thoroughlymonitoring the drilling.

In case of basaltic rocks, occurrence of multiple aquifers is common. Generally, the upperweathered zone in such rocks is totally clayey and impervious and the first aquifer in suchformations may occur at different depth as vesicular zones. Each vesicular flow should betapped by an independent piezometer. In areas where more than one vesicular flow has tobe monitored, piezometer nests or a group of piezometers within a limited area (village,watershed) need to be installed, tapping different aquifers. Care has to be taken in properlysealing the upper aquifers while tapping the deeper zones. Typically, contractors who drilldrinking water wells are not the best suited for drilling such piezometers. Departmentaldrilling rigs should be mobilised for taking up such drilling.

In the case of hard sedimentary rocks, good understanding of the stratigraphy is critical inunderstanding the different potential aquifers. Sandstone, shale and limestone occur insequences. The sandstone in many cases have copious supplies. The limestone rockspossess both primary and secondary porosity in the form of fractures, solution cavities andcavernous zones. Shale have limited discharge. Good understanding of the startigarphy,combined with judiciously used geophysical surveys and profiling, the depth of the aquifer tobe monitored can be inferred. Confined aquifers when met with produce artesian free flowingwells, should be, anticipated at the design stage itself. Methods to monitor the pressurechanges should be part of the design.


The depth has to be accurately measured after the piezometer construction is complete byusing a weighted tape. The measurement should also be compared with the total number ofdrill rods used during the piezometer construction.

3.3 Diameter of piezometer

A piezometer is a non-pumping well and ideally needs to be as small in diameter aspossible. The diameter should be such that it shall facilitate measurement of water tableusing a variety of measuring devices and collection of water sample. The diameter will alsobe dictated by the diameter of the measuring device, such as the probe of the Digital WaterLevel Recorder, diameter of the water quality sampling pump. Piezometers having adiameters of 100 mm are the most suitable. Shallow piezometers having diameter of 50 mmare also uncommon. The utility of the piezometers, to carry out pumping tests, geophysicaldown hole logging and hydrofracturing should also influence the diameter of the piezometer.In the case of deep tube wells (>100 mtrs) in the alluvial areas, the cost will be a majorconsideration in deciding the diameter of the piezometer. In such situations telescopicdesign of 100-150mm down to 30 mtrs followed by 50mm dia till the bottom should also beseriously considered. Inclined piezometers can reduce the diameter considerably and causemajor problems during lowering of DWLR probe or the sampling pump etc.

The diameter of the hole is often critical and is recorded based on the diameter of the drillingbit. The hole diameter is best measured using a calliper log.

The piezometer is intended to be vertical, however it does not always stay vertical but driftsfrom verticality. To understand the drift use of a mirror should be made to reflect sunlightdown the hole to enable a visual check on the straightness of a hole. Visibility of half hole isan indication of loss of verticality. The exact point of deviation can be checked by measuringthe depth with a tape, while reflecting light down the hole.

3.4 Actions to be taken prior to drilling

• Confirm landowner's/concerned government agencies, permission to enter the propertyfor drilling.

• If the location is within a school/office/hospital discuss with the authorities to confirm theappropriate time when the drilling can be carried out without disturbing their functioning.

• Check the marking at the site and confirm with the geographical co-ordinates.

• Locate any subsurface power lines, waters lines, telephone cables, sewer etc.

• Locate water sources for drilling purposes and secure permission for their use.

• Prepare the drainage channel for draining of water.

3.5 Piezometer construction in unconsolidated formations

Unconsolidated formations in peninsular India are largely localised to coastal tractscomposed of beds of sand and clays, and sedimentary beds of Gondwana and Tertiaryformations made of alternate layers of sandstone and shales. Piezometer construction inthese areas is through rotary drilling. In the unconsolidated formation, rotary drilling has tobe adopted. Rotary drilling makes use of viscous bentonite mixed fluid as medium of drilling.The mud fluid acts as coolant to the rotating drilling bit as well as a medium for bringing outdrill cuttings outside the borehole. Use of bentonite clay has been banned for water well


drilling in many countries, as they are not bio-degradable. Organic materials like guar gumare replacing bentonite clay as popular bio-degradable drilling fluid.

The main components (see figure 3.1) of a piezometer in an unconsolidated formation are:

Borehole: This is the primary component of a piezometer and acts as a host to the othercomponents.

Well assembly: This is essentially the hardware of the piezometer and is accommodated inthe borehole and also protrudes above the ground. Depending upon location of the aquifer inthe vertical section, it may comprise one or more of the following parts:

Figure 3.1:Piezometer components in unconsolidated rocks

Blank casing pipe: A blank casing pipe is provided to serve one or more of the followingobjectives:

• To prevent caving-in/sloughing of the drilled formation.

• To prevent a hydraulic connection between the piezometer and the drilled formationother than the aquifer to be monitored.

• To collect the fines entering into the screen. As debris sump.

Screen: A screen provides a hydraulic connection between the piezometer and the aquiferto be monitored.

Gravel pack and seal: Gravel is provided in the annular space between the borehole andthe well assembly around the screen and beyond, extending preferably over the entirethickness of the aquifer to be monitored. The gravel pack serves the following purposes:

• inhibits the entry of the fines into the screen.

• enhances the hydraulic connection between the piezometer and the aquifer


Cement seal:is provided just above and just below the gravel pack to preempt any hydraulicconnection between the piezometer and the overlying/ underlying formations, through thegravel pack and screen perforations.

Sanitary seal: A 50cm thick concrete seal is provided at the ground surface to prevent theentry of surface water into the piezometer. The seal should be in the form of a cone aroundthe casing to drain the water away from the well. The seal is underlain by a clay fill/packingfor a more effective isolation of the aquifer to be monitored.

3.6 Sampling procedures during drilling

Examination of drill cuttings is very critical part of piezometer design in the un-consolidatedformations. Some formations are better aquifers than others. Grain size have to beinterpreted during the examination of the lithology (see figure 3.2).

Clean gravel have large pores and hold large quantities of water.

Sand and gravel mixture are very good aquifers. When percentage of gravel to sand is veryhigh the aquifer will permit copious discharges.

Coarse sand are potential aquifers

Figure 3.2:Grain size classification

Fine sand are poor aquifers

Clays hold lot of water but cannot flow. In some situations the clays when tapped can yieldpoor quality water.


Sandy aquifers when overlain by thick impermeable clay and when penetrated by thepiezometer can result in flowing wells.

Standarised sampling procedures have to be adopted by all agencies:

• Collect the samples for every meter.

• Lay the samples in succession, as obtained, and mark the depth

• Dry the samples for accurate identification and classification.

• Describe the samples precisely before and after washing and record any additionalinformation.

• Look out for fossils and identify them

• Compare all samples with previous samples.

• Place the samples in plastic wrap and label legibly for any future identification/test.

• Sample boxes with pigeon hole windows are best suited to transport and for preservingthe samples.

3.7 Down hole inspection

In order to take a decision on the design the piezometer assembly, downhole geophysicallogging needs to be carried out. Logging should be used for providing additiona informationthan gained from examination of drill cuttings. Th details to be collected shall include theformation penetrated, formation characteristics modified as a result of drilling, condition ofthe hole, the exact depth and thickness of the aquifers and water quality of the aquifers. Thestandard probes to be used shall be electric, SP, Gamma, calipper, temp and fluidconductivity (refer Annexure-II). The geo-physical logging, examination of drill cuttings andthe objective of monitoring should form the basis for finalising the piezometer design.

3.8 Piezometer Completion

Piezometer completion should ensure that the hydraulic head measured in the piezometer isthat of the aquifer of interest. Ensures that only the aquifer of interest contributes water tothe piezometer and prevents the annular space from being a vertical conduit for water andcontaminants. Such completion steps are critical to the long-term goals of groundwatermonitoring. It has to be remembered that the investments made in the construction ofpiezometers are part of network monitoring programme that have to last for decades. Wellcompletion in unconsolidated deposit rocks consists of installing the well casing and screen,filling and sealing the annular space between the well casing and piezometer wall.

3.8.1 Piezometer Design

Good design and careful well construction can only ensure good hydraulic flowcharacteristics in the aquifer. The screens should be lined up exactly with the permeableportion of the aquifer. The screens should provide the same hydraulic conductivity of theaquifer. The design should prevent entry of fines and sand particles into the piezometer.The piezometer should completely seal the aquifer which are not to be monitored. The wellassembly should be able to withstand any corrosion or physical damages during pumpingand logging.


Unconfined aquifer For monitoring the piezometric head of an unconfined aquifer, thepiezometer essentially comprises a cement seal at its bottom followed by the well assembly,resting on the seal, comprising of (starting from the bottom) a bail plug, screen and finally awatertight casing pipe extending above the ground surface

Confined/leaky-confined aquifer: For monitoring the piezometric head of a confined/leaky-confined aquifer, the piezometer essentially comprises a borehole drilled through theoverlying formation and the entire thickness of the aquifer, into the lower formation toaccommodate a cement seal at its bottom. The well assembly, resting on the seal,comprises (starting from the bottom) a bail plug, screen and watertight casing pipe extendingabove the ground surface

3.8.2 Screen length

The well screen should be long enough to ensure that the piezometer records the verticallyintegrated piezometric head of the investigated aquifer. Thus, there must be a perfecthydraulic connection between the piezometer and the aquifer over the entire aquiferthickness. Ideally, this requires a fully penetrating piezometer, that is, the screen providedover the entire thickness of the aquifer.

In case of thin aquifers, a fully penetrating piezometer may be provided. However, in case ofthicker aquifers, a fully penetrating piezometer may not be economically feasible, and assuch, a partially penetrating piezometer may have to be provided. But even a partiallypenetrating piezometer can provide an almost perfect hydraulic contact, if it is surrounded bya fully penetrating (that is, extending over the entire aquifer thickness) gravel pack of largeenough thickness and hydraulic conductivity. The length of the screen, in such a case mustbe large enough to ensure a free inter-flow of water between the piezometer and the aquiferthrough the gravel pack. A screen length of two meters surrounded by a fully penetratinggravel pack may provide the necessary hydraulic contact and ensure the free inter-flow.

The gravel pack should have a greater grain size than that of the aquifer material in thevicinity of the screen. The gravel pack grain size and gradation should be so designed tostabilize the aquifer material adjacent to the screen and permit only the finest grains to enterthe screen during development, finally providing sediment-free water into piezometer (seefigure 3.3). The gravel pack must not intersect multiple aquifers and should not cross-confining units, otherwise they would establish vertical, hydraulic connection along theannulus between the two aquifers, thus defeating the whole concept of piezometersmonitoring single aquifers.

Figure 3.3:Idealised arrangement of gravel around thefilter assembly for increasing porosity andhydraulic conductivity


Specific details of completion require consideration of several hydrogeologic factors,including the depth to water, to the top of the aquifer of interest, and to the zone in theaquifer to be monitored;

• the nature of materials that make up the aquifer to be monitored and that overlie theaquifer

• expected water-level fluctuations

• expected direction of the vertical head gradient--down ward,

• whether the aquifer is confined or unconfined

3.8.3 Design of gravel size and screen slot size

Particle sizes are to be determined in the field after sieve analysis of the aquifer material(see figure 3.4). Before sieve analysis the samples need to be dried and weighed. Thestandard sets of required sieves need to be placed one above the other in the order ofincreasing sieve diameter. The sample should be placed in the top sieve and shaken toseparate the various grain sizes. The weight of the material retained in each sieve should bemeasured and expressed in percent of the initial weight.

Figure 3.4:Standard sets of sieves

The percentage weight should be plotted against the sieve size, on a logarithmic scale. Theresultant curve that is obtained gives information about the uniformity of the aquifer material.Use of screen having a median size of the aquifer material is generally preferred. Sincepiezometers are not pumping wells the main concern should be top have a good hydraulicconnection while at the same time preventing any entry of fine material into the piezometer.

The slot size of the screen should be so designed that the aquifer material does not enterinto the piezometer. Assuming that fractions greater than or equal to the d60 of the aquifermaterial are to be retained, a slot size of d60 may be provided. (d60/d10) gives theuniformity coefficient The higher the uniformity coefficient, the higher would the efficiencyand vice versa. Thus, depending upon the uniformity coefficient and the extent of theexpected well development, the usually recommended slot size is d40 to d60 of the aquifermaterial.


The average size of the gravel should be 4 to 6 times the d50 size of the aquifer. The gravelshould be as uniform as possible to avoid segregation during the placement.

3.8.4 Annular seals

Annular seal(s) are to be installed from above the gravel pack to near land surface, in orderto seal the annular space between the casing and borehole wall. These seals should prohibitvertical flow of water between aquifers and prevent mixing and cross-contamination ofaquifers. They also protect against infiltration of water and contaminants from the surface.

3.8.5 Surface Seal

The surface seal prevents surface runoff down the annulus of the well and, in situations inwhich a protective casing around the well is needed, holds the protective casing in place.The depth of installation of a surface seal can change from area to area. The surface sealshould be a mixture of cement and gravel.

3.8.6 Protective Cover

A protective cover should be installed around the piezometer to prevent unauthorizedaccess, house the measuring device as well as to protect the piezometer from damage. Theprotective cover should be installed at the same time as the surface seal and should extendto below the ground. Many designs of protective casing are already available. Essentially itshould be a large diameter casing or a GI sheet with locking protective cover and weep hole,which permits condensation to drain out.

3.8.7 Development

The development of the piezometer, is primarily aimed at ensuring an efficient hydraulicconnection between the aquifer and the piezometer. The development e is very crucialsince the drilling mud, which inevitably sticks to the walls and invades into the aquifer inhibitsthe hydraulic connection between the aquifer and the piezometer. The invasion of the drillingmud and thickness of the cake depends upon the hydraulic conductivity of the aquifer. Thehigher the hydraulic conductivity, the higher is the mud invasion and mud cake thickness.The development should completely remove the invaded/sticking mud and also the fines(see figure 3.5). Under-developed piezometers will fail to provide the true information of theaquifer being monitored and the water level data emerging from such piezometers can leadto wrong understanding of the system.

Figure 3.5:Effective development by pumpingwater under pressure through thescreens


The mud cake around the screen should be dissolved using sodium tripolyphosphate.Sufficient volume of solution of sodium tripolyphosphate should be made and circulated todisplace mud around the screen area as well as a portion of the casing for disaggregatingthe clays. The polyphosphate solution should be allowed to act for at least 24 to 36 hours.The solution should be circulated through the well screen that effectively acts on the mudcake. This should be followed by washing.

The development should be carried out through air compressor by alternatively surging andpumping with air. The air should be injected into the piezometer to lift the water. As the waterlevel reaches the top of the casing, air supply should be shut off allowing the aerated watercolumn to fall. Use of eductor lines is recommended when the static water level is deep.

High velocity jetting is another development technique that consists of a jetting tool fitted tothe bottom of the drill string. The jetting tool should be lowered and washed all along thescreen length using fresh water. This should be followed by airlift. Careful jetting of thescreened area is required. Jetting combined with airlift should be continued till pumped wateris free from fine sand and bentonite, and the discharge from the piezometer stabilizes.

Development can also be done through back washing. In back washing, there is a reversalof flow through screen opening, which agitates the sediments and leads to the removal ofthe finer fraction and rearrangement of the formation particles. As a part of back washing thewater column should be alternatively lifted and allowed to fall back. The pump should initiallybe started at a reduced capacity and gradually increased to full capacity.

Mechanical surging needs to be carried out at times using surge blocks attached to drill rods.The surge block forces water into and out of the screen similar to a piston in a cylinder. Thesurging process at times forces fine material back into the screens and hence the finesshould be removed before taking up surging.

3.8.8 Pumping Test

A pumping test is conducted with constant discharge or variable discharge with constanthead for estimating hydraulic parameters of the aquifer tapped in the piezometer. The testinvolves monitoring of the time variation of drawdown in one or more observation wells inresponse to a pumping at a known discharge, from the piezometer. The observation must bein the vicinity of the piezometer and must be tapping the same zone. If no such observationwell is available, the drawdown can be monitored in the piezometer itself. Details of pumpingtest procedures is enclosed as Annexure-III


4 Piezometers construction in consolidated formations

The drilling of piezometers in consolidated formations are different from the construction ofunconsolidated and semi-consolidated formations. The groundwater occurrence in theconsolidated rocks is in the weathered zone and fractured zone. The consolidated rockshave negligible primary porosity and it is only the secondary porosity, like fracturing andweathering, that provides the porosity and permeability necessary for the storage and flow ofgroundwater.

Groundwater discharges are largely dependent upon the rock type. In granite, gneiss andkhondalites highly productive groundwater zones are found in the vicinity of largelineaments, fractures and deep weathered areas. The lava flows are mostly horizontal andoccasionally gently dipping and as such, groundwater occurrence is controlled by the waterbearing properties of the vesicular zones. In carbonate rocks like limestone, marble anddolomite, solution cavities serve as large repositories of groundwater. In all these rocks thedrilling is usually carried out by the Down The Hole (DTH) drilling technique or a combinationof DTH and rotary drilling.

For monitoring the piezometric head of an unconfined aquifer, the design should be a casedborehole drilled through the top collapsible/weathered rock zone, overlying the unconfinedformation to be monitored and is hard enough to stand on its own without the casing. Thecasing should stand above the ground by 0.3 to 0.5 m.

For monitoring the piezometric head of semi-confined aquifer, which has differentpermeability from the top weathered zone then the design should be a cased hole, drilledthrough the entire weathered rock zone, overlying the fractured/hard formation to bemonitored. The depth of drilling should be such that it taps the most productive part of thefractured zone. Geophysical resistivity surveys should provide the value for the depth ofdrilling. DTH drilling is very fast and completion of one piezometer of 100-m depth takesonly 12-18 hours.

4.1 DTH drilling characteristics

The drilling being very fast, supervision of DTH drilling becomes very important. The sitehydrogeologist has to ensure that the compressor is in good condition to deliver the requiredair pressure and that the drill bit is of the required diameter. The site hydrogeologist has toensure that the drilled hole is constantly cleaned of the drill cuttings. During the change ofthe drill rod as well as when a water bearing zone is met, the well should be adequatelydeveloped and the discharge measured using a V notch. The drill cuttings should becollected and studied continuously. At the end of drilling to the desired depth, the well shouldbe cleaned for at least two hours. The cleaning should lead to de-clogging of all the fracturesdrilled through, and removal of all fines and cuttings.

4.2 Sampling procedures for consolidated rocks

The drill cuttings should be sampled for every one-meter frequency and whenever there is achange in lithology. The samples obtained in the DTH drilling are due to the action of the drillbit, which should be kept in mind while examining the sampled cuttings. Further, the depthsof the formations as revealed by the cuttings may not always be accurate - though they canbe generally relied upon. The drill cuttings have to be classified on the basis of megascopic


observations using hand lens, both for texture and mineral constituents. The descriptionshould identify the rock, colour, grain size, shape, fossils, trace minerals, etc. The drillcuttings should be dried, packed in polythene bags, marked with well number and depthinterval, date and time. The samples should be stored in a box with numberedcompartments. A correct procedure for collection and storage of drill cuttings ensures goodcorrelation between the drillers log, VES interpretation, downhole logging and samplescollected. The recorded drilling data should include the following:

• A drill log (time taken for drilling each meter of the drilled depth)

• A description of drill action (such as nature of drilling noise and motion of the rig)

• Depths at which moisture is struck

• Depth at which water flows

• Depths at which discharge increases

• Colour, pH and EC of the water

4.2.1 Removal of fines during drilling

Air drilling causes plugging of fractures and crevices with fines of drill cuttings. The cloggedmaterial until removed the hydraulic conductivity with theaquifer cannot be established. Thewater level measurements from such piezometers give erroneous data and theinterpretations of such data gives a wrong picture about the groundwater reservoir. Cleaningand development of the drilled hole should be part of drilling activity and should be carriedout simultaneously with the drilling operations. After change of every drill rod, cleaning andflushing of the hole should be carried out. On completion of drilling to the desired depth afinal development should be done by running the compressor of the rig till the water is free ofcuttings and the water is clear. The development should be carried out using educator pipesif the piezometer is very deep. Jetting should be carried out where the depth of drilling islarge, the discharge is low and the drilling speed is very high.


5 Measuring water levels

After completion of piezometer construction the piezometer should be surveyed to determinetheir X-Y coordinates and elevation. A surveyed reference mark should be placed on theNorth side of the piezometer at the ground elevation and on top of the well casing. Thisreference point should be used for all depth-to-water measurements. Its height should bedetermined within 1mm in relation to mean sea level.

Figure 5.1:Water level measurement using water level indicator

The depth to water in the piezometer shall be measured by more than one instrument, suchas a steel tape coated with chalk/ electrical tape. Use of DWLR could be another option.Regardless of the equipment used, the depth to water should be measured to the nearestmm. The measuring devices should be chemically inert. They should be thoroughly cleanedafter each measurement

Water levels measured in piezometer should be used to construct a water table map usingthe data from the other existing piezometers in the neighbourhood. Before constructing thepotentiometric map fresh water level measurements should be taken from the all thepiezometers and observation wells in the neighbourhood that will be used in the constructionof the potentiometric map. The potentiometric surface should be in consistent with theneighbouring observation wells. Any inconsistency could probably be also due to poordesign, improper development, and establishment of vertical hydraulic gradients. Suchpiezometers should be inspected through geophysical down hole logging or additionaldevelopment.


Figure 5.2: Example hydrograph

The development of the piezometer will result in lowering of water levels. On completion ofthe piezometer the water level need not have fully recovered. It will take considerable timefor the water level to become stable. Continue water level measurements in the piezometerperiodically after completion until it becomes stable and it no more shows any recovery. Thefinal water level to be recorded in the well completion report is the stable water level.


6 Groundwater sampling

The chemical composition of the groundwater should represent mineral composition of thecomposition of aquifer through which the water moves. In addition it is influenced by therecharging waters, which can be simple precipitation, or biological and chemical reactionsoccurring on the land surface and in the soil zone. The piezometer if not completelydeveloped then is likely to reflect the chemical characteristics of the drilling mud and sometimes drilling oils used. If the piezometers tap more than one aquifer then the samplecollected will be a mixed sample reflecting the characteristics of all the aquifers tapped.

Several water quality parameters should be measured directly in the field during the processof piezometer construction. These include the electrical conductance, pH, and dissolvedoxygen content of the groundwater.

Electrical conductance measurements can be used to estimate the concentration of totaldissolved solids (TDS) in a water sample. However, it should be noted that natural watercontains a variety of both ionic and uncharged chemical species. Therefore, conductancedeterminations cannot be used to obtain highly accurate estimates of TDS.

Two steps should be taken before collecting the sample. First, the depth to water and totaldepth of the piezometer should be measured from the reference point. After this step waterin the piezometer should be evacuated to removed any stagnant water, which may not berepresentative of flowing groundwater in the aquifer. The standing water may contain foreignmaterial that was inadvertently or deliberately introduced during drilling. Removal of allstagnanat water is performed by removing three times the volume of water standing in thecasing. The evacuation procedure can be performed with a low speed pump or bailingdevice. The water level in an evacuated piezometer should be allowed to recover to theoriginal level before sampling.

The type of analysis for which a sample is being collected determines the type of bottle,preservative, holding time, and filtering requirements. Generally, samples should be placedin a cool place or in an ice box. Ideally, the samples, should be transported to the analyticallaboratory within 24 hours of sample collection.

Sample labels should include the identification number, name of the collector, date and timecollected, place of collection, and parameters to be analyzed by the laboratory.

Analytical results of the first sample should be made available to the site hydrogeologistwithin 24 hours to guide remediation activities in the piezometer development or design. Tocheck Laboratory accuracy duplicate samples can be collected and sent to two laboratories,wherever possible.


Figure 6.1:Small diameter sampling pumpfor purging and collectinguncontaminated water samples


7 Documentation of piezometer construction

The piezometer drilling information should be documented in permanent files. Careful andcomplete documentation aids in interpretation of groundwater data and provides historicalreference for future use of the piezometer. The groundwater data collected must be storedin the computer files of HIS.

Documentation of the methods and materials used for well installation is required for eachpiezometer. Documentation of all the details should be completed at the time of piezometercompletion. Documentation should include lithological log, driller's log, piezometer designand record of well development. A record should be kept of other logs collected during andafter drilling, such as geophysical logging, pumping test, discharge test, packer test andwater quality field analysis. Part of tye documentation should include location map and sitesketches. The location map and site sketch need to be of sufficient detail and scale toenable field location of a well by field personnel unfamiliar with the site. Information on thelocation map. This should typically include roads, topography, water bodies, and culturalfeatures. Compass directions or latitude/longitude and a horizontal scale need to beindicated on the location map. Distances from milestone markers or other permanent culturalfeatures to the piezometer site should also be included. A sketch of the piezometer wellhead, should be available, identifying features of the outer cover, such as the height of thetop of the casing in relation to land surface, the locations of measuring and sampling points,and general characteristics of the protective cover and fencing details. Written descriptionsof the site and piezometer characteristics should compliment the sketch.

Photographs of each piezometer should be documented in the well file as well as digital dataalong with the database. The purpose of documentary photographs of each piezometer siteis to provide a visual record of land use near the piezometer, which can aid in theexplanation and interpretation of the water level data, and can aid in locating the piezometer.When changes occur at or near the piezometer that might affect hydrological interpretationof data from the well, a new set of photographs is required to document those changes. Forexample, changes in the reference datum of the well or changes in land use near the wellmight warrant a new set of photographs. The set of photographs should be, one photographof the piezometer and surrounding area as seen when approaching one close-upphotograph and water-level measuring point and any additional photographs to documentfeatures that might influence the chemistry of water in and from the piezometer Generalinformation and the identification of important features shown on the photographs need to berecorded. The minimum general information should include the date of the photographs, andlocation and identification of the piezometer (site identification number or station name,latitude and longitude, written description). Features identified on the photographs willinclude at least the measuring point used for water-level measurements and the samplingpoint.


Figure 7.1:Photograph of the piezometer with theprotection cover showing the landusein the neighbourhood


8 Piezometer nest

In areas with multiple aquifers, piezometer nests or cluster piezometers have to beconstructed for monitoring the piezometric head of independent aquifers. In theunconsolidated formations the different aquifers to be tapped by individual piezometer aspart of the cluster has to be based on good understanding of the different verticallydistributed aquifers. The design of the cluster piezometers should be such that there is novertical connection between the different aquifers. Systematic collection of drill cuttingsfollowed by electrical logging of the borehole is very important in delineating the exactthickness of independent aquifers. In the event of construction of nest the deepest aquifershould be drilled first. The independent piezometers should be accommodated within a smallarea of 15-20m diameter.

Figure 8.1:Design of piezometer nest tapping multiple aquifers


Annexure – IHydrogeological Frame Work of Peninsular India

The Peninsular India is a vast region with diversified geological, climatological andtopographic set up, giving rise to varying ground water situations. The rock formations rangein age from Archean to Recent, and are widely varied in composition terrain's of the Easternand Western Ghats to the flat alluvial plains of the river valleys and coastal tracts. Thetopography and rainfall control runoff and ground water recharge to a large extent.

Almost the entire Peninsula is occupied by a variety of hard and fissured formations,including crystalline, trappean basalt and consolidated sedimentaries (including carbonaterocks), with patches of semi-consolidated sediments in narrow intra-cratonic basins. Ruggedtopography, compact and fissured nature of the rock formations, combine to give rise todiscontinuous aquifers, with moderate to poor yield potentials. The near surface weatheredmantle, forms an important ground water reservoir and is a source for circulation of groundwater through the underlying fracture systems. In the hard rock terrain, deep weatheredpediments, lowlands, valleys and abandoned river channels, generally contain adequatethickness of porous material, to sustain ground water development. Figure A.1.1 gives thegeological framework of Peninsular India.

Figure A.1.1:Geological map of India showingnatural recharge study area inGranite terrain

An assembly of geological formations in the earth crust containing ground water undersaturated conditions is called as ‘ground water basin’. Groundwater basins are made up ofthe rocks of geological formations. These rocks are composed of minerals, which will havetheir own particular physical appearance and chemical composition. The shape, size of theindividual minerals in rocks and their packing in rock units is very important. The rock as a


whole when considered may appear as compact, hard, loose, brittle, consisting of individualgrains, crystals or even an amorphous appearance.

Rock types are classified as ‘consolidated’ or as ‘unconsolidated ‘ types. The consolidatedtypes are referred to as ‘ hard rocks’. These rocks are composed of individual mineralswhich are fused together and can not be separated from each other. The unconsolidatedrock types or ‘soft rocks’ are made up of loose materials consisting of separated minerals.Consolidated rocks include granites, basalt, gneiss, sandstone, shale, limestone, quartziteetc. Unconsolidated rocks include sands, silt, gravel, clays and loams etc.

Rock Porosity:

Rock porosity refers to the open space in various rock types. In unconsolidated rocksopenings are present in between the grains due to loose packing. In consolidated formationsopenings are due to fractures, fissures and joints which are not uniform. Figure A.1.2 givesthe pore space arrangement in various rock types. Figure A.1.2, a,b,c,d shows the primaryporosity also referred to as intergranular porosity, which are openings between the rockgrains formed during the

Figure A.1.2:Pore space arrangement in variousrock types

Deposition of the unconsolidated sediments. Figure A.1.2e, f shows the decreasee primaryporosity in consolidated rocks.

The porosity of rock is the ratio of the volume of the openings to the volume of rock and isexpressed in percentages.

n= V0/ Vt

Where n= porosity in fraction, V0 = Volume of pores, Vt = Rock volume

Figure A.1.3:Flow velocities in rock


The effective pore space is always smaller than the total pore space. As such effectiveporosity can be defined as the ratio of the volume of effective pore space to the total volumeof rock. Figure A.1.3 gives the flow velocities in rock.

The porosity of unconsolidated rocks like gravel range from 0.2 to 0.4, sand 0.2 to 0.5 andfor silt 0.3 to 0.5 depending upon their grain size packing and sorting. For consolidatedformation the porosity is less than 0.1

Based on this the varied modes of ground water occurrence in the country may be broadlysummarized as below (CGWB, 1989):-

(a) Porous formations comprising unconsolidated and semi consolidated sediments.Aquifers interconnected, often extensive, both continuous and discontinuous, moderateto very high yield potential.

(b) Consolidated and fissured formations. Aquifers discontinuous; limited yieldpotential.

Porous Formations

Unconsolidated formations:

In unconsolidated formations rock openings are present between the individual grains andminerals. The distribution of these grains is far more even than in consolidated formations.The openings are known as primary porosity. The porosity depends upon grain packing andsorting.

The Quaternary sediments in the coastal and deltaic tracts bordering the peninsular andcoastal alluvium form important reservoirs of ground water. The horizons of sand and theiradmixture form potential aquifers. The aquifer materials vary in particle size and roundingand in their degree of sorting. Consequently, their water yielding capabilities varyconsiderably. The coastal aquifers show wide variation in the water quality, both laterally andvertically, thus imposing quality constraints for ground water development. Thus, in thesetracts, ground water withdrawal requires to be regulated so as not to exceed recharge and inorder to avoid sea water ingress into coastal aquifers. Sites for piezometer location can bereadily identified in these formations.

Semiconsolidated Formations:

The Semiconsolidated Formations belong to the group of rocks ranging in age fromCarboniferous to Mio-Pliocene. These are mainly composed of shale, sandstone, andlimestones formations. The terrestrial freshwater deposits belonging to Gondwana System ofthe peninsular shield are included under this category. The Gondwana sandstones formhighly potential aquifers, locally. Elsewhere, they have moderate potential and in places theyyield meagre supplies. These sediments normally occur in narrow valleys or structurallyfaulted basins. Though these formations have been identified to possess moderate yieldpotential, the physiography of the terrain restricts exploitation. Under favourable situations,these sedimentaries give rise to flowing well conditions as in parts of Godavari valley, Vellarbasin, Cambay basin and parts of westcoast.


Consolidated and Fissured Formations:

In consolidated rock openings are primarily present at fractures, at joints, along beddingplanes, and in the form of solution holes. Openings do not have an even distribution, but arerather localised phenomena. This unevenness is due to during and after consolidation of therock, cooling, compaction, tectonic activity, or dissolution process cause the unevendistribution of the openings. Such openings are refereed to as secondary porosity.

As indicated earlier, from hydrogeological point of view, these rocks cover over a widestretch of peninsular India and are broadly classified into the following four groups:-

i. Crystalline rocks.ii. Volcanic rocks.iii. Clastic rocks.IV. Carbonate rocks.

The nature of occurrence of rocks, its geological, lithological, mineralogical characteristics,structural/textural control in disposition of lithounits and consequent occurrence & movementof ground water in these groups of rocks are briefly described below:-

Crystalline rocks:

The major suits of crystalline basement rocks of Peninsular India are of Pre-Cambrian age.The predominant lithological types are granite gneisses and high grade metamorphic rockslike Charnockites, granodiorites and Khondalites. The ancient land surface has beenexposed to prolonged weathering, which has resulted in the formation of a mantle ofalternation products, normally more than 10 m thick. This alteration product is called regolithand includes both the residual soil and saprolite. The latter is derived from the in-situweathering and has become largely disaggregated. The residual soil is developed from theunderlying saprolite by further dissolution and leaching, combined with other chemical,physical, and bioturbation processes. The rocks are composed of predominantly alluminiumsilicates. Over long periods the infiltrating acidic rainfall has reacted with the alkalineminerals, leaching the more soluble and mobile components and reprecipitating less mobileminerals with the formation of kaolinite and Fe- Al oxides. In the extreme kaolinite dissolutionalso occurs and only quartz sand is left. The soil includes tropical oxysols, kaolinite quartzand oxydised Fe- minerals mostly biotites.

The relative depth and degree of weathering also depends on the mineral grain size of thecrystalline rocks, their intensity of fracturing and the relative proportion of Fe-Mg minerals.The transition to unweathered rock bedrock is generally gradual and occur over a fewmeters with saprock as intermediate layer. The upper part of saprolite is also with highkaolinite clay, while lower part contains more primary minerals. The boundary betweensaprolite rock and weathered regolith is more sharp in coarse grained granite than in finegrained banded rocks. A basal breciated zone is present with rock fragmentation but manytimes misinterpreted as saprock in bore hole logs. The coarse grained quartz rich crystallinerocks develop higher porosity/permeability, while schistose metamorphic rocks have lessporosity due to presence of Fe- Mg minerals which readily weather to secondary bi-products.

According to their modes of origin, these rocks can be divided into igneous (plutonic andhypabyssal) rocks and metamorphic rocks. The common rock types are granite, gneiss,charnockite, khondalite, quartzite, schist and associated phyllite, slate etc. These rocks


posses negligible primary porosity but at places, are rendered porous and permeable due tothe formation of secondary openings by fracturing and weathering. In moist areas havinghigher annual rainfall (like Western Ghats), these rocks are often capped by thick lateriticzones, especially in Karnataka.

Ground water yield depends on rock types and possibly upon the grade of metamorphism.The ground water studies carried out in the crystalline rocks have revealed the existence ofdeeply weathered and fractured zones along certain lineaments, locally forming potentialaquifers. These lineament zones are found to be highly promising for ground waterdevelopment. Further, following additional aspects of weathering profile in crystalline rocksare important:

Figure A.1.4: Schematic conceptual hydrologeological model of the weatheredcrystalline basement aquifer

(Source: P. John Chilton & Stephens D. Foster, Hydrogeology Journal v.3 no. 1, 1995)

Generally, little consideration is given to petrographical and textural aspects for siting ofpiezometers; yet, in the fine grained igneous rocks (like ‘aplites’), the weathered zone isoften clayey, whereas in coarse granite and gneiss, it can be productive.

In plutonic igneous and metamorphic rocks, the typical profile of the weathered zonebeginning with the fresh host rock at the bottom and progressing upwards, varies from A toD as follows:

Zone D: Sandy clays or clayey sands, commonly aluminous, ferruginous and oftenconcretionary. Only few metres thick, high porosity and low permeability.


Zone C: Massive clay alterites, commonly kaolinitic, in which some primary minerals maybe preserved in their original form. High porosity but low permeability. Mayrange upto 30 m thickness.

Zone B: Rock which is progressively altered upward to granular, crystal aggregates androck fragments. May range from a few metres to 30 m thickness. Low porosityand appreciable permeability.

Zone D: Fractured and fissured rock, may range from a few tens to a few metres thick. Ithas low porosity and moderate permeability in fracture systems.

The water is often struck at the junction of weathered Zone (B) with the fractured rock (Zone'A'). Following additional points may also be noted:

• The sheared and steeply dipping phyllites and schists are often found to be productive.Yet, in the piezometers the weathered zone of these rocks should be isolated due to itsclayey nature.

• The fractures in the fine grained igneous rocks are closely spaced but they may not beopen at depths and thus, may not contain water. Besides, wells located along thetensional, brittle (aC2 type) fractures will give reasonably good discharges. In manysituations, trends of intrusive bodies (like dykes and quartz/pegmatite veins) representdirection of tensional fractures and lineaments.

• The water level fluctuations in the crystalline aquifers are often sharp due to their lowspecific yield and poor yield.

• The piezometers are often located in the crystalline formations in the direction of theirfoliation or regional dip, due to the variations in the aquifer in the dip direction.

• In the metamorphic rocks, which are poor aquifers, the piezometers should be locatednear structural lineaments and at intersection of fractures to ensure all round availabilityof water.

In areas underlain by hard crystallines and metasedimentatries (viz., Granite, gneiss, schist,phyllite, quartzite, charnockite etc.), occurrence of ground water in the fracture system hasbeen identified down to a depth of 60 metres upto 200m, locally. It has been noted that thedeeper fracture systems are generally hydraulically connected with the weathered saturatedresiduum. Due to such factors, the depth of piezometers in crystalline formations is requiredto represent the aquifers being exploited and hence there is a need to drill piezometers todepths greater than 50 m below ground surface coinciding with the fracture zones beingtapped in the neighbourhood. A detailed inventory of the operational borewells in the area isa pre-requisite for deciding on the depth of drilling of the piezometer. The yield potential ofthe crystalline and meta-sedimentary rocks will show wide variations. Very food developmentof the piezometer is a pre-requisite after completion of drilling.

Volcanic rocks

The Deccan Trap lava flows are mostly horizontal but occasionally, are very gently dipping.Minerologically basalts are composed of Fe-Mg minerals and epidotes. The epidotes areeasily dissoluble and susceptible to weathering due to its hydroxyl group. The basalts formalternate layers of compact and vesicular beds of lave flows. The topography, nature andextent of weathering, presence of vesicles and lava tubes (often interconnected byfractures), thickness, number of flows and the nature of intertrappean layers (red boles) arethe important factors which play a major role in the occurrence and movement of ground


water in these rocks. The 'basic' type of volcanic rocks like basalts exhibit higher productivitythan the acidic (siliceous) varieties like rhyolites due to lesser viscosity in the former type oflava, resulting in larger openings between successive lava flows. The basalts have usuallymedium to low permeabilities depending on the presence of primary porosity of vesicles andsecondary porosity arising due to fracturing during cooling of lavas, tectonic disturbancesand weathering. Ground water occurrence in the Deccan Traps is controlled by thecontrasting water bearing properties of different flow units, thus, giving rise to multipleaquifer system, at places.

The important hydrogeological features of volcanic rocks are as under:

• The weathered zones in basalts in the high rainfall areas like Konkan region ofMaharashtra are often clayey with considerable thickness upto 25-30 m and should beisolated by casing.

• In a given watershed, a ‘Recharge Zone’ having moderately dissected plateau (MDP)with low to medium erosional characteristics are preferred for location of piezometersites over the 'Run off' zones situated in high relief areas where groundwaterdevelopment is also poor. However, piezometers can also be located in the lower partsof a watershed, (termed ‘Storage zone’) occurring in the valley areas, wheregroundwater development is also good. Many a times, in the Recharge zone of awatershed, the weathered zone is restricted in thickness, beneath which a fracturedzone are encountered. The construction of the piezometers tapping the fracture zoneshould correspond to the depth of operational borewells in the area. In areas wheremore than one lava flows are being penetrated it has to be ensured that independentpiezometers have to be conducted for monitoring the groundwater levels in the differentlava flow zones.

Carbonate Rocks

Carbonate rocks include limestones, dolomite and marble. Among the carbonate rocks,limestones have the greatest distribution. In the carbonate rocks the principal aquifers arethe fractured zone and solution cavities. Solution cavities develop due to circulation ofwater. This process leads to widely contrasting permeability within short distances. Thecavities are irregular in size and shape and are perpendicular to general direction ofdrainage. These are referred to as storages with ‘roofs’. The cavernous zones often getenlarged in the vicinity of the water table in these formations due to which ground water flowcan be turbulent with flow direction not remaining normal to the water table elevationcontours. The occurrence of solution cavities in these formations can be highly erratic, butmay tend to be localised along bedding planes especially in rocks with clayey intercallations.Potential karstified aquifers are found to occur in these formations in which the discharge islikely quite appreciable.

Clastic Sedimentary Rocks

Consolidated sedimentary rocks occur in Cuddapah and Vindhyan subgroups and theirequivalents. The formations consist of conglomerates, sandstones, shales, states,quartzites, apart from the limestones/ dolomites. Locally, they contain phyllites and schists.The Cuddapahs and their equivalents were subjected to low grade metamorphism in placeswhile the Vindhyans and their equivalents do not show any evidence of metamorphism. Theoccurrence and movement of water in them is governed by bedding planes, cleavages,fractures, joints, faults, contact zones, degree and magnitude of weathering, topography andclimate. When interbedded with clay, sandstones can form a multi aquifer system. In case


of dipping formations, piezometers located along dip direction may tap different aquifers,thus giving rise to artesian conditions along dip slopes. Also, at any given location, thealtitude of piezometric head usually increases in the lower aquifers. The clastic formationspossess higher porosity and permeability than the crystalline rocks due to its intergranularporosity, though limited due to cementation and fracture systems. As a result, they yield upto moderate quantities of water under favourable conditions. However, their porositydecreases with depth due to compaction. Piezometers sites in these formations have tocarefully selected keeping in kind the structural controls.

Chemical/mineral composition of Rock types:

Commensurate to rock types the mineralogical composition of rocks also varies very greatly,which in turn effect the flow regime as well as quality of water depending upon thesusceptibility of minerals for easy decomposition. The granite and granite gneiss iscomposed of aluminium silicates with potassium, sodium, calcium and magnesium hydroxylgroups. 60% of mineral is composed of silica, 30 % feldspar and rest others like mica, basicminerals etc. The aluminium silicates with hydroxyl group are highly unstable and easilydissociate into soluble salts. The orthoclase mineral comprise, potassium silicates and arehighly susceptible to weathering under oxidised condition, giving rise to kaolin clayformation. The Charnockites and granodiorites are high degree metamorphosed rockscomprising hypersthene and feldspar minerals. Due high-grade metamorphism and high-grade temperature weathering of the mineral assemblages is not easy. Similarly fracturing ofrocks is not very prolific in these rocks except in highly structurally disturbed areas. Theschist rocks contain mica, chlorite and hornblende. Sandstone rocks are composed ofquartz, feldspar and micas. Limestone is composed of calcite, which is easily susceptible todissolution forming calcium and bicarbonates. Basalts are composed of Plagioclase,feldspars, Augite and olivine. Olivine easily disintegrate to form diopside, an unstablemineral.

Consolidated Unconsolidated

Rock Type (Minerals) Rock Type (Minerals)Basalt Plagioclase, Feldspar, Augite, olivine Gravel Quartz main Mineral

Granite Quartz, Orthoclase, Feldspar, Mica Sand Quartz, Feldspar, Mica

Schist Mica, Chlorite, Hornblende Loam Quartz, clay, mica

Sandstone quartz, Feldspar, Mica Clay Kaolinite, illite, etc.

Limestone Calcite - -

Table A.1.1: Mineral assemblages in different geologcal formations

The varied assemblages of minerals in the rock types in an area gives rise to a host ofmineral bi-products when disintegration takes place due to reaction with water in oxidisingand reducing conditions. The interaction of minerals in oxidizing conditions produces kaolinclays, which create retardation/blocking to ground water flow conditions. The sodium andcalcium group feldspars form a series of metasomatic group of rocks with phased changes inlithology over a period. The water from schists through fractures, rich in potassium gives riseto sodium rich kaolinite. In the process such series of mineralogical changes the groundwater regime gives rise to new hydrogeological environment and consequent different flowpattern.


Similarly olivine and hypersthene bearing basaltic rocks get easily converted to diopsidewhich is unstable hydroxyl magnesium mineral. The magnesium is replaceable by iron (Mg-Fe) in basic rocks forming unstable mineral like gibsite. Similarly granite contains orthoclaseand mica minerals and both are unstable, susceptible to easy weathering into soluble kaolinminerals. Mica, particularly of phlogopite variety dissociates and forms soluble underoxidising conditions.

In lime stone and dolomite rocks the calcium mineral gets easily dissociates into calcium bi-carbonates, forming stalactites and stalgamites; this no doubt creates solution channels butalso blocks the flow path.

The basalts, granites, granitegneiss and lime stone rock formations have another aspect offluoride contamination due to alteration of rocks containing fluorite, fluor apatite, cryoliteminerals as a secondary bi-product. Contamination of water by fluoride is a serious healthhazard for drinking purposes.

These minerals get activated with proper environment in a reacting chemical environ, whenthey come in contact with water, affecting both textural and chemical changes in the flowregime of the area.

The coastal alluvial and stratified semi-consolidated formations formed as a fresh water andlittoral deposits exhibit large variation in vertical as well as lateral distribution and extension.Abrupt pinching and swelling of aquifers and lateral changes in lithology due to facieschange is very common and as such building up of a subsurface lithological section basedon archaic data is essential to target aquifer for monitoring. The attitude of aquifer dispositionin space also can be visualized by constructing Fence diagram. Aquifer with regionalextension and of appropriate thickness can be identified and selected for monitoring. Anyabnormal behaviour in the piezometric head can be deciphered with the knowledge of thehydrogeological regime frame work.

Textural framework of rock types

The texture of hard rocks is generally euhedral crystals when fresh and on weathering yieldsto irregular, sub angular to subrounded grains of quartz, feldspar and mafic minerals theinsitu location. Sediments involved with transportation acquire roundness and the degree ofroundness described as sphericity is related to length of transportation from source to thesite of deposition. The river sediments and littoral, lake deposits are sub-rounded to roundedin nature and give maximum effective porosity in a stratified layer. On the contrary the sub-angular grain packing gives minimum effective porosity. In regoliths and tallus deposits theeffective porosity is minimal and ground water movement is retarded one, perhaps with nolateral continuity. These process have a bearing in the selection of piezometer sites andinterpretation of monitored data.

The actual visualization of structural and textural features is possible from the study andexamination of drill cuttings of the bore well site. The presence of clay in cuttings indicates insitu weathering with no virtual flow but also a fair degree of clay exchange and with doubtfulquality of water. Subangualr and subrounded grains reflect not much transportation of grainsand hence limited extension of aquifer and site close to recharge area. As a result rise inwater table could be expected to be synchronous to recharge in the area instead of delayed.


It is essential to understand the grain size and texture of the lithology of a bore well in anarea to decipher and interpret the hydrological behaviour of the particular litho unit. Forunderstanding the texture a textural scale diagram is essential as a field guide.

Such a complex geological scenario with diversified lithological units will represent a verycomplicated and varied hydrogeological situation in a region. With the coverage of variedtopographical, climatic, hydrogeological and hydrochemical conditions in the area, it isdifficult to comprehend to a simple general classification. As such taking recourse to texturalframe work the entire range of formations have been grouped in to three categories fromhydrogeological point of view, which will also broadly fit into the ground water regime of thearea.

Ground water occurrence is confined to weathered and fractured/fissured zones withmoderate to moderately low ground water prospects. The weathered zone serves as themother recharge zone receiving recharge directly from the precipitation and acts as a feederto the deeper confined fracture zones at the same place or from a distant recharge zone. Assuch in case the deeper aquifer is not connected to the phreatic zone it is very essential todesign network stations to monitor the behaviour in both the aquifer separately. The phreaticzone often is connected to deeper fractured zone through a semi-confined layer with verylow permeability causing head difference over a period with time lag. Hence it is essential toassess their interrelationship for planning towards exploitation.

The identified piezometer site should provide an overview of the hydrogeolgic characteristicsof the proposed piezometer site as well as its surrounding region also. It makes use of theavailable local information so as to fit in the regional ground water regime information systemand compatible with to other areas with similar hydrogeologic conditions. The local selectioncriteria essentially comprises:

• Identification of potential aquifer horizons and their regional continuity

• The hydrogeological and hydrochemical data of the aquifer

• Monitoring protocol – long term or short term

A local assessment comprises data from the drilling, hydrogeological and geophysicaltesting right at the site comprising following elements:

• Potential zone targeted for piezometer construction

• Hydraulic characteristics of the identified zone

• Hydrochemical characteristics of the specific zone

Nature and protocol of the monitoring program of the piezometer, a long-term or short termhas to be known.


Annexure – IIGeo-physical bore hole logging

Geophysical bore hole logging is a cost effective way of providing vital subsurfaceinformation required by geoscientists for piezometer design and evaluation.

Logging of piezometers in the construction stage is:

• To guide the design of piezometer and its completion (construction logging)

• To identify the lithological sequence and the vertical and lateral variations in rockproperties (formation logging)

• To identify the fluid bearing potential of the individual layers (fluid logging)

Figure A.2.1:Geo-physical down the logger

The logging provides a correlation of the newly drilled section with unknown geology nearbyso that the lateral extent and thickness of aquifers and aquicludes can be determined.Where loose or unconsolidated materials are penetrated logging is essential:

• To guide the design of the piezometer and the selection of well screen and gravel packand their proper placement (formation, construction, fluid logging)

In existing piezometers as part of quality check geophysical logging is used to:

• Provide information on the piezometer construction (construction logging)

• Monitor performance

• Identify the position and relative magnitude of water inflows including pollutants(Formation and fluid logging)

• Provide information to correlate similar rock and aquifer horizons from site to site(formation logging and fluid logging)

• Provide data to examine horizontal, vertical and time varying water quality changes(fluid and geochemical logging)

• To identify fractures and fissures and distinguish those that are ground water active(formation and stress fluid logging)

The geophysical logging, used alone or in conjunction with other data, like fluidgeochemistry, hydraulic properties, pump test data, provides a powerful tool for investigatinghydrogeology and resolving problems in both the saturated and unsaturated zone.


Geophysical logging is carried out by lowering a sensing devise called logging tool (probe orsound) in the hole on a cable with continuous measurements of geophysical properties likeresistivity, self potential, natural gamma, temperature (differential and absolute), caliper(borehole diameter), and others. The logs are record as a continuous graphically recordedcharts at the surface by the logger unit.

Required Information Possible Logging TechniquesLithology and stratigraphic correlation ofaquifers and associated rocks

Resistivity, caliper logs made in open holes; radiationlogs made in open or cased holes

Total porosity or bulk density Calibrated sonic logs in open holes; calibrated neutronor gamma-gamma logs in open or cased holes

Effective porosity or true resistivity Calibrated long-normal resistivity logs

Clay or shale content Natural gamma logs

Permeability Under some conditions long-normal resistivity logs

Secondary porosity/permeability - fractures,solution openings

Caliper or sonic, logs

Specific yield of unconfined aquifers Calibrated neutron logs

Grain size Possible relation to formation factor derived fromresistivity logs

Location of water level or saturated zones Resistivity, temperature, or fluid conductivity logs;neutron or gamma-gamma logs in open or casedholes

Moisture content Calibrated neutron logs

Infiltration Time-interval neutron logs

Source and movement of water in a well Fluid velocity or temperature logs

Salinity, temperature, density and viscosity ofwater

Calibrated fluid conductivity or temperature logs ;resistivity logs

Construction of existing wells, diameter andposition of casing, perforations, screens

Gamma-gamma, caliper logs

Guide to screen setting All logs indicating lithology, water-bearingcharacteristics, and correlation and thickness ofaquifers

Cementing Caliper, temperature, or gamma-gamma logs

Casing corrosion Caliper logs under some conditions

Table A.2.2: Summary of Logging Applications to Groundwater Hydrology(after Keys and MacCrary, 1971)


Spontaneous - Potential (SP) log: The SP logs are records of the natural potentialsdeveloped due to the ionic exchange between the bore hole fluid and the water in the waterbearing horizons. The spontaneous potential is due to membrane potential and the diffusionpotential. The membrane potentials are formed at the junction of impermeable zone anddrilling fluid; while the diffusion potentials are formed due to ionic exchange between drillingfluid and ground water in the aquifer. The SP is well developed against permeable zone andhence helps in identifying aquifer horizon and its salinity status. The SP log is used foridentification of boundaries between porous and non-porous rocks such as clays/s d. Inconsolidated rocks the SP log is not of much help, but shale-sandstone and shale-limestonesequences may be determined. SP logging is helpful only in open, uncased and fluid filledboreholes. Figure A.2.2 gives the electrical log of the borehole at Pipli in puri district ofOrissa, along with lithological log, drill time log for better comparision.

Figure A.2.2: Typical subsurface records of piezometer drilling information

The resistivity log: The resistivity log is a record of the apparent resistivity of thesubsurface formation with depth. Resistivity logging is carried out in uncased wells, bymultiple electrode and single point resitivitance probe. Single point resistance log representschanging electrical resistance between a single downhole electrode and a fixed surfaceelectrode. It does not measure the true resistivity and is strongly affected by change in borehole diameter. Yet, it gives a clear and good contrast of resistance of the formations and bedboundaries. Figure A.2.2 is an example of use of point resistance, resistivity andspontaneous potential log of a piezometer tapping unconsolidated coastal alluvial formationsin Puri district of Orissa state.

The multiple electrode logging principle is similar to surface resistivity survey except that theelectrodes in the bore hole in mud fluid medium and the current electrodes effect the currentpenetration inside the formations in the bore hole. The multiple electrode devices performsshort normal, long normal and lateral logging. These devices essentially measure the


apparent resistivity of a volume of rock/material surrounding the hole. The short normalsdevice has an electrode spacing of 0.4 m gives good vertical detail and record the apparentresistivity of the mud invaded zone and are best suited to determine formation boundaries.The long normals with electrode spacing of 1.6 m and laterals with an electrode spacing of5.7 m record the apparent resistivity beyond the 'invaded' zone. The long normal resistivitiesreflect better formation resistivities. These logs can be run only in open, uncased and fluidfilled boreholes. The factors that influence formation resistivity are the nature andtemperature of the water in the formation pores and rock structure. The range of resistivity inhard rocks is quite large. Fractures filled with water decrease the apparent resistivity in hardrocks.

Geophysical logs provide continuous graphical records of subsurface attributes (physicalproperties) constitutes as valuable tools in lithological and geological correlation of variousbore data and in establishing regional continuity of the formations.

Interpretation of geophysical logs in terms of lithology is difficult from logs alone andsupporting information is usually required from the lithologs and drill time logs (orhydrogeologic logs) prepared during drilling of the piezometer. Quantitative interpretationabout the quality of groundwater in the aquifers and its effective porosity and permeability isalso possible using the principles of formation factor analysis and certain empiricalrelationships

Natural - gamma log: Natural- gamma logs (or gamma-ray logs) are records the amount ofnatural-gamma radiation emitted by radio active minerals in the rocks/material. Gamma raylogs are useful in identifying the lithology and clay layers in the bore hole and forstratigraphic correlation. High gamma logs are associated with clay layers. These can be runin open as well as cased boreholes filled with, or without water. The radius of investigationextends to about 6-12 inches from the borehole wall.

Neutron - gamma log: These logs record the intensity of gamma radiation from a neutronsource in the probe after it is back-scattered and attenuated within the borehole andsurrounding rocks. The main uses of neutron-gamma logs are for identification of lithologyand measurement of bulk density and porosity of rocks/soils. They are also used for locatingcavities and cement outside the casing. The radius of investigation is about 6 inches fromthe borehole wall. These logs are run and recorded as a continuous graphical chart in openor cased boreholes, with or without water.

Caliper log: Caliper log is records the average diameter of the bore hole. Its major use is toevaluate the structural and textural nature of rock units, their physical/environmentconditions, which effect the hole-diameter. These logs are useful to identify presence ofcavities, solution channels, fractures and faults/joints etc. The caliper has to be calibratedwith different diameter rings before lowering in to the bore hole. Caliper logs are run duringupward logging from the open bore holes filled with drilling fluid (as a continuous graphicalrecord).


Figure A.2.3:Idealised diagram of geo-physical logging tool-caliper

Temperature log: Temperature logs provide a continuous record of the fluid temperaturewithin the bore hole immediately surrounding the sonde as a continuous graphical chart indegree Celcious. The differntial temperature log gives difference in temperature of fluidsexisting in the bore hole due to different flow conditions, and thus the influent or effluentnature of aquifer flow can be deciphered. The gradient temperature log gives thermalgradient existing in the bore hole. Temperature logs are best recorded in fluid filled openboreholes.

Fluid - Conductivity logs: These logs provide a continuous measurement of theconductivity of the borehole fluid filled in bore hole. When properly corrected, they provideinformation on the chemical quality of the drilling mud. They can be recorded in open, fluidfilled boreholes.

Fluid flow meter: A spinner type fluid flow meter measures the velocity of ground water flowoccurring at different depths contributing to the total flow from the tube well/bore well. It isconducted in completed well only. The log helps in identifying the relative potential ofdifferent aquifers tapped in the bore well. It is run in production or developed well duringpumping process. The flow meter needs to be calibrated for different flow velocities in a tankbed before it is lowered in the bore well. The combination of geophysical logs selecteddepends upon data requirements and the local geological and hydrological conditions.


Annexure – IIIAquifer parameters and well characteristics

General

After construction of a piezometer, it is necessary to evaluate the efficiency of the well andevaluation of hydraulic properties of the aquifer encountered in the piezometer by conductingPumping Tests. Efficiency of a well is expressed in percentage as the ratio of theoreticaldrawdown to observed drawdown. The well efficiency depends upon two factors, namelywell loss and formation loss.

Well Loss Characteristics:

The drawdown in a pumped well consists of two components, the aquifer losses and the welllosses. The aquifer losses are the losses due to the hydraulic head that occurs in theaquifer, where the flow is laminar. The hydraulic head depends on the time of pumping andvaries linearly with the well discharge. Well losses are caused due to deviation in flow of thewell from a linear to non- linear conditions, accordingly losses categorised into linear andnonlinear losses. Linear well losses are caused by damage to the aquifer during drilling andwell completion:

• head losses due to compaction of aquifer material during drilling;

• head loss due to plugging of aquifer with drilling mud which results in reduction ofpermeability near the bore hole;

• head losses in the gravel pack;

• head losses in the screen;

Among the non-linear head losses, following are included:

• friction losses occurring inside the well screen,

• the suction pipe where the flow in turbulent, and

• the head losses that occur in the zone adjacent to the well where flow is usually alsoturbulent.

All the linear and non-linear well losses are responsible for the drawdown inside the wellbeing much greater than one would except otherwise. The steady state drawdowns insidepiezometer is expressed by following general expression.

S = B *Q + C * Q2

Where BQ is approximately the formation loss and C Q2 is approximately the loss of head asthe water enters into the well, B is the formation loss coefficient and C is the well entry losscoefficient. Jacob(1947) suggested the well loss to be proportional to the square of thedischarge (Q) as indicated above.

Rorabaugh (1953) expressed the total drawdown Sw in the well loss (for steady state flow ina confined aquifer) treating the Q in the well loss term as an unknown exponent.


The well loss can be a substantial fraction of total drawdown when pumping rates are largeand can be minimized with proper design and development. In the old piezometers andwells, clogging or deterioration of screens can increase well losses. The condition of apiezometer (or a well) can be evaluated from the Table

Well loss coefficient (C)in mm2/m5

Well condition

< 0.5 Properly designed and developed

0.5 to 1.0 Mild deterioration or clogging

1.0 to 4.0 Severe deterioration or clogging

> 4.0 Difficult to restore well to original capacity

Table A.3.1: Relation of Well loss coefficient to well condition (after Walton, 1962)

In-situ Slug Test

Usually two types of in-situ Slug (or bail) tests can be carried out in a piezometer forevaluation of hydraulic characteristics of aquifer tapped. In practice, such tests areapplicable in unconsolidated and consolidated formations as these can be conducted for apoint-determination as well as a larger aquifer horizon.

1 Point Piezometer Test: As a point piezometer is open only over a short interval towardsits base, the test may not yield representative values of tranmissivity for the entireaquifer.

2 Test for Screened Piezometers, which are open over the entire thickness of a confinedaquifer.

Both the tests involve inducing instantaneous change in water level in the piezometerthrough a sudden addition (Slug test) or removal (Bail test) of a known volume of water. Therecovery/drawdown of the water level with time is than observed.

Piezometer Test

For this Test, Initially, a rigid pipe is driven (removing the soil below the pipe with an auger)or jetted into the soil. When the pipe has reached the desired depth, a cavity (naked hole) isaugered below the bottom of the pipe. After the water level in the pipe has reachedequilibrium, the slug of water is removed and the subsequent rise of water level is measured.The K value of the soil around the naked hole (cavity) is calculated as below (Luthin andKirkham, 1949):

where Ap = a factor depending on the shape of the cavity and the depth of the lowerboundary (of impermeable or very permeable layer).

π

= yylog

tpA

wrK 0

e

2

nw CQBQs +=


In unstable or loose formations the piezometer cavity must be screened (at the bottom of thepipe) to prevent caving.

The volume of soil on which 'K' is measured with this method is smaller than with othertechniques.

The height (Lc) of the cavity usually varies from 20 to 30 cm. If Lc = 0, the piezometerbecomes a cased auger hole, and this technique is called 'Tube' method.

Bail/Slug Tests in a Screened Piezometer Tapping a Confined Aquifer:

This method is useful in those cases where pumping tests can not be conducted, due tocertain logistic problems. The analysis is valid under same assumptions as applicable inTheis method. The method involves a curve matching procedure to determine aquifertransmissivity.

For the bail test, the method involves the preparation of a plot of recovery data in the form(H-h)/(H-H0) verses t on a semilog paper, with time ‘t’ on log scale. The field curve is thensuperposed on the Type Curve.

Keeping the axes parallel the data curve is best fitted to one of the Type curves and a matchpoint is chosen in the graph (in matched position).

For case of calculation, it is common to choose a match point so that W = 1.0, hence

where parameters are expressed in any consistent set of units.

However, in the above method, a unique value of storage coefficient cannot be evaluateddue to similar slopes of the type curves.

The main limitation of slug/bail tests is that these are highly dependent on an intake portionwhere the screen is noncorroded (or nonclogged) should be done only in a newlyconstructed piezometer. On the contrary, in case of an old piezometer, if a piezometer hasbeen developed by Surging or Backwashing before testing, the measured values oftransmissivity may reflect the increased conductivity of the AGP around the intake. Use ofDWLR for monitoring the water levels for slug test has proved very useful in picking up smallvariations also.

1WfortrTor

2==

t

WrT

2

=


Pumping Test

A pumping test is conducted with constant discharge or variable discharge with constanthead for estimating hydraulic parameters of the aquifer tapped in the piezometer. The testinvolves monitoring of the time variation of drawdown in one or more observation wells inresponse to a pumping at a known discharge, from the piezometer. The observation must bein the vicinity of the piezometer and must be tapping the same zone. If no such observationwell is available, the drawdown can be monitored in the piezometer itself.

Constant Discharge Tests in Unconsolidated Aquifers:

In the case of homogeneous and isotropic aquifers mostly encountered in unconsolidatedformations, the assumptions made while analyzing the aquifer pumping test data can beassumed to be valid to a great extent. The general assumptions are as under:

– The aquifer is homogeneous and isotropic.– The flow of water in the aquifer is horizontal.– The aquifer is of infinite aerial extent and is uniform in thickness.– The production well taps full thickness of aquifer.– Prior to pumping, the piezometric surface is nearly horizontal over the area of influence.– The flow is in steady state.

The following additional assumptions are made in case of unsteady state of flow:

1 The water removed from the storage is discharged instantaneously with decline of head.2 The diameter of the well is small so that the well-storage can be ignored.

Though many of the assumptions are valid to a fair approximation, certain assumptions likeinfinite extent of aquifer, full penetration of aquifer (by the piezometer) and instantaneousdischarge of water are often violated even in unconsolidated aquifers.

Single well drawdown test for Hydraulic Conductivity

In cases, when the aquifer is made up of coarse sand and gravels, a pump test can be madeto obtain a rough estimate of hydraulic conductivities in these materials.

The equipment required for the test includes a pump, a calibrated bucket and a stopwatch todetermine the flow rate.

After measuring the static water level in the piezometer, water is pumped from the well at aconstant rate. After sometime, the water level in the hole will reach a steady-state level.(Steady state can be assumed to exist when the water level in the hole drops less than 3 cmin 2 hours). At steady state, the flow rate and depth of water in the well are recorded. Thesedata alongwith depth of static water level from bottom of the piezometer are used in thefollowing equation:


For Unconfined Aquifer:

If the aquifer is unconfined, the hydraulic conductivity

where R (radius of influence) = 500 r, generally and r = radius of piezometer(m)

Q = flow rate at steady state conditions (m3/day)

H = height of static water table above bottom of piezometer (m)

h = depth of water in piezometer (above the bottom) at steady state (m)

For a Confined Aquifer:

where s = drawdown of water level = (= H - h), m

d = thickness of confined aquifer (m)

Recovery Method:

This is a good method for estimating transmissivity of a confined aquifer tapped in apiezometer, which has been pumped sufficiently for a longer duration and the pumping hasbeen stopped to allow recovery of the water levels in the piezometer to the original staticwater level.

The residual drawdown (i.e. the difference between the drawdown component and therecovery component) s' is given as under (Theis 1935):

where t is the time since pumping started and t1 time since pumping stopped.

So a plot of residual drawdown (s1) verses log of t/t1 forms a straight line. Thus, for �t1,

the residual drawdown for 1 log cycle of log t/t1 (= 1) becomes

( ) ( )( )22

e

hH

r/RlogQday/mK

−π=

( ) ( )ds2

r/RlogQday/mK e

π=

( )1ttlog

T4

Q3.2's

π=

1s4

Q3.2Tand

∆π=

T4

Q3.2s1

π=


As distance (r) of observation well from pumped well is not involved in the above equation,the observations of pumped piezometer alone serve the purpose. In case, the piezometer istapping an unconfined aquifer, the observed drawdown (so) values should be corrected asper the following formula:

where scor is corrected drawdown and d is the initial saturated thickness of the aquifer.

Variable Discharge (or Step Drawdown Test) Test:

The variable discharge test, also called Step Drawdown Test is necessary to evaluate theextent of development of a piezometer as well as to evaluate well losses. This test involvespumping of the piezometer at a progressively increasing rate. The pumping should becarried out at a rate till the drawdown is stabilized. Thereafter, the rate of pumping shouldbe increased in steps with the corresponding drawdowns being stabilized.

Evaluation of Well Loss by Hantush - Bierschenk Method

The well loss can be readily evaluated by analyzing data of a step-drawdown pumping testconducted on a piezometer. However, this method is especially valid for piezometers inunconsolidated formations where the groundwater flow is largely laminar and follows Darcy'slaw.

Procedure:

For analyzing the step drawdown test data, the incremental drawdowns (si) for each step aredetermined from drawdowns for equal time intervals.

– The time-drawdown curve for each step is plotted on a semilog graph with time (in min.)on log scale.

– The individual drawdown curve should be extrapolated with a slope proportional to thedischarge in order to measure the incremental drawdowns.

– The equation rewritten as sw/Q = B + CQ represents a straight line with a slope C andintercept B, at Q = 0.

Determine the values of si corresponding to the discharge at each step, i.e. sw(3) = ∆sw(1) +∆sw(2) + ∆sw(3) and so on. Subsequently, calculate the ratio sw(i)/Qi for each step.

– On arithmetic paper, plot the values of si/Qi versus the corresponding values ofdischarge (Qi). Fit a straight line through the plotted points . (If the data do not fall on astraight line, a method based on the well loss component CQn, where n > 2) should beused.

– Determine the slope of the straight line which is the value of C.

d2

sss 02

0cor −=

i

)i(w

Q

s

∆

∆


– Extend the straight line until it intercepts the Q = 0 axis. The interception point on thevertical axis gives the value of B.

)i(

)i(w

Q

s


The above procedure is applicable if

– the basic assumptions made earlier for groundwater flow about the infinite areal extentof aquifer and its uniform thickness and the piezometric surface/watertable beinghorizontal are satisfied;

– In addition, the assumptions listed below are also made;

– The aquifer is confined, leaky or unconfined;

– The aquifer is pumped step wise, at increasing discharge rates;

– The flow to the well is in an unsteady state;

– The nonlinear well losses are appreciable and vary according to the expression CQ2.

However, the limitation of the above method is that values of sw(i) depend on extrapolateddata and therefore, can be erroneous.

Further, in case of consolidated formations, the method is likely to give only approximatevalues of well loss coefficient as assumption of homogeneity & isotropy of the aquifer areclearly violated. In such aquifers, Rorabaugh method is applicable as the value of 'n' can beconsidered greater than '2', indicating turbulent flow especially around the piezometer.

Evaluation of Well Loss in Consolidated Formations: (Rorabaugh's Method)

In this method, the value of n can be assumed greater than 2, so the equation is of the form

This equation can be written as

or (sw(i)/Q(i) - B) =

or log (sw(i)/Q(i) - B) = log C + (n-1) log Q(i)

A plot of (sw(i)/Q(i) - B) versus Q(i) on log-log graph will yield a straight line

In addition to the assumptions made for Hantush-Bierschenk method, the assumptions forapplication of this technique include replacement of last assumption(s) as under:

– The non-linear well losses are appreciable and vary according to the expression nCQλ .

( ) ( ) ( ) ( )niiiwiw

n

1i

CQBQss +==∑=

( )

( )( )( )1ni

i

iw QCBQ

s −+=

( )( )1niCQ −


Procedure:

– On semi-log paper, plot the drawdowns sw against the corresponding time t (T on thelogarithmic scale) ;

– Extrapolate the curve through the plotted points of each step to the end of the next step;

– For each step, determine the increments of drawdown ∆sw(i) by taking the differencebetween the observed drawdown at a fixed time interval ∆t, taken from the beginning ofthat step, and the corresponding drawdown on the extrapolated drawdown curve of thepreceding step;

– Determine the values of sw(i) corresponding to the discharge Qi from sw(i) = ∆sw(1) +∆sw(2) + .. + ∆ sw(i);

– Assume a value of Bi and calculate [(sw(i)/Qi) - Bi] for each step (for this, value of Bcalculated by Hantush-Biersehenk method can be used as the starting values);

– On log-log paper, plot the values of [(sw(i)/Qi) - Bi] versus the corresponding values of QI.Repeat this part of the procedure for different values of Bi. The value of Bi that gives thestraightest line on the plot will be the correct value of Bi;

– Calculate the slope of the straight line ∆[(sw(i)/Qi - B]/ ∆Qi. This equals (n - 1), fromwhich the value of ‘n’ can be obtained;

– Determine the value of the interception ‘B’ on the straight line with the Qi = 1 axis. Thisvalue of ∆[(sw(i)/Qi - B] is equal to C.

Remarks:

As the Rorabaugh method allows consideration of turbulent flow in the aquifer, especially inthe vicinity of the production peizometer, it can be readily used in case of fractured aquifersencountered in the consolidated formations.

The aquifer parameters usually required to be estimated are the hydraulic conductivity,Tranmissivity and Storativity. These parameters, if estimated on regional basis, can beutilised to optimize ground water withdrawal from the aquifer for a given drawdown. If thepiezometer is tapping an aquitard only, such parameters can be all the more relevant for arealistic perception of the hydraulic situation.

Time Drawdown Method (for observation piezometer)

This method involves measurements of drawdown at an observation piezometer withincreasing time of pumping at the nearby piezometer tapping a confined aquifer.

The relevant equations used for computing T & S are given below:

where t0 is the time for zero drawdown and r is distance between production piezometer andobservation piezometer.

If t/t0 = 10 and hence log t/t0 = 1, s can be replaced by ∆s, i.e. by the drawdown differenceper log cycle of time and it follows that

2

0

r

Tt25.2S =


where Q is well discharge (m3/day) and ∆s is drawdown for 1 log cycle of time.

The following assumptions and conditions should be satisfied:

– All the conditions as enumerated for Unsteady State of flow.– The values of u = r2S/4Tt are small (u < 0.01) i.e. r is small and t is large. Here S is

aquifer storativity, r is distance of observation piezometer from pumping piezometer welland t is time of pumping.

The condition that u is small will be satisfied in confined aquifers for moderate distancesfrom the pumped well within an hour or less.

Procedure:

– Plot for one of the piezometers (r = constant) the values of s versus the correspondingtime t on single logarithmic paper (t on logarithmic scale), and draw a straight linethrough the plotted points.

– Extend the straight line till it intercepts the time-axis where s = 0, and read the value of t0(= time for zero drawdown).

– Determine the slope of the straight line, i.e. the drawdown difference ∆s per log cycle oftime.

– Substitute the values of Q and ∆s and solve for T. With the known values of T and t0,calculate S.

Remarks:

– When the values of T and S are determined they are introduced into the equation u =r2S/4Tt to check if u < 0.01, which is a condition for the applicability of the method.

– Before being substituted all numerical values should be expressed in the equivalentunits. If necessary, conversion factors should be introduced. For example: for the sakeof convenience the time in the time-drawdown diagram is often plotted in minutes whilethe transmissivity is expressed in m2/day. So when introducing the value of t0 the valueread from the graph should be divided by 1440 to convert the minutes into days.

For application of the method to piezometers tapping unconfined aquifers, all values ofobserved drawdowns (s0) should be corrected by using following equation:

where scor = corrected drawdown

d = initial saturated thickness of the aquifer.

In such a case of unconfined aquifer, the condition of u being small (< 0.01) will generally besatisfied after about 12 hours of pumping.

s4

Q30.2T

∆π=

dx2

sss

20

0cor −=


Time-Drawdown Curve Method (for observation piezometer tapping semi confined aquifers):

This method is valid for analysing time-drawdown data of peizometer tapping semiconfinedaquifer.

The relevant equation for finding transmissivity (T) and storativity (S) are as under:

L = leakage factor = c.T , in m

c = hydraulic resistance of aquitard = D'/K', in days

D' = thickness of aquitard (m) and

K' = hydraulic conductivity of aquitard (m/day)

r = distance between production piezometer (well) and observation piezometerin (m)

For use of this method, the following limiting conditions should be satisfied.

– The aquifer is semi-confined.– The flow to the well is in an unsteady state (i.e. the drawdown differences with time are

not negligible nor is the hydraulic gradient constant with time).– The removed from storage is discharged instantaneously with decline of head.– The well diameter is very small, so that the storage in the well can be neglected.

In this technique, instead of one type curve, there is a type curve for each value of r/L.These type curves are known as Walton Type curves for leaky confined aquifers.

Procedure

– Plot Walton type curves on double logarithmic paper [W(u, r/L) versus 1/u for differencevalues of r/L] using the type anves.

– Plot on another sheet of double logarithmic paper of the same scale s versus t if onlyone piezometer is used; this gives the observed data curve.

– Superimpose the observed data curve on the family of type curves and adjust, whilekeeping the coordinate axes parallel, until a position is found by trial where most of theplotted points of the observed data curve fall on one of the type curves.

– Select a match point A on the superimposed sheets and note for A the values of W(u,r/L), 1/u, s and t/r2 (or t).

– Substitute the values of W(u, r/L) and s and the known value of Q and calculate T.– Substitute the value of k and the reciprocal values of 1/u and t solve for S.

Tt4

sruwhereL/r&uoffunctionWellais)L/r,u(W,Here

2

=

( )Lr,uW

s4

QT

π=

2r

Ttu4Sand =


– The numerical value of r/L belonging to the type curve to which the observed data curvefits in the best way, makes it possible to calculate the numerical value of L and

onsequently c because L = c.T .

Remarks

– It is difficult to obtain a unique fitting position of the data curve with a type curve unlessa sufficient number of the observed data fall within the period during which leakageeffects are negligible.

Constant Discharge Tests in Consolidated Formations:

A majority of piezometers in the HP States, are located in consolidated formations. Thus,the analysis of pumping test data by conventional methods for aquifers in homogeneousmedia are not valid for the fractured aquifers as groundwater flow in such heterogeneousaquifers is complex. Due to lack of precise knowledge about nature of fracturing in variouskinds of consolidated formations, the identification of the fracture system is often relied onthe use of certain models of fracturing. Out of many models available, the more commonlyused concept of flow in fractured rocks is the Double Porosity Model.

The Double Porosity concept considers a fractured rock formation as consisting of twomedia: the fractures and the matrix blocks, both having their own, distinct hydrauliccharacteristics. The permeability of the block is small in comparison to that of fractures,whereas storage of blocks is larger than that of fractures. During pumping of a piezometertapping such a system, water will be first released from fractures and a pressure differenceis created between the fractures and the surrounding blocks. Consequently, water movesfrom the blocks to the fracture, a phenomenon similar to release of water from an aquitard tothe aquifer. The flow through the fracture to the peizometer/well is radial and in anunsteady state.

As there is little likelihood of two operating piezometers being available in close vicinity,methods of analysis of pumping test data of observation wells will not be commonly used foraquifer characterization. Yet, in case of exceptional situation with two functionalpiezometers being available in close vicinity, the pumping test data of observationpiezometers can also be analysed.

The methods of analysing flow in fractured media are based on the following assumptionsand conditions:

– The aquifer is confined;

– The aquifer has an infinite areal extent;

– The thickness of the aquifer is uniform over the area influenced by the test;

– The production piezometer fully penetrates a fracture;

– The production piezometer is pumped at a constant discharge rate;

– Prior to pumping, the piezometric surface is horizontal;

– The flow towards the well is in an unsteady state;

– The aquifer is of the double-porosity type and consists of homogeneous and isotropicblocks or strata of primary porosity (the aquifer matrix), separated from each other either


by an orthogonal system of continuous uniform fractures or by equally spaced horizontalfractures;

– Any infinitesimal volume of the aquifer contains sufficient portions of both the aquifermatrix and the fracture system;

– The aquifer matrix has a lower permeability and a higher storativity than the fracturesystem;

– The flow from the aquifer matrix into the fractures (i.e. the interporosity flow) is in apseudo-steady state;

– The flow to the well is entirely through the fractures, and is radial and in an unsteadystate;

– The matrix blocks and the fractures are compressible;

– Interporosity flow coefficient λ = α rw2 Km/Kf < 1.78, where α = a shape factor, rw =

radius of piezometer, Km and Kf = hydraulic conductivity of matrix and block respectively(m/day).

It has been shown that the double-porosity behaviour of a fractured aquifer only occurs inrestricted area around the pumped well. Outside that area (i.e. for λ values greater than1.78), the drawdown behaviour is that of an equivalent unconsolidated, homogeneous,isotropic confined aquifer, representing both the fracture and the block flow.

Warren & Root Method for estimation of Transmissivity of Fractured Aquifers(without observation piezometer)

Following procedure may be followed for evaluating transmissivity and storativity of fracturedaquifers tapped by piezometers in consolidated formations:

– On a sheet of semi-log paper, plot drawdown s versus time of pumping t (t onlogarithmic scale);

– Draw a straight line through the early-time points and another through the late-timepoints; the two lines should plot as parallel lines ;

– Determine the slope of the lines (i.e. the drawdown difference ∆s per log cycle of time);

– Substitute the values of ∆s and Q into Tf = 2.30 Q/4π ∆s, and calculate Tf where Q =well discharge (m3/day);

– Extend the early-time straight line until it intercepts the time axis where s = 0, anddetermine t1

– Substitute the values of Tf, t1, and r into Sf = 2.25 Tft1/r2, and calculate Sf (Storativity of

fracture);

– Extend the late-time straight line until it intercepts the time axis where s = 0, anddetermine t2 ;

– Substitute the values of Tf, t2, r, and β into Sf + β Sm = 2.25 Tft2/r2, and calculate Sf + Sm

(where β = a factor; for early time analysis it equals zero and for late time analysis itequals 1/3; and Sm = Storavitiy of the matrix block);

– Calculate the separate values of Sf and Sm.


Remarks:

The two parallel straight lines can only be obtained at low λ values (i.e. λ < 10-2). At higher λvalues, only the late-time straight line, representing the fracture and block flow, will appear,provided of course that the pumping time is long enough. The analysis then yields values ofTf and Sf + Sm.

To obtain separate values of Sf and Sm when only one straight line is present, followingprocedure can be applied

– Follow above given procedure to obtain values of Tf and Sf from the first straight line, orif it is not present, values of Tf and Sf + Sm from the second straight line;

– Determine the centre of the transition period of constant drawdown and determine 1/2∆s ;

– Calculate the value of ω using Equation ω = 10-∆sv/∆s;

– Substituting the values of ω and β into Equation [ω = Sf / (Sf + β Sm)], determine thevalue of Sm if Sf is known, or vice versa.

Further, to estimate the centre of the transition period with constant drawdown, thepreceding and following curved-line segments should be present in the time-drawdown plot.

Curve-fitting Method (with observation piezometers)

In a fractured aquifer, the drawdown response to pumping as observed in observation wellscan be expressed as under (Bourdet and Gringarten, 1980)

where

For small values of pumping time the equation reduces to

where

Equation is identical to the Theis equation. It describes only the drawdown behaviour in thefracture system (β equals zero). For large values of pumping time equation is same as to

( ) 2mf

f

rSS

tT*u

β+=

( )tT4

rSSu

f

2mr β+

=

( )ωλπ

= ,*,uFT4

Qs

f

( )uWT4

Qs

fπ=


Theis equation which describes the drawdown behaviour in the combined fracture and blocksystem (β equals 1/3 or 1).

For λ values less than 0.01, Equation for drawdown reduces as under:

A simple method based on matching both the early and late-time data with the Theis typecurve, which yields values of Tf and Sf, and Tf and Sf + Sm, respectively is described below.From the steady-state drawdown at intermediate times, a value of λ can be estimated.

Procedure

– Prepare a type curve of the This well function on log-log paper by plotting values ofW(u) versus 1/u from values;

– On another sheet of log-log paper of the same scale, plot the drawdown s observed inan observation well versus the corresponding time t;

– Superimpose the data plot on the type curve and adjust until a position is found wheremost of the plotted points representing the early-time drawdowns fall on the type curve;

– Choose a match point A and note the values of the coordinates of this match point,W(u), 1/u, s, and t;

– Substitute the values of W(u), s, and Q and calculate Tf ;

– Substitute the values of 1/u, Tf, t, and r and calculate Sf (β = 0);

If the data plot exhibits a hozizontal straight-line segment or only an inflection point, note thevalue of the stabilized drawdown or that of the drawdown at the inflection point. Substitutethis value into the equation and calculate λ:

– Now superimpose the late-time drawdown data plot on the type curve and adjust until aposition is found where most of the plotted points fall on the type curve;

– Choose a matchpoint B and note the values of the coordinates of this match point, W(u),1/u, s, and t;

– Substitute the values of W(u), s, and Q into equation and calculate Tf ;

– Substitute the values of 1/u, Tf, t, and r into equation and calculate Sf + Sm (β= 1/3 or1).

Remarks:

– For relatively small values of ω, matching the late-time drawdowns with the This typecurve may not be possible and the analysis will only yield values of Tf and Sf ;

– For high values of λ (i.e. for large values of r), the drawdown in an observationpiezometer no longer reflects the aquifer's double-porosity character and the analysiswill only yield of Tf and Sf + Sm ;

λπ=

26.1log

T4

Q30.2s

f

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