resource assessment situation · pdf file4.2.1 water balance and resource assessment ......

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CHAPTER 4:

GROUNDWATER RESOURCE ASSESSMENT /SITUATION ANALYSIS

DWAF – DANCED Groundwater Management

Chapter 4: Resource assessment

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CONTENTS

4.1 INTRODUCTION........................................................................................................ 1

4.2 DETERMINATION GROUNDWATER FLOW COMPONENTS........................ 24.2.1 Water Balance and Resource Assessment ......................................................... 24.2.2 Recharge Estimation.......................................................................................... 44.2.3 Hydrocensus .................................................................................................... 114.2.4 Numerical Groundwater Modelling................................................................. 154.2.5 Strategic Environmental Assessment .............................................................. 184.2.6 Analysis of opportunities and constraints........................................................ 194.2.7 Groundwater Vulnerability.............................................................................. 20

4.3 DATA REQUIREMENTS FOR RESOURCE ASSESSMENT ............................ 21

4.4 STEPS IN RESOURCE ASSESSMENT/SITUATIONAL ANALYSIS ............... 23

4.5 INITIAL/CONCEPTUAL PLANNING .................................................................. 244.5.1 Hydrogeological terrains ................................................................................. 264.5.2 Developing a Conceptual Model ..................................................................... 28

4.6 DETAILED PLANNING AND RECONNAISSANCE .......................................... 294.6.1 Procedure ......................................................................................................... 294.6.2 Tools for Planning and Reconnaissance .......................................................... 304.6.3 Target Appraisal Procedure ............................................................................. 324.6.4 Tools for Target Appraisal .............................................................................. 33

4.7 AQUIFER CHARACTERISATION........................................................................ 414.7.1 Pumping Tests ................................................................................................. 414.7.2 Downhole Geophysics Methods ...................................................................... 494.7.3 Tracer Tests ..................................................................................................... 50

REFERENCES..................................................................................................................... 51

FURTHER INFORMATION SOURCES.......................................................................... 55

LIST OF FIGURES

Figure 4.1: An illustration of the water cycle, showing some of the components that should beconsidered in water balance models. (From USGS – educational website:http://wa.water.usgs.gov/outreach.html ) .................................................................... 3

Figure4. 2: Graphical representation of the characteristics of gaining (left) and losing (right)streams. The bottom images show the groundwater flow lines................................. 10

Figure 4.3: Flow diagram showing the step to the groundwater resource assessment process. Theshaded areas represent the tools used at different phases of the process................... 24

Figure 4.4: Stages of a groundwater exploration project (modified from Moon and Whateley,1995) ......................................................................................................................... 25

Figure 4.5: Steps typical of a groundwater exploration programme, with special emphasis on theuse of geophysics. ..................................................................................................... 30

Figure 4.6: Log-log and semi-log plots of the theoretical time-drawdown relationships ofunconsolidated aquifers (From Kruseman and De Ridder, 1991)............................. 48



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LIST OF TABLES

Table 4.1: Types of Groundwater Data that are typically required during a hydrocensus. .......... 14Table 4.2: Summary of typical opportunities and constraints for different water sources. ....... 19Table 4.3: Examples of different approaches to vulnerability assessment (Barber et al., 1993) .. 21Table 4.4: Information typically required (and possible sources), as part of a resource

assessment/situational analysis programme. ............................................................. 21Table 4.5: Summary of targeting tools for hydrogeological terrains of SA ................................. 38Table 4.6: Recommended Time Intervals for Measuring Drawdown in the Pumped Well during

a Pumping Test (Driscoll, 1989). .............................................................................. 44Table 4.7: Recommended Time Intervals for Measuring Drawdown in the Observation Well(s)

during a Pumping Test (Driscoll, 1989).................................................................... 45Table 4.8: Defining Aquifer type based on overlying layer response to pumping..................... 47Table 4.9: Methods of aquifer response analysis (modified from Kruseman and De Ridder,

1991) ......................................................................................................................... 47



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EXECUTIVE SUMMARYExpected context

The development of CMAs offers an opportunity for integrated assessment and optimal use of waterresources. It is hoped that investigation of groundwater potential in the WMAs will result in anappreciation of the strategic value of groundwater and will encourage improved investment ingroundwater feasibility studies.

In this scenario, the groundwater coordinator will need to develop a sound understanding of theaquifer potential of the WMA, and will be provided with the resources necessary to determine with,reasonable confidence, the volumes of allocatable groundwater available on a sustainable basis.

The role of the Groundwater coordinator

It is likely that most of the CMAs will appoint someone to specifically handle and coordinate thegroundwater component of its water resources management function. The specialised nature ofgroundwater management means that such a person should ideally be a trained hydrogeologist, or ascientist or engineer with a good understanding of hydrogeology.

The groundwater coordinator will need to act as a central point of understanding in balancing thepredicted potential for groundwater delivery, against the demands of water users. The coordinator willneed to commission and coordinate resource investigations, many of which will be undertaken byspecialist consultants (such as geophysicists). It is the role of the coordinator to ensure that bestpractices are followed in these investigations and that the most appropriate and cost effective tools andtechnologies are employed. A phased approach is generally used, as described in this chapter. Thegroundwater resources in many catchments remain undefined in terms of the water balance of thegroundwater resource, and its role in the larger water cycle and environment. Tools for assessing thesecomponents of the resource is described. Where the location and extent of the groundwater resourceremains undefined, groundwater exploration and aquifer characterisation is required. An overview ofthe tools and methods for exploration and aquifer characterisation is given.

At the end of a groundwater resource assessment the catchment manager/stakeholders/etc shouldunderstand the basic functioning of groundwater in the catchment and the sustainable yield of theresource.

The groundwater coordinator needs to ensure that investigation results are captured and stored in thecurrent WMA and national databases.

This chapter identifies the key hydrogeological terrains of South Africa and recommends appropriategeophysical investigation tools for each. The country is divided into surficial deposits; basementcomplex; fractured sedimentary; carbonate; extrusives; Karoo dykes and sills; and younger granites,gneisses and the Bushveld Igneous Complex. Each group has a distinctive set of hydrogeologicalcharacteristics and targets.

The groundwater coordinator is directed through exploration best practice at a level of detail thatshould help to manage and commission appropriate projects. Available tools and sources of moredetailed information are given. Particular emphasis is given to the assessment of fractured aquiferyields and reference made to recent work by the WRC (e.g. van Tonder 2001).



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The following diagram illustrates the phases that would typically be expected of a groundwaterresources assessment study:

The final decision on the development of a groundwater resource is generally a comparative one,assessing the costs and benefits of groundwater development against those for other available watersources. Key criteria are identified on which these decisions are often made are described and sometypical issues for different sources listed. The groundwater coordinator needs to understand how thesedecisions are taken so that he/she can fully represent the opportunities and constraints presented byaquifer development.

Key Recommendations.

The investigation of aquifer potential is the core of hydrogeological science and much ofhydrogeologists’ formal training is focussed on this technical area. If we hope to be able to make fullassessments of available groundwater resources around the country we will need ‘more of the same’ interms of strong hydrogeological expertise, mainly in the consulting field, and a significant increase ingeophysical capacity. If the groundwater coordinator is to effectively play a coordinating role theywill need project management skills.

An area where improved approaches and capacity is required is in recharge estimation, which will becritical for sustainable yield determination. We need to improve the accuracy of recharge estimationaround the country and in a greater variety of hydrogeological settings.

An area where most hydrogeologists need additional training and exposure is in cost-benefitcomparisons of groundwater with other water sources. Hydrogeologists need to be able to understandand speak the language of resource planners in order to articulate the benefits of groundwaterdevelopment, as well as understand the long-term socio-economic constraints.

SituationalAssessment

Reconnaissance/Target Identification

Desktop studyReview of Literature& data bases (e.g.NGDB)

Demand for newresources

Sustainable ResourceDevelopment

Field Assessment(Remote Sensing,Geological mapping,Hydrocensus, andGeophysics)

Aquifer assessment

Target Appraisal

Geophysics, Drilling,Pumping testsRecharge Estimation.

Resource SelectionCost-benefitcomparison of watersources



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In summary, improvements in capacity are required in the hydrogeological community in:

• Recharge estimation

• Project management

• Cost-benefit analysis

• Geophysics



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4.1 INTRODUCTION

General The extent of groundwater resources can be complex to assess and difficultfor non-specialists to picture. As a result groundwater resources have oftennot been included in the assessment of regional water resources. TheNational Water Act requires water managers to consider all the waterresources of a region. This has coincided with the recognition ofgroundwater’s ecological functions and its valuable role in rural supply,drought relief and supply security.

Aims This chapter aims to help water managers in CMAs to assess thegroundwater resources of their region and to plan with a full appreciation ofits value and vulnerability. This is done through a description of thegroundwater provinces of South Africa in terms of their hydrogeologicalcharacteristics and exploitability, susceptibility to pollution, and issues ofsustainability.

Document Content Tools for the assessment of aquifers at both the regional and local levels arediscussed and briefly described. The information required for watermanagers to develop a holistic appreciation of the groundwater resources ofa region is listed, as well as its place in water balance models. Tools for thedevelopment and assessment of aquifers are discussed in terms of theirapplication, what they measure, and the steps required for its applicationand use.

An appreciation of the dynamics of hydraulic systems and the roleintegration of groundwater with other sources of supply is vital if theexploitation of groundwater resources is to be sustainable. Re-use and theconservation of water are discussed in Chapter 3 of this document. Thischapter will provide a perspective on the determination of recharge and itsrole in the sustainable exploitation of groundwater resources. As a follow-on from this a description is provided of the main hydrogeological terrainscommon to South Africa. This description helps in the easy assessment ofregional recharge characteristics, and serves to guide groundwatercoordinators in the design of an appropriate exploration, assessment andmanagement strategies for their regions. Tools for aquifer assessment anddevelopment are also discussed. Special emphasis is placed on pump-testing techniques in the characterisation of aquifer characteristics.

Approach It should however be noted that the generic nature of this document meansthat a lot of the information contained here is of a general nature. The mostappropriate strategy would be determined by case specific circumstances;including, available budget, water quality variations, and the location ofrecharge and discharge areas.

Integration It is important to recognise that where Groundwater Resource Assessmentsare conducted, it would form part of assessing the total water resources ofthe catchment. The results of assessing the groundwater resources of anarea should thus be integrated with the results of the surface waterresources studies. In such instances care should be taken that doubleaccounting does not take place where groundwater supplies baseflow to




surface water.

Role of ResourceAssessment/SituationalAnalysis in RDM

An understanding of the sustainable yield of an aquifer and anunderstanding of its aquifer characteristics and area of recharge anddischarge are critical to the RDM. The RDM aims to ensure protection ofgroundwater resources at a level of impact which is acceptable tostakeholders in the catchment. The process of classification (part of theRDM, see chapter 6) includes an understanding of the present state of theresource, its unimpacted, or pristine, state and the integrated water balanceof the catchment. These elements are inherent with resource assessment,therefore the two processes should happen parallel.

At the end of a groundwater resource assessment the catchmentmanager/stakeholders/etc should understand the basic functioning ofgroundwater in the catchment and the sustainable yield of the resource.

At the end of implementation of RDM the catchment manager should have:

• An understanding of the level of impact acceptable to stakeholdersand a classification of the resource accordingly;

• A determination of the Reserve necessary to sustain basic humanneeds and aquatic ecosystems;

• The Resource Quality Objectives which will provide themanagement goals for the aquifer.

4.2 DETERMINATION GROUNDWATER FLOW COMPONENTS

4.2.1 Water Balance and Resource Assessment

Background The management of groundwater entails assessing and controlling the degree offluctuation that can be tolerated in an aquifer to ensure that water levels remainabove a critical level, below which further pumping could cause harmful andoften irreversible effects (Bredenkamp, et al., 1995). The National Water Actrecognises the integrated nature of water resources. As such the management ofgroundwater needs to be integrated with the management of all other waterresources. Significant in the management of groundwater resources is therecognition of groundwater’s function in maintaining the ecological reserve.

Water BalanceModels

The use of water balance models offers a means of integrating the management ofgroundwater and other water resources; in that way addressing the mis-accountingand mis-allocation of water resources. Water balance approaches are based on theprinciple of the conservation of mass. Although the catchment is part of the widerhydrological cycle (or system) which extends beyond catchment boundaries, theinputs (precipitation, groundwater and surface water inflows) balsnce the outputs(evapotranspiration, groundwater and surface water outflow) plus the change instorage.

Inputs = Outputs ±�S

A diagrammatical illustration of the hydrological cycle is shown below. The




development of a water balance model starts with an understanding of thehydrological cycle.

Figure 4.1: An illustration of the water cycle, showing some of the components that should beconsidered in water balance models. (From USGS – educational website:

http://wa.water.usgs.gov/outreach.html )

Groundwater inthe hydrologicalcycle

The management of the groundwater resource takes place within the recognisedcontext of groundwater’s role in the water cycle. In a groundwater mass balancemodel, inputs and outputs to and from the groundwater storage component arerepresented by groundwater recharge and discharge respectively. These fluxes areoften difficult to quantify. The reasons for this lie in the unique characteristics ofgroundwater compared to surface water, including its slow flow velocity, its longresidence times, the delayed nature of responses of the groundwater system toinputs, and the low assimilative capacity for pollutants (Braune and Dziembowski,1997).

Exploited aquifer systems need to be managed to ensure that such exploitation canbe sustained over an extended period, and that associated impacts are withinacceptable limits. Wright and Xu, (2000), note that the impacts of exploitation arelikely to include a reduction in the overall discharge from the groundwater unit, adecrease in overall storage and a modification of overall recharge.




Wright and Xu (2000) warn that the isolated use of the water balance approach may result in theseveral important issues being overlooked. The following focus areas are modified from Wright andXu, 2000:

Location ofimpacts

The purpose of the water balance model is to ensure that groundwater is allocatedsustainably. However, this process overlooks issues concerning borehole location.It is therefore suggested that, in addition to application of the water balancemodel, care should be taken to ensure boreholes are optimally located with regardto minimising the localised impacts of pumping on sensitive ecosystems. Theviability of different pumping scenarios could be assessed through numericalmodelling of the aquifer. A groundwater protection strategy should includeregulations regarding the total quantity that can be pumped for various uses anddetails of the areas within the unit that are to be protected (e.g. groundwater levelsshould not be allowed to drop below a certain level, at a certain distance from asensitive aquatic ecosystem). These will be among the Resource QualityObjectives set for aquifers.

Storage Part of utilisable groundwater within a region may be identified as neither enteringor leaving a geohydrological unit (i.e. may not be in a state of flux). Suchgroundwater could be considered as being held in storage. If abstraction ispossible, without impacting upon the surface environment, this additional volumeof groundwater could be used and, subsequently, included in the availableresource of an area. However, care must be employed to ensure that this valuableback-up resource is not degraded over the long term.

Water qualityissues

The Resource Directed Measures and the Reserve encompasses quality as well asquantity issues. Water quality and quantity are inextricably linked factors and it ispossible that quality thresholds could be surpassed prior to quantity thresholds. Itwill therefore often be necessary to adjust the volumetric-focused water balanceequations, to account for water quality issues, to ensure that the resource is of anacceptable standard with respect to both criteria. These will also be protectedunder the RQOs.

Rechargecharacteristics

Effective recharge to an area can be highly variable, temporally and/or spatially,particularly in southern Africa. Such variations are overlooked when using asingle average annual value for effective recharge of an aquifer. In situations suchas this, it is likely that the water balance approach would require adjustment, toaccount for the long periods of low or no recharge. A possible solution would beseparate management strategies for ‘wet’ and ‘dry’ seasons, with reference tolocal monitored groundwater levels, or to account for drought cycles over periodsof several years.

Integration withsurface watermanagement

The new Water Act requires water managers to consider groundwater as part ofthe larger hydrologic cycle, in a continual state of flux with surface water. It istherefore essential that any groundwater planning method fits with local surfacewater strategies, ensuring the two linked resources can be conjunctively used in apragmatic manner, minimising adverse impacts on the overall water resource (i.e.surface and groundwater).

4.2.2 Recharge Estimation

Background Though pumping test analyses provide valuable information on the hydraulic characteristics ofaquifers, the sustainable exploitation of that aquifer is subject to understanding the amount ofrecharge that it receives over an extended period.




RechargeProcesses

The principle recharge mechanisms as defined based on its source has been described byLerner, et al. (1990). These are:

Direct Recharge: This is water added to the groundwater reservoir in excess of soil moisturedeficits and evapotranspiration by direct vertical percolation through the unsaturated zone.

Localised Recharge: This is an intermediate form of groundwater recharge resulting from thehorizontal (near-) surface concentration of water in absence of well defined channels.

Indirect recharge: This is percolation to the water table through the beds of surface watercourses.

Simmers (1996) note that such definitions represent a simplification of reality. He notes somepoints to keep in mind when doing recharge evaluations:

1. Recharge occurs to some extent in even the most arid regions;

2. As aridity increases, direct recharge is likely to become less important than localisedand indirect recharge in terms of total aquifer replenishment;

3. The estimation of direct recharge is likely to be more readily derived than the estimatesfor localised and indirect recharge.

ChallengesinEstimatingRecharge

Semi-arid zone recharge tends to be highly variable. It is known that such variability tends toincrease with increased aridity (Simmers, 1996). It is thus important to note that under suchcircumstances water balance calculations done on an annual basis will not reflect actualgroundwater recharge. The reason for this the fact that significant recharge only results frominfrequent large events. In would thus be highly misleading when discussing recharge, to talk interms of mean annual recharge, or of recharge as a proportion of mean annual rainfall.

A further challenge in recharge estimation is the issue of spatial variability. Not only do naturalvariations in soil type, vegetation, aspect, and others, influence recharge figures over an area;but also human activities. Care should thus be taken in relating point calculations to an area ofinterest.

Simmers (1996) eludes to the use of the combination of field measurements, remote sensingand geostatistics as a powerful tool in understanding recharge processes, if short time periodsare considered and the variability of the water balance components are taken into account.

RechargeMethods

A description of the recharge methods adapted to South African conditions is provided by VanTonder and Xu (2001). Generally, an integrated approach that uses more than one method ofrecharge calculation is advocated. A summary of the more common methods suited to SouthAfrican conditions are given below (as modified from Van Tonder and Xu, 2001).

TheChloridemethod

This is the cheapest method to estimate recharge. The Cl in rainfall and in the saturated zonemust be known as well as the dry deposition of Chloride (only very important at the coast andwhere dust is a factor).

The following assumptions are made in the application of the Chloride method (Van Tonderand Xu, 2001):

1. There is no source of chloride in the soil water or groundwater other than that fromprecipitation;




2. Chloride is conservative in the system;

3. Steady-state conditions are maintained with respect to long-term precipitation andchloride concentration in that precipitation; and

4. A piston flow regime, which is defined as downward vertical diffuse flow of soilmoisture, is assumed. However, this assumption may be invalidated if the flow throughthe unsaturated zone is along preferred pathways.

It should be noted that if flow through the unsaturated zone is bimodal (i.e. diffuse pluspreferential flow) the Cl content in the soil water in the unsaturated zone will be different thanthe Cl content in the groundwater. If the Cl in the groundwater is less than the Cl in the soilwater, preferential flow paths exists (Van Tonder and Xu, 2001).

The SVFand CRDmethods

The Saturated Volume Fluctuation Method and the Cumulative Rainfall Departure Methodexamines the inflow and outflow from the aquifer as well as the spread of boreholes.

Both methods use rainfall, abstraction and discharge volumes and groundwater levels tocalculate recharge.

They are limited by the uncertainty on the volumes of in and outflow from the aquifer. TheSVF-method is further limited in that it requires a good spread of boreholes over the area of theaquifer. The CRD method on the other hand may be less accurate for aquifers with deep watertables.

TheEARTHmodel

The Extended model for Aquifer Recharge and soil moisture Transport through the unsaturatedHardrock method is used in cases where only data of 1 borehole is known. Only monthly waterlevel, evaporation and rainfall data is required for application of this method. The methodrepresents an improvement on the conventional estimation of regional recharge in that anestimation of regional recharge is based on a classification of groundwater level characteristics:the greater the area covered by a class the more dominant is that class for areal recharge. Thesize of the area can be used as a weighting factor for each class. The Thiessen polygon methodis used to determine the area represented by each borehole (Gehrels and Van der Lee, 1990).

Isotopes Oxygen-18 and Deuterium (2H) will fingerprint the origin of the water while the Deuteriumdisplacement method will give an estimate of recharge if the recharge is generally below 20mm/a (Van Tonder and Xu, 2001).

Moisture fluxes or recharge estimates may be derived from a relationship between 2ddisplacements of isotopic compositions of soil moisture from the local meteoric line and theinverse of the square root of recharge (Allison, et al., 1984). It was found that in a 18d - 2dplot, the displacement of soil moisture was represented by a line parallel to the local meteoricwater line (MWL) and was proportional to the inverse of the square root of the recharge rate.

The amount of displacement from the local MWL is controlled by a balance between theisotopic enrichment attained in the upper layers of the soil (due to evaporation) and dilution ofthis isotopic enrichment by rainfall. For a uniform soil with a constant evaporative demand,cumulative evaporation will increase linearly with the square root of time from the last rainfallevent; i.e. enrichment of 2H or 18O will be proportional to the square root of the elapsed timeafter the last rainfall event. For n rainfall events occurring in a year with a constant amount ofrainfall for each event, the period between each event will be 1/n year, provided that the eventsare evenly distributed over the year. The total annual isotope production (dm) is thenproportional to the number of rainfall events multiplied by the isotopic enrichment at the end of




the dry period (Allison et al., 1984).

Carbon-14 method is also included.

Qualifiedguesses

Qualified guesses on the percentage of recharge in an area are based on the study of existingmaps on soil type, geology, vegetation and land-cover. The use of multiple criteria could beused to refine the recharge estimates for an area.

The following soil types are used with the assumed recharge percentage on a flat bare areagiven in brackets (Van Tonder and Xu, 2001)

% Clay

Soil Type

0 – 10

Sand (50)

10 – 20

Sandy loam (20)

20 – 35

Sandy Clay loam (5)

> 35

Clayey loam, clay (3)

*Depending on the vegetation in the are the following percentage corrections are made to theabove percentages:

Wood/Tree (90)

Grass (40)

Bare (0)

Geology may be used in a similar manner. In this case the qualified guess is dependent on thepercentage of the area covered by a specific type of formation or soil cover and the slope of thetopography.

Geology

% Recharge (soil cover <5 m)

% Recharge (soil cover >5 m)

Sandstone, mudstone, siltstone




5

2

Hard Rock (granite, gneiss, etc.)

7

4

Dolomite

12

8

Calcrete

9

5

Alluvial sand

20

15

Coastal sand

30

20

Alluvium

12

8

Coastal sand

30

20

* A correction of 40% is made if the surface slope is more than 5%.

Discharge Estimation




It is essential to gain a good understanding of the groundwater discharge processes in a catchentinorder to balance the estimates of inflows and outflows. Groundwater discharge may occur through anyof the following:

• Abstraction from boreholes;

• Baseflow to rivers;

• Baseflow to springs;

• Baseflow to wetlands;

• Discharge to the sea;

• Transpiration from vegetation;

• Evaporation from the capillary zone.

Various methods are available to either directly measure or infer discharge volumes. These aresummarised below.

Abstractionfromboreholes/wells

May be calculated through:

• Records of pumped volumes;

• Records of power or diesel consumption;

• Records of the irrigated area, crop types and crop coefficients;

• Records of the number of people/livestock dependent on a source.

Baseflow Groundwater fed baseflow to surface water features (springs, rivers, wetlands,lakes) may be calculated from stream hydrographs. This requires the volume offlow in the stream or spring to be measured. The separation of a streamhydrograph into its components is based on the assumption that the differentcomponents of flow arrive at the stream at different intervals. Overland flowwould arrive most rapidly, through-flow next, and groundwater flow at the slowestrate. Two methods exist to differentiate between baseflow and stormflow:graphical methods and Isotopic/chemical methods.

Graphical methods

In graphical hydrograph separation an arbitrary separation is made between quick-flow and slow-flow. A widely used approach is that proposed by Hewlett andHibbert (1967) in which quick-flow is separated from delayed flow by a line ofconstant slope (0.0472 mm/day) projected from the beginning of a stream rise tothe point where it intersects the falling limb of the hydrograph.

Hydrometric methods alone, however, fail to adequately explain stream chemistryduring storm runoff events. The main failing of the hydrometric method is thatalthough they quantify discharge, they fail to adequately explain stream chemistryduring storm runoff events.




Isotope/Chemical Separation

Hydrograph separation using stable environmental isotopes has been widelyapplied in hydrological process studies (Saayman and Scott, 2002). The basis ofthis method is temporal variations in the isotopic composition of precipitation,resulting in the isotope composition of the event water differing from thecomposition of the water already in the catchment (Genereux and Hooper, 1998).In numerous studies, stable isotopes (Oxygen 18 and Deuterium) and chemicalelements (for example Cl and Si) have been used as tracers to infer the relativecontributions to runoff from soil water, groundwater, storm precipitation, andsnowmelt. Temporal and geographic variations in the precipitation chemistry withindividual rainfall events however complicate use of these methods.

Where hydrograph separation methods are used, the surface hydrograph dataneeds to be checked against relative groundwater/surface water levels and theoccurrence of permeable zone, in order to confirm the feasibility for discharge.

Gaining and losing reaches of rivers should be identified as well as areas wherethe surface and groundwater systems are isolated from each other by impermeablemedia.

Figure4. 2: Graphical representation of the characteristics of gaining (left) and losing (right)streams. The bottom images show the groundwater flow lines.

The baseflow separation technique used will depend on the type of data that isavailable. Xu et al. (2001) give an assessment of the methods in a Water ResearchCommission report on the reserve determination, and recommend a modifiedHerold (1980) approach. (Xu, et al., 2001).




Actual measurements of groundwater inflows to surface water can be made usinga simple drum and plastic bag (Bokoniewicz and Zeithin, 1980).

Continuous measurements of groundwater seepage can be made using anultrasonic groundwater seepage meter. A transient time seepage meter usesultrasonic signals and two piezoelectric transducers to determine flow rate(Paulsen, et al., 1997). As water enters the flow tube, it passes through theultrasonic beam path. The direction and velocity in which the flow is moving isdirectly proportional to the difference in travel times of the ultrasonic signals. Asshown in Figure 3, the ultrasonic signal that travels with flow will arrive soonerthan the signal travelling against flow (O’Rourke, et al., 1999).

Discharge tothe sea

Discharge to the sea usually occurs in large volumes which are inferred from theconceptual understanding of groundwater gradients and aquifer thickness. Pointmeasurements can be made using either the ‘drum method’ or an ultrasonicgroundwater seepage meter (see above).

4.2.3 Hydrocensus

Background Existing and historical data often do not provide adequate information on thedistribution of water sources and the water quality variations that occur in an area.In the design of a resource development programme, it is often valuable foradditional information to be gathered from aquifers and surface water bodies.

The design and execution of a programme that gathers field data in order todevelop a more complete understanding of the hydrological systems within a studyarea is known as a hydrocensus. An example of a hydrocensus questionnaire/ datasheet is given in the appendix of Chapter 7 of these guidelines.

In groundwater studies this generally involves the collection on data ongroundwater levels, chemistry, and level of use (Step 6); but during thehydrocensus it may also be important to note the interactions between groundwaterand surface water. A risk-based approach can be followed to identify the type ofdata that should be collected (See Section 7.2.3. of Chapter 7). This can beincorporated in a Data Quality Objectives (DQO) approach.

Data QualityObjectives

A strategic, systematic process for planning and executing a hydrocensus isproposed, based on the Data Quality Objectives (DQO) approach (US-EPA, 2000).The DQO process aids in answering the following basic questions:

• Why do we need data?

• How will we obtain this data?

• What must the data represent?

• How will we use the data?

The process ensures that the data collected for decision-making are of the righttype, quantity, and quality. The following steps are usually part of a hydrocensus:

State the Problem

Identify Inputs

Define Study Boundaries

Develop a Decision Rule

Collect Field Data

Collation, Manipulation & Analysis of Data




Step 1: Statethe Problem

The most important step in the process is to define the problem that initiated thestudy. Problems can be complex and this often requires examination of a variety ofpolitical, economic, scientific, technical, legal, and social factors. A concisedescription of the problem should be developed. This description includes studyobjectives, the regulatory context, groups who are involved or who have aninterest in the study, political issues, funding, previous study results, and anyobvious existing sampling design constraints.

Step 2:IdentifyInputs

The next step is to identify the different types of information needed to resolve theproblem. A risk-based approach may be followed (as discussed in Chapter 7). Hereenvironmental variables and other information required are listed, and the sourcesof this information identified. Data from previous studies or investigations may beavailable, or new data may need to be collected. As part of this process themethods available to provide acceptable environmental measurements should beidentified and the most suitable methods selected. The available skills andequipment may constrain the available choices.

Step 4: DefinetheBoundaries ofthe Study

As a next step the spatial and temporal boundaries of the study is defined and thedata collection process placed within this context. The spatial boundaries includethe geographic area of the study. The temporal boundary provides a time framefor the data collection as well as its manipulation and interpretation. Factors suchas seasonal or daily variations, as well as weather and temperature conditions thatmay affect the data collected must be considered. Obstacles to data collection,such as the availability of sampling equipment or personnel or gaining permissionto investigate private property needs to be identified.

Step 5:Develop aDecision Rule

In this step, the attributes of the problem and how the information collected willguide the team to choose a course of action that will solve the problem aresummarized. The parameter of interest, the scale of decision making, the actionlevel, and the alternative actions all play a role in the decision rule.

The action value is determined by regulatory standards in some cases. A decisionrule is developed. The decision rule is an "if ... then..." statement that incorporatesthe parameter of interest, the scale of decision making, the action level, and theactions that would result from the decision.

Step 6: CollectField Data

A decision had already been made on what data needs to be collected (Step 2) andthe area over which the data is to be collected – geography and time (Step 4). Theparameters that will be measured vary depending on conditions prevailing atspecific sites as well as the type of data needed to develop a model or formonitoring. To physically collect the data in the field is the execution of theplanning contained in the preceding steps. Proper planning can save time andexpense when things go wrong in the field. Important points to note beforeembarking on the field data gathering is:

• That arrangements had been made for analysis of the water or soilsamples.

• That access to the measuring/sampling points has been arranged with theproper authorities or landowners.

• That you have all the necessary field equipment, and that everything is inworking order. A checklist of necessary equipment can be very helpful




and may eliminate the likelihood of finding yourself in the field withoutthe right equipment. An example of such a list is given in Appendix D.

• That transport and accommodation has been arranged, and thatarrangements have been made for the samples that are collected to bereturned to the lab for analysis, or a place of storage.

An example of a data collection form is given in appendix C. This may be alteredto meet the specific requirements of the study. The type of data that may becollected is listed below. See Chapter 7 of this document for more detail and inputon the design of groundwater monitoring networks and sample collection.




Table 4.1: Types of Groundwater Data that are typically required during a hydrocensus.

Site constants

Once-off data pertinent to the construction and geometry of the groundwater monitoring network

Coordinates of monitoring points

Coordinates of point sources and sinks of water (production boreholes, springs, etc.)

Geometry of the aquifer system (lateral boundaries, aquifer outcrops, surface water bodies)

Elevation of datum (usually the top of the borehole casing, used for water level measurements)

Elevation of ground surface

Elevation of top and bottom of aquifers and aquitards

Elevation of top and bottom of perforated sections of borehole casing

Diameters and depths of boreholes

Geology and lithology

Variable parameters

Dynamic data representing the status of the system at a particular point in time1

Piezometric head (groundwater level)

Pumping rate/ injection rate

Spring flow rate

Surface water level (for water bodies in hydraulic connection with aquifers)

Precipitation depth over an area

Rate of water abstraction from overlying aquifers

Rate of application of irrigation water or liquid effluents

Temperature

Electrical conductivity

pH

Concentrations of chemical elements, ions and compounds (macro and trace elements)

Stable and radioactive isotope concentrations

Microbiological variables

Constant parameters

Slowly varying and slowly accumulating data describing the characteristics of the hydrogeological system.

Hydraulic parameters used in groundwater flow models e.g. hydraulic conductivity, transmissivity, storagecoefficients




Step 7:Collation,Manipulationand Analysisof Data

Once the data has been collected and the results of the sample analysis received, itneeds to be entered into a database. Once in the database the data can betransformed and manipulated to highlight trends. Geographically referencing thedata and mapping it usually proves valuable in the interpretation of the data. It isimportant to note in the hydrocensus report the operational details and theoreticalassumptions of the selected design in the sampling and analysis plan. This isnecessary for the efficient and valid statistical interpretation of the data.

4.2.4 Numerical Groundwater Modelling

Background Groundwater models play an increasingly important role in groundwatermanagement. Through the construction of numerical models we are able tosimulate the response of aquifers and surface water resources to decisions on use,land-use change and the effect of climate change, long before such eventsbecome a reality. Numerical modelling offers the opportunity to generate asimulation of field conditions. When considering issues of groundwatermanagement and groundwater contamination, models help to develop anunderstanding of the problem and to define the best solution, in terms of botheffectiveness and cost-efficiency.

NumericalModels

A numerical model attempts to simulate groundwater flow indirectly by meansof a governing equation thought to represent the physical processes that occur inthe system, together with equations that describe heads or flow along theboundaries of the model. For time-dependent problems, an equation describingthe initial distribution of heads in the system is also needed (Anderson andWoesner, 1992). Generally, the fewer the simplifying assumptions in formulatingthe model, the more complex the model.

Limitations ofHydrogeologicalmodelling

One of the main difficulties in modelling groundwater flow and mass transport isthe heterogeneity of flow systems, both in terms of the system’s characterisationthrough in-situ measurements and its conceptualisation and simulation (Tsang,2000). Measures to overcome such heterogeneities have been developed for thelarge, intermediate and small scales. When dealing with large scale structures weuse in situ surveys and borehole measurements, followed by deterministicmodelling; while for small scale structures we use some kind of averaging andrepresentation (Tsang, 2000).

Morrissey (2000) lists three areas of concern in the development ofHydrogeological Modelling practice:

1. Uncertainty exists on the reliability of the software used in numericalmodels. With the proliferation of Graphical User Interfaces (GUIs), themechanisms of model construction has been simplified. This has resultedin the inner workings of the underlying models not being understood byhydrogeologists and modellers. At the same time GUI pre-processors arenot being subjected to the same level of peer review as the underlyingcore models. This could result in incorrect results being used to makemanagement decisions.

1 Variables, particularly water quality variables, can also represent a particular point in space in the case ofvertical logging down the borehole.




2. Numerical models are increasingly being misused by inexperiencedusers. Many models have default values for parameters used in thenumerical processors. Using these without understanding its implicationmay introduce error into the model results.

3. Training of modellers places too much emphasis on use of GUIs. Thisplaces hydrogeologists under the impression that they are receivingtraining in modelling when they are only being trained in the use of aGUI. This can result in the inappropriate application of numericalmodelling in groundwater management.

Steps toconstructingHydrogeologicalmodels

The development of hydrogeological models can be complex and timeconsuming. An important requirement for the construction of thehydrogeological model is that a thorough understanding of the hydrogeologicalsystem exists. The conceptual understanding is often informed by the processesdescribed in some of the other section of this chapter, such as the hydrocensus,exploration drilling, hydraulic testing, and recharge estimation. The complexityof the model to be developed will depend on the scale of the study site and thelevel of detail required from the results; while the availability of budget, time,expertise and infrastructure/software will limit the model construction options.

The following steps are proposed for the development of a hydrogeologicalmodel (modified from Anderson and Woessner, 1992, and Tsang, 2000):

1. First the purpose of the model needs to be determined. This willdetermine the governing equation to be solved and the code to beselected.

2. A conceptual model of the system needs to be developed (see section4.4.2). This includes the identification features, events, and processes(FEPs) relevant to the study site. The interplay of these features, eventsand processes with each other needs to be evaluated and stated in theconceptual model.

3. Conduct geological surveying and site evaluation, with particularemphasis on the identification of faults and fractures.

4. Select the governing equation and a computer code . Both the governingequation and the code should be verified. Verification demonstrates thatthe governing equation accurately describes the physical processes thatoccur, while verification of the code ensures that the computer programaccurately solves the equations that constitute the mathematical model.

5. Use multidisciplinary data and information (geology, geophysics,hydrology and geochemistry) for input to the model. Investigate theirconsistency for the development of the site characteristics.

6. Analyse the system evolution under different scenarios with time.

7. Calibrate the model to establish whether the model to establish whetherthe model can reproduce field-measured heads and flows. This is doneby trail-and-error adjustment of the parameters or by using an automatedparameter estimation code.




8. Perform a sensitivity analysis to establish the effect of uncertainty on thecalibrated model. Evaluate the uncertainty in the parameter values, FEPs,and in scenarios and external processes that impact the system.

9. Do model verification by using a set of calibrated parameter values andstresses to reproduce a second set of field data.

10. Predict the response of the system to future events. The model is runwith calibrated values for parameters and stresses.

11. Predictive sensitivity analysis is done to quantify the effect ofuncertainty in parameter values on the prediction. Ranges in estimatedfuture stresses are simulated to examine the impact on the model’spredictions.

12. Ideally every model should be subjected to a post-audit several yearsafter the model study is completed. This serves to establish whether thepredictions made in the model are correct. Where the post-audit leads tonew insight into the behaviour of the system, it should lead to theredesign of the conceptual model and changes in the model parameters.

SustainableYield estimation

The estimation of the sustainable yield of an aquifer depends on anunderstanding of the long-term average recharge of the aquifer. Numericalmodelling offers a method for obtaining dynamic and cumulative rechargeestimates (Merrick, 2000). Such long-term averages are however sensitive to thelength of the averaging period and the start date of the averaging. In order tocounter such uncertainty Merrick (2000) proposes the use of all possiblecombination of averages for a specified minimum averaging period. Thismethodology is limited by the integrity of the numerical groundwater model, itsunderlying conceptualisation and the representivity of the simulation period of afull climatic cycle, and by an arbitrary choice of the averaging period (Merrick,2000).

Models are also available to assist in the design of an optimal groundwaterabstraction system. Such models (like MODAFC) uses optimisation techniquesto find the best combination of pumping rates and well locations the achievecriteria specified by the user (Zhang and Chiang, 1999). In a similar optimisationmodel (MODMAN) a typical objective that may be specified is to maximise thetotal pumping rate of a group of wells, while constraints might include upper andlower limits on heads, gradients or location and pumping rate of a single well(Zhang and Chiang, 1999). Most of the management models available today aredesigned to be coupled to MODFLOW, making MODFLOW a very practicalmodelling package for use in groundwater modelling exercises.

Interaction ofsurface waterandgroundwater

Assessing the interacting of groundwater and surface water forms an importantcomponent of resource accounting, especially in the determination of thegroundwater reserve. Numerical models offer a means of quantifying thecontributions of groundwater to stream flow, and the sustenance of water levelsin surface water bodies such as lakes and wetlands. In groundwater - surfacewater interaction models, the surface water features are usually treated as fixedheads, and Darcy’s law used to assess the water exchange. In this way the modeltakes account of the different flow characteristic of groundwater and surfacewater. This includes the low velocity of groundwater flow and its slow response




to pressure changes.

A number of packages have been developed to simulate the interaction ofgroundwater and surface water. These include the River and Lake Packages(Cheng and Anderson, 1993), the Streamflow-Routing Package (Prudic 1989),the Reservoir Package (Fenske et al. 1996), the Wetland Package (Restrepo, etal., 1998), and others. Care is advised in using these packages, as they have beendeveloped with a particular field situation in mind that may not necessarilyreflect local conditions. Groundwater – Surface water modelling is a specialisedfield that requires the participation of a modeller experienced its application.

The work of Kelbe and Germishuyse (2000) serves as an illustration of the wayin which Groundwater - Surface water models can be applied to address waterresource management issues in South African coastal aquifers.

4.2.5 Strategic Environmental Assessment

Definition Strategic Environmental Assessment offers a process that assesses regional orlarger scale projects or developments in a holistic manner, by taking account ofenvironmental, social and physical factors in the development of natural resources.The holistic nature of SEAs makes it ideal for WMA level planning of waterresources development. It is thus important that the insight and understanding ofthe groundwater resources of an area be placed within the SEA context. Box 1 liststhe guiding principles for SEA.

Box 1 Guideline Principles for SEA in South Africa

1. SEA is driven by the concept of sustainability.

2. SEA identifies the opportunities and constraints, which the environment places on the development ofpolicies, plans and programs.

3. SEA sets the criteria for levels of environmental quality or limits of acceptable change.

4. SEA is a flexible process which is adaptable to the policy, planning and sectoral development cycle.

5. SEA is a strategic process, which begins with the conceptualisation of the policy, plan or programme.

6. SEA is part of a tiered approach to environmental assessment and management.

7. The scope of a SEA is defined within the wider context of environmental processes.

8. SEA is a participative process.

SEA is set within the context of alternative scenarios.

SEA is based on the principles of precaution and continuous improvement in achieving sustainability objectives.

Source: SEA Guidelines prepared for Department of Environmental Affairs and Tourism by CSIR, 2000 as

sighted in DWAF (2001).

It is envisioned that SEAs would have the following functions when applied to waterresources management (DWAF, 2001):

• Complex hydrological data and models are simplified and interpreted in a form whichis readily understood by ordinary people;

• Background information is gathered and interpreted through easily understood mapsand plans;

• Opportunities and constraints in making better use of land and water are clearlyidentified;




• public awareness of choices is increased, and local people are encouraged to take aninterest in the possibilities for long term water use in their own area (i.e. catchment),

• the costs and benefits of taking particular courses of action are clearly articulated, andthe information is presented using a simple negotiation and decision supportframework,

• Decision-makers are presented with both the basic information, and the expression ofpublic opinion on key issues before they take decisions affecting future water use;

• All decisions are transparent and the underlying reasons are published; and• The effectiveness of decisions on water use can be monitored over time, and

corrective measures can be introduced where necessary to avoid social, economic orenvironmental damage.

4.2.6 Analysis of opportunities and constraints

Use of‘OpportunitiesandConstraints’Analysis

Once the total water resources of an area have been assessed, the use ofopportunities and constraints analysis may provide valuable insight into the mostappropriate resource for exploitation. Such an analysis is area specific, and followsfrom a needs determination.

Table 4.1 summarises the most important criteria to be considered in such ananalysis. Each of the criteria listed is evaluated in terms of its sustainability and anevaluation of the costs and benefits that would follow from the development of aparticular water resource. It should also be emphasised that conjunctive use oftenoffers an opportunity to maximise the benefits associated with different sources ofsupply. The table below summarises the costs and benefits that are typicallyassociated with a water resource derived from: Water Conservation/Water DemandManagement (WC/WDM), Surface Water, Groundwater and the Desalination ofSeawater.

Table 4.2: Summary of typical opportunities and constraints fordifferent water sources.

WC/WDM (WaterRecycling)

Surface Water Groundwater Desalination ofSeawater

Maximises use ofavailable resourcesreducing need for newdevelopments

Associated uses fordams (recreational,fishing, etc.) and floodcontrol

Often mostinexpensive option,with higher assuranceof supply

Small scale possiblefor coastalcommunities

EconomicCost of treatment is highfor some uses

Generally expensive;Infrastructure costsincreasingly high aseasy targets disappear

Resource assessmentcosts high

High operational andcapital costs

Educates communitieson the value of water

Is often preferredsource and damscreate recreationalenvironment.

Localcontrol/managementis possible

Relatively secure

Social

Cultural aversion to useof recycled water. ?

Hand or windpumpdelivery perceived assecond rate

Alienating technology

Availability

Possible in most urbanand some irrigationareas

Allow storage andplanning

Distributed widely.

Seasonally reliable

Seawater quantity nota limiting factor




WC/WDM (WaterRecycling)

Surface Water Groundwater Desalination ofSeawater

?

Limited distribution;Evaporation losses;

Seasonally variable

Hidden from thelayman

Restricted to coastalareas

Education is mostimportant. Technologyfor retro fitting isavailable

Tried and testedextensively

Readily available, withvarious energysources(hand/wind/diesel)Technology

RequiredRe-use of waste waterrequires advancedtreatment

Specialised Some prone tobreakdown withoutproper maintenance

New technologyrequired to improvefeasibility

Can be behaviouralrather than engineered

Long life expectancy Usually only localinfrastructure needed

Many sites available

Engineering/Infrastructure

Retro fitting limited toindustry and higherincome groups

Requires extensiveconstruction and oftenlengthy pipeline

Vulnerable to sabotage

Increased uncertaintyin development ofresource

Big plants are mostviable

Reduces resourceimpacts

Increased aquatichabitat with damconstruction

No impact on non –GDE* ecosystems

Limited impact onwater source

EnvironmentTreatment may beenergy intensive

Potentially hugeenvironmental impactwhen dam building

Reduced dischargemay impactenvironment

Waste by-product andsignificant energyimpact.

* - Groundwater dependent ecosystems

Constraint issues are shaded.

4.2.7 Groundwater Vulnerability

Components of the physical environment and the polluting substance need to be considered whenassessing the vulnerability of a groundwater resource to pollution. A review of the present state ofknowledge on the assessment of groundwater vulnerability to pollution in the South Africanenvironment has been compiled by Sililo, et al. (2001). The most important components to consider inevaluating vulnerability include (Sililo, et al, 2001):

1. the time of travel of infiltrating water (and contaminants)

2. the relative quantity of contaminants that can reach the groundwater, and

3. the contaminant attenuation capacity of the geological materials through which the water andcontaminants travel (Sililo, et al., 2001)

The travel time, attenuation capacity and quantity of contaminants are a function of the followinggeological and hydrogeological attributes of any area (Sililo, et al., 2001):

1. The subsoils that overlie the groundwater

2. The type of recharge – whether point or diffuse

3. The thickness of the unsaturated zone through which the contaminant travels.




A description of the methods available for groundwater vulnerability assessment is presented in Sililo,et al., 2001. The reader is referred to this text for a complete discussion of the topic. The followingtable summarises the tools available for the definition of groundwater vulnerability. Many of thesehave been available for a long time, and are being strengthened through numerical groundwatermodelling, and the use of GIS systems.

Table 4.3: Examples of different approaches to vulnerability assessment (Barber et al., 1993)

Type ofassessment

Scale ofApplication

PollutionHazard

Example Identifier Reference

Empirical Local

Local

Regional & Local

Regional

Regional/National

UST -petroleum

Landfill leachate

Universal

Universal

Universal

Aldicarb

MATRIX

LeGrand

DRASTIC

GOD

Orgon DEQ, 1991

LeGrand, 1983

Aller et al., 1985

Foster, 1987

NRA, 1991

Lover et al., 1989

Deterministic Local/regional

Regional

Specificpollutants

Pesticides

LPI Bachmat & Collin,1987

Meeks and Dean,1990

Combined EmpiricalDeterministic

Regional

Regional

Pesticides

Pesticides

DRASTIC-CLMS

DRASTIC-PRZM

Ehteshami et al.,1991

Banton & Villeneuve,1989

Probabilistic Regional Pesticides VULPEST Villeneuve et al.,1990

Stochastic Regional

Regional possibleNational

Pesticides

Universal/Pesticides

Discriminant

Analysis weight ofevidence models

Teso, 1989

New LWRRC project

4.3 DATA REQUIREMENTS FOR RESOURCE ASSESSMENT

The following Table summarises the kinds of data that are typically required for groundwater resourceassessments.

Table 4.4: Information typically required (and possible sources), as part of a resourceassessment/situational analysis programme.

Category Information Source

Geological maps Geological Survey

Existing reports DWAF, Consultants, Gov.Departments

Aerial photos Surveyor General

Satellite imagery Surveyor General, CSIR, NASA

Geology and Structure

Field geological mapping Field Reconnaissance Phase




Category Information Source

Geophysical surveys Target Appraisal Phase

Lithological and geophysicalborehole logs

Aquifer Assessment Phase

Interpretation of pumping test data Aquifer Assessment Phase

Packer tests in boreholes Aquifer Assessment Phase

Flow, fluid conductivity andtemperature logs


Laboratory measurements on cores

Interpretation of peizometricgradients

Interpretation of well hydrographs

Aquifer Properties

Calculation of water balances

PiezometryWater level measurements inobservation boreholes andunpumped wells


Topographic maps Surveyor General

Aerial photographs Surveyor GeneralSurface features

Satellite imagery Surveyor General, CSIR, NASA

Rainfall records DWAF, Weather Bureau

Meteorological data (evaporation,sunshine, wind, humidity)

Weather Bureau, ARC

Land-use and vegetation Satelite imagery, TopographicMaps

Soil type and hydraulic properties

Leakage losses from water supply Local Authority

Irrigation return flow (from croptype, soils, agricultural practices,climate, irrigation techniques)

ARC, Dept. Agriculture

Recharge

Sewage leakage flows (frompopulation, water use, recordedsewer flow)

Local Authority

Pumped abstraction (fromflowmeter records, abstractionreturns, pump capacity, hour/fuelconsumption)

DWAF, WUA

Spring flows DWAF

Discharge

Flow in drains and sewers Local Authority

Losses/gains to or from streams RDM determination

Water levels in lakes or rivers DWAF

Rating curves to convert levels toflows

Interaction with surface water

Lithologies and inferred hydraulicproperties of bed material

Council for Geosciences




4.4 STEPS IN RESOURCE ASSESSMENT/SITUATIONAL ANALYSIS

The following steps are necessary to assess groundwater resources in an area. The steps to be followedin the assessment of the groundwater resources of an area will depend on the extent to which theresources have already been developed. Where no development has taken place , the resourceassessment will follow the steps from 1 to 6.

1 Initial/Conceptual Planning This includes the understanding of the problem, and scoping ofpossible solutions. Existing knowledge is used to develop aconceptual understanding of the groundwater system.

2 Detailed Planning andReconnaissance

Based on the conceptual understanding of the system, moredetailed planning and reconnaissance is done. Groundwatertargets are and methods selected.

3 Characterisation of theAquifer

Use pumping tests, geophysics and tracer tests to determine theaquifer characteristics. Also assess water quality throughhydrocensus and analysis.

4 Determine Flowcomponents

Use water balance models to conceptualise system. This requiresan understanding of inflows (recharge) and outflows (discharge)to and from the system.

5 Groundwater Surfacewater Interaction

Develop model ( conceptual and/or numerical) to illustrate theinteractions of groundwater and surface water resources.

6 Strategic EnvironmentalAssessment

The results of the Resource assessment should form part of theregional scale planning process. Decisions on the development ofthe groundwater resource should be done with due considerationof physical, environmental, economic and social factors.

Groundwater resource development projects are typically composed of the first three componentslisted. The processes involved in them is fairly well known among hydrogeologists. The steps 4 to 6have only relatively recently received attention and have till now formed part of very few resourceassessment/situational assessment studies. The NWA (Act No. 36 of 1998) now requires that steps 4 to6 also be followed when groundwater resources are assessed. It is in recognition of this fact that steps4 to 6 will be discussed first. Steps 1 to 3 is discussed later in the document (sections 4.7 – 4.9).

Each of these will be discussed in detail in this chapter. The following flow diagram illustrates howeach of these components fit into the groundwater resource assessment/situational analysis.




Figure 4.3: Flow diagram showing the step to the groundwater resource assessment process. Theshaded areas represent the tools used at different phases of the process.

4.5 INITIAL/CONCEPTUAL PLANNING

Assessing theresource

Where new groundwater resources are needed, an assessment of potentialsustainable groundwater, including exploration and pilot abstraction will beneeded. In many cases some exploitation or monitoring will be already takingplace. In that case, a consolidation of the resource assessment data will be needed.

GroundwaterExploration

Most South African aquifers are of a secondary nature, occurring under confinedor semi confined conditions. This means that in most cases, defining the aquifersgeometry equates to the location of aquifers. The following section thereforedescribes the steps to be followed in groundwater exploration.

Phases ofExploration

Exploration can be divided into a number of interlinked and sequential stages,which involve increasing expenditure and decreasing risk. The early phases areknown as the planning and reconnaissance phases. These phases cover the stages

Initial/ConceptualPlanning (incl.Thickness, extent,boundaries, etc.)

Determine aquifercharacteristics (T& s) and Quality

Determine theflow componentsof the system

Define the groundwater –surface water interactions

Place resultswithin SEAcontext

InformationSystems

Sustainablegroundwaterresource use

Hydrocensus,ContinuousMonitoring

Existing Reports,Geophysics, Pump-testing, Tracer tests

Numerical Modelling

Existing maps andReports, Geophysics

RechargeDetermination

Resourcedevelopment/managementneed

DetailedPlanning andReconnaissance




leading to the selection of an area for detailed ground work. The special nature ofwater (i.e. the need for the water source to be located reasonably close to thesettlement that requires supply), tends to limit the area of exploration.

The first phase of the exploration study is the planning phase. This may bedivided into conceptual and detailed planning phases. The planning phases coverthe review of existing literature, maps and aerial photographs, and the selection ofan appropriate exploration technique and strategy.

The next phase in the exploration project is the reconnaissance phase, whichinvolves the gathering and interpretation of focussed field data, and culminates inthe selection of exploration drill sites. The detailed geophysics and pinpointing ofthe drilling target is the target appraisal phase. Aquifer assessment follows onfrom a successful target-drilling programme. The Development phase follows thelocation of successful boreholes during the exploration drilling and aquiferassessment. The development phase includes the construction of the productionboreholes and the installation of the pumping and distribution infrastructure.

Figure 4.1 illustrates the different phases of the groundwater exploration study, with the relativeexpenditure required during each phase. The figure also illustrates the time at which decisions on thecontinuation of the study should be made.

Exp

endi

ture

– L

ogar

ithm

ic S

cale

ConceptualPlanning

DetailedPlanning

ReconnaissancePhase

TargetAppraisal

AquiferAssessment

Development Decommissioning

Act

ivit

y

Literaturereview, andDiscussionwith Peers

Literaturereview,field visit

Remote sensing,Hydro-census,Geophysics.

Detailedgeophysics,Geologicalmappingverification.

Drilling, TestPumping,Chemicalsampling andanalysis

Boreholeconstruction,pumpinstallation,Supplyinfrastructure

Removal ofequipment andclosure of boreholes.

Figure 4.4: Stages of a groundwater exploration project (modified from Moon and Whateley, 1995)

High Risk

Decreasing Risk

Key Decision node

Rate of expenditure

Regional Selection

Area Selection

FormationSuitability

Feasibility




Constraints The planning of an exploration programme needs to identify and account for theconstraints on exploration. Such constrains on the execution of a groundwaterexploration programme may include:

Quantity and quality of water required,

Preferred sources of supply,

Location of the demand,

Resources available to assess and develop a source,

Environmental considerations.

Tools If it is expected that groundwater can supply the quantity and quality of waterrequired in a feasible location, planning for resource development needs to takeinto account the local geological targets. An understanding of the hydrogeologicalterrains will direct the exploitation, assessment and interpretation methods whichare suitable. Broad categories of South African hydrogeological terrains are givenbelow. The planning of an exploration programme needs to include ahydrogeologist, catchment manager and engineer who can take account of theacceptable spending and risk for the project.

Information All available information on the study area should be reviewed including boreholelogs, maps (including topocadastral, geology and geophysical maps whenavailable), existing reports, borehole logs, test-pumping data, water quality data,etc (SADC Water Sector Coordination Unit, 2000). Information on dry boreholescan provide invaluable information about drilling targets (DWAF, 1997). Thegeology information should also be used to determine the geological terrain of thestudy area. This data should then be used in the preliminary groundwaterresource assessment to define the areas or zones of potential groundwateroccurrence that will be investigated further. The applicability of geophysicalsurveying must be established at this stage and the applicable techniques defined.According to DWAF, 1997 “It should be emphasised that the nature of thetargeted geological structure must be identified in order to assess the applicabilityof the particular geophysical exploration techniques and the methodology used inits identification.” This first phase investigation should be conducted by aqualified hydrogeologist, assisted by a qualified geophysicist if geophysics is tobe used during the exploration phase.

Hydrogeologicalterrains

A number of distinct hydrogeological terrains exist in South Africa. The toolsused in the groundwater exploration programme should be selected based on thehydrogeological characteristics of the exploration area. A description of the SouthAfrican hydrogeological terrains is presented in the following section to assist inthe exploration planning.

4.5.1 Hydrogeological terrains

From the interpretation of the geology of South Africa, a number of hydrogeological terrains can bedefined. The following section serves to briefly describe these terrains in terms of their occurrence,geology, and yield. A map showing the distribution of these terrains is included as Appendix A.

Surficialdeposits

Surficial deposits cover approximately 40 % of the geology of South Africa.Surficial deposits are young, unconsolidated to lightly cemented deposits




overlying the older geological formations. These, mostly unconsolidatedsediments, predominantly consist of sand, gravel, conglomerate, soft sandstone,limestone and calcrete. Some clay and silt, ferricrete and silcrete also fall in thisgroup. Saturated silty/clayey sediments often form storage, replenishing moretransmissive underlying fractured rock aquifers (e.g. in Karoo environments).Surficial deposits may be marine, fluviatile or aeolian in origin. The mostsignificant of these deposits is the Kalahari Group sediments with thicknesses inexcess of 280 m in places (Geological Survey Republic of South Africa, 1984).This geological terrain also includes alluvial deposits in palaeo-river channels andcoastal sands. The thicker, more extensive deposits of coarse sand and gravel arethe main targets for groundwater exploitation. Fine sand deposits can also bedeveloped if the sand horizon is thick enough (Tredoux, et al., 1998). Softsandstone and limestone may also constitute viable groundwater explorationtargets provided the cementation is not excessive (Geological Survey, Republic ofSouth Africa, 1984). It is important to note that where coastal sands are exploited,the freshwater-seawater interface should be carefully monitored and abstractionmanaged to prevent saltwater intrusion into the aquifer.

Basementcomplex

This geological terrain consists mainly of Archaean granites and gneisses,however the greenstone belts of the Murchison and Barberton sequences are alsoincluded. These rocks are characterised by intense folding, faulting and varyinggrades of metamorphism (Geological Survey Republic of South Africa, 1984).This hydrogeological terrain is characterised by deeper occurring groundwater,under confined or semi-confined conditions. The groundwater yield is directlyrelated to the fracture frequency, the connection between fractures, and theopenness of the fractures. The groundwater yield from a borehole will increasewith an increase in the number of fractures intersected in the borehole. Thisgenerally means that the greater the thickness of fractured rock, the greater thegroundwater exploitation potential. A major controlling factor is the amount ofrecharge that an area receives. It is important to note that most igneous andmetamorphosed igneous rocks are associated with low- to medium-yieldingaquifers.

FracturedSedimentaryterrains:

The main sedimentary sequences like the Karoo, the Cape, the Waterberg, theTransvaal, the Nama, and the Witwatersrand Supergroups were deposited in largeinland or intracratonic basins. The Nama Group sedimentary succession wasdeposited on a tidal plain, and the Malmesbury Group sediments were depositedin a geosyncline and consist mostly of shale. The Waterberg Group sediments arealso a major aquifer group and were deposited on a flood plain. (GeologicalSurvey Republic of South Africa, 1984). The Table Mountain Group is unique inthat it consists predominantly of sandstone and quartzite with minor shale,siltstone and conglomerate layers. Studies indicate that the secondary openingscreated in this way have a high storage capacity (Weaver and Talma, 2000).

Most of these sedimentary rocks were subjected to folding and faulting. The mostvaluable aquifer material in sedimentary sequences are the sandstone layers asthese have a higher inherent hydraulic conductivity. Fracturing in sandstone isalso more extensive because it is a more competent rock and will preferablydeform in a brittle rather than plastic manner. Fractures in sandstone are morelikely to stay open than fractures in shales. The Malmesbury Group is theexception to this rule. High yielding boreholes have been established in fracturedshale where sufficient fractured thickness occurs (Tredoux, et al., 1998).




Carbonateterrains:

The Malmani Subgroup of the Transvaal Sequence and the Campbell Group of theGriqualand West Sequence consist of extensive carbonate rocks (GeologicalSurvey Republic of South Africa, 1984). Most carbonate rock systems arecharacterised by well-developed fracture and joint systems. Rainwater combineswith carbon dioxide in the atmosphere as it falls to the ground and forms weakcarbonic acid. When this weak acid enters fractures or joints in the carbonaterocks, the rock is dissolved. The dissolution of carbonate rocks over time cancreate large voids, thereby forming reservoirs for groundwater storage (Driscoll,1989). The targets for groundwater exploration in such terrains will thus be largevoids in the carbonate rock.

Extrusives: The Ventersdorp Supergroup, Transvaal Sequence, Karoo Sequence, Richtersveldand the Barberton Sequence contain extrusive lithologies. The VentersdorpSupergroup is a layered sequence of andesite, tuff and agglomerate, with somesedimentary layers. The extrusive Drakensberg formation of the Karoo Sequenceoccurs in Lesotho and neighbouring areas and also in Central Northern Provinceand in the Soutpansberg (Lurie, 1994). The Drakensberg rocks are mainly basalticlavas consisting of multiple flows. The Orange River Group of the Richtersveldconsists of mafic lavas, andesites, dacites, rhyolites and tuffs. Intruded by graniticrocks. Folding and faulting in these rocks provide good pathways for the flow ofhydrothermal solutions as well as groundwater.

Karoo dykes,sills andintrusives:

Dolerite sills and dykes have been extensively intruded into the Karoo sediments.The intrusion of the doleritic material and the associated tectonic action thatresults from the cooling of the magma, tends to result in the development offracture zones in the surrounding rock (Chevallier et al, 2001). Such fracturezones are the targets for groundwater exploration. The horizontal Karoo doleritesills, inclined sheets and “ring” sheets also represent an important explorationtarget. The exploitable groundwater resources in these rocks are usually wherefracturing has occurred in the surrounding country rock (Chevallier et al, 2001).Storage of water may also occur in weathered dolerite. Magmatic material tend tohave lower porosity and hydraulic conductivities than sedimentary deposits.

Young Granitesand Gneisses

The Cape granites intrude the Malmesbury Group rocks, apart from the northernplutons, which intruded into the Gariep Complex. The Cape granite suite containslimited granodiorite and diorite in addition to granite. Most of these granites arealso characterised by several phases of intrusion.

Igneous rocks usually offer limited aquifer yields. This is due to the tightcrystalline nature of the rocks. The only groundwater typically associated withgranites will be in fractures, or in the weathered zone. This hydrogeologicalterrain is also characterised by deeper groundwater under confined and/or semi-confined conditions. As with other fractured rock aquifers, the aquifer yield willbe directly proportional to fracture frequency, connection between fractures andopenness of the fractures. A greater thickness of fractured rock will increase thefrequency of fractures intersected in a borehole, and a greater yield may beexpected.

4.5.2 Developing a Conceptual Model

A conceptual model is a descriptive (or pictorial) representation of a regional groundwater system andthe flow system, including the dimensions of the system and its functioning. Existing data and expert




knowledge provide sufficient information in most areas for the construction of a conceptual model.Based of the understanding of the system that is developed during the conceptual modelling exercise,decisions may be made on the exploration/management model to be followed.

The first step in the development of the conceptual model is to state the nature of the groundwatersystem. This involves its description in terms of the hydrostratigraphic units in the area, the types ofaquifers, their interconnectedness with each other and surface water sources, and the boundaries to thesystem. Existing reports, maps and first hand experience may provide sufficient insight for thegroundwater system to be described to a reasonable level of accuracy.

As a next step the flow systems that exist needs to be defined. This may require information and adescription on precipitation, evapotranspiration, runoff, groundwater head data, probable areas ofgroundwater recharge and discharge, and geochemical data.

A preliminary water budget for the area, with specific emphasis on the aquifer systems may also beconstructed. This includes a description of the inputs and outputs to the system; including natural andanthropogenic inputs. The water budget estimation should include estimates on the magnitudes offlows and changes in storage.

The conceptual model provides the foundation for developing a detailed plan for the resourceassessment study. The conceptual model informs the planning process on the most appropriate tools touse during the study, areas most suited for exploration, and gives an indication of the yields that maybe expected and its quality. It should be noted that the conceptual model represents the initialunderstanding of the groundwater system. As the study continues and more data is gathered thisunderstanding will broaden and refine.

4.6 DETAILED PLANNING AND RECONNAISSANCE

4.6.1 Procedure

DetailedPlanning

Following from the conceptual planning, areas for further investigation aredelineated and more detailed information gathered. Such information may includeairborne geophysics, remote sensing and aerial photography data andhydrocensus/database information. The insight and experience of hydrogeologistswho have worked on groundwater problems in the study area will provideessential insight and detail, and should be consulted. The regional offices of theDepartment of Water Affairs and Forestry can be a valuable reference point.

In areas where sufficient outcrop is available, field verification of air photos (andremote sensing) and mapping may be adequate for making a borehole sightingdecision. Geophysics is usually necessary where information on the targetstructure is obscured by cover material. Aerial photographs with stereographicanalysis are the most commonly used in planning the further exploration strategy,but other methods including satellite imagery are also used. Airborne and regionalgeophysics data – airborne magnetics, regional gravity and some airborneelectromagnetics – is available for large parts of South Africa and this data shouldbe used to aid the identification of targets. It is important to note that airborne andregional data often lack resolution due to the scale of survey. It should always beconsidered that the regional anomaly could result from a number of smalleranomalies on the surface. These target areas should be mapped on a base map.This can be done by the trained field hydrogeologist, but should be controlled bythe hydrogeologist in charge of the project.




Figure 4.5: Steps typical of a groundwater exploration programme, with special emphasis on theuse of geophysics.

Figure 4.3 shows the steps typical of detailed planning and reconnaissance. A description of the toolsthat are typically used during the detailed planning and reconnaissance phases are given in thefollowing subsections. The tools that are selected for use in any exploration programme depends onthe conditions unique to the area, and the constraints within the project functions.

4.6.2 Tools for Planning and Reconnaissance

Remote Sensing and GIS

Remote Sensing Remote sensing is the science of obtaining information about a phenomenawithout being in contact with it. Remote sensing deals with the detection andmeasurement of phenomena with devices sensitive to electromagnetic energy suchas light (cameras and scanners), heat (thermal scanners), and radio waves (radar).Remote sensing has application in hydrogeological exploration and resourcemanagement because of the large amount of information that can be captured inan image. Sensors have the ability to measure energy at wavelengths that are

Desk top study(Including airborne geophysical data)

Target zone identification

Field reconnaissance(viability of geophysical methods)

Pilot field testing(initial reconnaissance geophysics)

Detailed surface geophysics(to select optimal drilling locations)

Geophysical logging(Aquifer characterisation andrefine exploration programme)




beyond the range of human vision (ultra-violet, infrared, microwave), and allowfor global monitoring from nearly any site on earth.

AerialPhotography

Aerial photography is the most common remote interpretation method used ingeological investigations. Panchromatic film, producing a print of grey tonesbetween black and white in the visible part of the spectrum is by far the mostcommon and cheapest form of aerial photography. The advantage of using aerialphotographs is that the high resolution allows detailed investigation of soil, rockand vegetation. Overlapping images can be viewed in stereo, which allows evenslight differences in relief to be observed.

The relatively low cost of such photos and the level of detail they contain makethem ideal for localised exploration. The aerial photographic coverage for most ofSouth Africa is in black-and-white and analogue format. This complicates theintegration of such photos with other digital GIS data. Efforts are howeverunderway to provide digital ortho photo coverage for the whole country. Aerialphotos often form an ideal backdrop to GIS images in that it provides informationfree of interpretation.

The whole of South Africa is covered by a scale of 1:50000. For groundwaterwork the most useful scale for stereo airphoto interpretation of geology withrespect to groundwater occurrence is 1:30000, which is extensively available,while the 1:50000 prints can be used for those not covered at the 1:30000 scale.

Appendix B provides a description of the aerial photographs that can be orderedfrom the Office of the Surveys and Mapping.

SatelliteImagery

Satellite imagery is a valuable tool in groundwater exploration studies. The natureof satellite image data allows it to be manipulated to show structural lineaments.These represent important groundwater exploration targets. Satellite data alsooffers a means to identify vegetation distribution and monitor land-use changes.

Appendix C provides a description of some of the satellite products available inSouth Africa from the Satellite Application Centre, CSIR.

GIS Geographical Information Systems (GIS) has proven itself as a valuable tool inboth the planning of resource assessment studies, and the interpretation ofphysical exploration and demographic data. Data bases with geo-referenced data(see Chapter 8: Groundwater Information Systems for IWRM) makes it possiblefor the hydrogeologist and field technician to design a exploration programme thatconsiders borehole location, as well as property and natural boundaries.Geographically referenced data on groundwater quality and water levels givesinsight to the development of a conceptual understanding on the working of thegroundwater system.

Steps are underway to develop a georeferenced groundwater database for SouthAfrica (discussed in Chapter 8). It is hoped that such a GIS system would allowwater managers to visualise data like topography, geology, and remote sensingdata and would provide users with a complete picture of the resource to bemanaged and allow a spatial analysis and decision making.

Airborne Geophysics

Aeromagnetic Airborne magnetic and electromagnetic data can be acquired with reasonable ease




andElectromagnetic

and accuracy. Aeromagnetic data is the most commonly used data in groundwatersurveys, mostly because it can be used to delineate fault zones that have beenintruded by dykes. These form major zones for groundwater development, butthat these structures are limited and thus the application of aeromagnetic data islimited. With the advance in computer technology, shaded relief maps can bedrawn from traditional aeromagnetic data. These images enhance preferredorientation structures like structural trends, etc. (Jamal et al., 2001). Spatialstatistical analysis can also be applied to the aeromagnetic data based on theassumption that fracture density and direction is related to the stress regime. Byfiltering the data, optimum fracture direction can be determined and intersectiondensities determined (Antoine and Andersen, 2001). Optimal density areas can bedelineated for further investigation. Airborne electromagnetic data has beenapplied, until recently, to mineral exploration only (Parasnis, 1997). A newdevelopment is to use airborne electromagnetic data to delineate areas of highersalinity groundwater in the subsurface (Campbell et al., 2001). Zones of higherresistance (lower conductivity) are indicative of fresh water and can be mappedby using different time-windows for the airborne electromagnetic data. Airborneelectromagnetics has not yet been applied to groundwater exploration.

AirborneGravity

Airborne gravity has been acquired in the past, but because the data is affected byaircraft altitude, speed and heading the data reduction is almost impossible. Thisreduces the accuracy of the data and airborne gravity is not widely used forexploration. However, regional gravity data is available for most of South Africa.Surface gravity measurements were made on a wide grid and this regional datacan be used to delineate large structures. However, these structures are veryrarely related to groundwater features and the application of regional gravity datato groundwater exploration is limited. Some large sinkhole structures that canhave vast quantities of groundwater have been delineated in the past.

4.6.3 Target Appraisal Procedure

TargetAppraisal

The target identification should be followed by a focussed field reconnaissanceexercise for target appraisal. The field visit should includegeological/hydrogeological field mapping where necessary and applicable (e.g.where little geological information is available). Further targets should be addedto the survey if needed. The target areas should be identified on the surface and itshould be ensured that geophysical investigation of the targets is plausible. Spaceand layout should be considered. If necessary, the geophysical methods to beused can now also be reviewed. The theoretical design of the geophysical surveyshould also be defined at this stage. The station spacing, line length and spacingshould be such that the geophysical target can be delineated. The geophysicist,field hydrogeologist and the main hydrogeologist should be involved in this phaseof the study.

The field geophysics should then be conducted by a qualified geophysicist. It isgood practice to use at least two different geophysical methods to complimentinterpretation. A proper understanding of the geology, associated geohydrologyand geophysical method is needed to successfully conduct a geophysical survey.A number of possible drilling locations should be delineated and numbered inorder of preference. A trained field geophysicist, assisted by a more seniorgeophysicist should conduct the geophysics.

Detailed TargetAppraisal

The drilling can then commence under supervision of a qualified hydrogeologist.The borehole log information is needed to calibrate the geophysical data.




Geophysical logs of the boreholes should also be obtained as this stage if thescope and budget of the project allows. The geophysical data should then berevisited and refined to update the borehole-siting programme.

4.6.4 Tools for Target Appraisal

Background Borehole siting is the process of locating potential drilling locations, bydelineating targets that offer the best possibility for the establishing of asuccessful borehole (SADC Water sector Coordination Unit, 2000). Successfulborehole siting is based on an understanding of the geology, structures,geohydrology and geomorphology of an area (DWAF, 1997). Severalgeophysical tools are available to maximise the success rate of locating successfulboreholes.

Note that most geophysical tools are mainly used for investigating local features,i.e. to obtain more information on the geological structures that likely conveygroundwater. Groundwater occurrence in these structures is inferred fromexperience, but groundwater cannot be directly quantified in most cases.

Geophysical Methods for Target Appraisal

Application Geophysical surveying is not applicable to or viable in all terrains, and a properstudy of available geological and terrain information should be conducted beforedeciding on the geophysical method to be used. The success of the geophysicalmethod is reliant on a contrast between the physical and/or physico-chemicalproperties of the geology (Parasnis, 1997). Every method is specific in thedelineation of certain stratigraphic, structural and lithologic or other aquiferproperties. Stratigraphic information may include the vertical and lateral extent ofsurficial deposits, bedrock morphology, etc. Fault and fracture zones, folds, andigneous intrusions are some structures that can be delineated. Some geophysicaltechniques can also distinguish between different rock types, thicknesses, dips, etc(Kearey and Brooks, 1991). This information is used to make assumptions aboutthe presence of groundwater (Parasnis, 1997). Geophysical surveying is based onthe geological information available and should never be used without ageological and hydrogeological site assessment (DWAF, 1997).

GeophysicalTechniques

A short description of the geophysical techniques commonly used in groundwaterexploration is given below. These methods have been grouped into surface,airborne and down-hole geophysics. Airborne and geophysical logging is basedon the same principles as surface geophysics and will be briefly discussed.Detailed information can be obtained from the references listed in thebibliography. Magnetic, electrical and electromagnetic methods are often used ingroundwater exploration, while others such as the gravity and seismic methodshave limited potential for application.

It is important to note that geophysics is a specialised field and acquisition andinterpretation of geophysical data should be conducted by a qualified andexperienced geophysicist (Driscoll, 1989). However, all geophysicalinterpretation is subject to ambiguity. Several different geological features cancause the same geophysical anomaly and all available information should be usedto constrain the interpretation. “Noise” resulting from the geophysical instrument,man-made features or the operator can severely impact on the data quality.

The magnetic This method is based on measuring small changes in the Earth’s magnetic field.




method: These changes indicate differences in magnetization of the subsurface materialthat result from changes in the magnetic susceptibility of different rock types.The magnetic susceptibility is controlled by the magnetic content of the rock. Themost common magnetic mineral is magnetite (Parasnis, 1997). Mafic igneousrocks (e.g. basalt) normally contain large quantities of magnetite and are highlymagnetic. Some metamorphic rocks are also magnetic, but most sedimentaryrocks have no magnetic properties (Kearey and Brooks, 1984). This limitsmagnetic surveying to igneous and certain metamorphic rocks. Magneticanomalies can result from different adjacent lithologies, igneous intrusions, faults,etc. The inherent limitation of the magnetic method is that it can only be appliedto rocks with a magnetic susceptibility (Parasnis, 1997).

The resistivity(electrical)method:

This method uses direct current introduced into the subsurface via groundedelectrodes to study vertical and horizontal changes in the electrical properties ofthe subsurface (Kearey and Brooks, 1991). Resistivity is the inverse ofconductivity. Rocks in general, are highly insulating and electrical current flow iselectrolytic through ions in the pore water rather than electric. Resistivity is thuscontrolled by rock porosity and cementation, water saturation and water quality(resistivity) (Kearey and Brooks, 1991). The resistivity method can be used todelineate changes in electrical properties resulting from changes in rock properties(Driscoll, 1989). Resistivity surveying is reliant on good grounding with thesurface. In areas where high contact resistance can occur (e.g. sands) theapplicability of the method is limited. Elevation differences along a survey linecan influence sounding data (depth sounding). The resistivity method can be usedboth for traversing and for soundings. Multi-electrode methods can produce acombination of both traversing and sounding, i.e. a two-dimensional output.

Theelectromagnetic(EM) method:

The electromagnetic method is used to measure the apparent conductivity changesin the subsurface. Apparent conductivity is the inverse of apparent resistivity.The lateral and vertical changes in apparent conductivity can be related togeological features (Wiegmans, 1985). The same principles as for resistivitysurveying apply. However, the electromagnetic method does not use directground coupling to measure the electrical properties of the subsurface, but ratherinduction (Parasnis, 1997). Induced current flow can be limited in highly resistiveareas. Time domain electromagnetic methods can be very time consuming,however frequency domain electromagnetic methods are often limited in terms ofdepth penetration (Wiegmans, 1985). The ground penetrating radar method is aspecial application of the electromagnetic method, where very high frequencyelectromagnetic energy is used to determine changes in electrical properties of thesubsurface. This high frequency signal is attenuated rapidly and the groundpenetrating radar method does not have much application in groundwaterexploration (Parasnis, 1997). Electromagnetic methods can be used for traversingand for sounding.

The gravitymethod:

This method relates small measured changes in the earth’s gravitational field tochanges in density in the subsurface. The density changes can be related tochanges in geology. The gravity method does not have wide application ingroundwater exploration and is a time consuming surveying method. In southAfrica it is mostly used to delineate sinkhole structures in dolimitic terrains. Theinstrument is very sensitive and data acquisition is generally influenced by diurnalvariation in the gravitational field, instrument drift, elevation changes, etc. Thereduction of data is essential and time consuming. The gravity method is thusvery expensive (Kearey and Brooks, 1991).




The seismicreflection andrefractionmethods:

These methods are based on differences in acoustic velocity and the density ofrocks. Acoustic energy is propagated into the subsurface and the reflected orrefracted data are recorded at the surface. This data is then interpreted in terms ofchanges in geology. Seismic data acquisition is expensive and time consuming.The raw data also needs to be processed before interpretation.

Other methods The magnetic resonance sounding and the electrokinetic sounding methods arenewly developed techniques that directly detect the occurrence of groundwater.However, the applicability of these techniques to South African conditions is stillunder investigation.

Geophysical Applications for Hydrogeological Terrains

The following section suggests guidelines for the use of geophysical methods in groundwaterexploration for specific geological terrains. It is important to note that gross generalisations have hadto be made in writing this section. Site-specific information is thus all the more important to improvethe confidence of the geophysical survey.

The appropriate geophysical technique for each geological terrain is recommended based on the basicinformation available on these terrains. Some of the characteristics relevant to the selection of the mostappropriate geophysical technique include its geology, geohydrology and geophysics. Limitations arementioned where necessary.

Surficialdeposits:

These deposits are characterised by a varying thickness of unconsolidated depositsoverlying weathered bedrock. The resistivity sounding method can be used forexploration of these unconsolidated sediments. In specific situations whereborehole data is available for ground-truthing, combined traversing and soundingusing the frequency domain electromagnetic method can be used. The resistivitysounding method will provide a vertical profile of change in resistivity with depthat a specific location. This change of resistivity with depth can then be interpretedin terms of the underlying geology. In dry, unconsolidated sands, high contactresistance at the electrodes can occur and this may complicate the resistivitysurveying. The resistivity method will only be successful if a sufficient differencein the resistivity exists between the unconsolidated sediments and the underlyingweathered bedrock. The resistivity associated with most of the exploitableunconsolidated sediments is high, whereas the resistivity of the underlying rockswill vary from low for clays, weathered granites and shales, to high for fracturedsandstones, etc. Thus in sandy terrain underlain by fractured sandstone, there willbe only a slight increase in resistivity between the overlying sand and underlyingsandstone. However, the resistivity sounding method can still distinguish betweenhigh and very high resistivities and the bedrock topography can be mapped usingthe resistivity sounding method. To determine the general bedrock topography ina specific area, resistivity sounding data should be acquired at several locations ona pre-determined grid. Similarly, where alluvial channels occur, the channelmorphology should be delineated by doing several soundings in a line across thechannel. The thickness of the unconsolidated sediments is qualitativelydetermined and can then be plotted. The locations where the thickestunconsolidated sediments occur should be targeted for drilling water supplyboreholes. It is important to note that the latest developments in resistivitysurveying allow for the acquisition of a pseudo-section of resistivity data along aprofile line known as multi-electrode resistivity surveying. Inversion softwarealso allows for determining the real resistivity of the subsurface along this profileline. This method is ideal for determining depth to bedrock.




Frequency domain electromagnetic sounding and profiling can be used inconjunction with the resistivity sounding method where groundtruth data isavailable. The electromagnetic method will only be useful where a significantdifference in conductivity between the overlying unconsolidated sediments andthe underlying bedrock exists. Conductivity is the inverse of resistivity and leastconsolidated sediments will have a low conductivity. Weathered bedrockconductivity can be expected to vary from high for clayey materials to low forfractured sandstone. The electromagnetic method cannot distinguish betweenvery low and low conductivities and should not be used in areas where sandoverlies weathered sandstone. The sounding and profiling method should becombined to allow a qualitative interpretation of zones of thickest surficial cover.Electromagnetic sounding will provide information on the change in conductivitywith depth and profiling will provide information on the change in conductivitywith depth. Combined sounding and profiling data will provide a cross-section ofconductivity information along the profile. This data can be interpreted in termsof zones of deeper weathering by using borehole information to relate changes inconductivity to changes in lithology. It is important to note that mostcommercially available electromagnetic equipment is limited in terms of depth ofinvestigation.

This hydrogeological terrain is characterized by shallow unconfined groundwateroccurrence. Groundwater yields tend to increase with an increase in thetransmissivity and thickness of the deposits. Geophysical exploration should befocused on delineating the thickest parts of the surficial deposits i.e. to determinethe greatest depth to “bedrock”. “Bedrock” in this case will be the in-situweathered underlying rock. This can be clay where the surficial deposits areunderlain by shale or fractured sandstone where the surficial deposits areunderlain by sandstone. Note that in cases where exploitable groundwaterreserves are not associated with the surficial deposits, the underlying bedrocklithologies can often be exploited. Low transmissivity deposits (like silt) oftenprovide storage for underlying fractured rock.

Basementcomplex:

The geological model will be a thin cover of unconsolidated recent sediments,underlain by weathered and/or fractured bedrock, grading into solid bedrock. Theonly geophysical method that can be used to explore this fractured rock is theresistivity sounding method. The resistivity method has the flexibility to increasethe depth of investigation as needed. The resistivity difference between water-saturated fractured rock and dry bedrock will be significant and an accuratethickness of fractured rock can be determined at specific locations. Thedifference in frequency of fracturing can also be determined between adjacentlocations, by comparing the resistivity of the target fracture zone. A highlyfractured rock will have a lower resistivity due to higher water content andconversely the resistivity will increase as the fracturing decreases. The resistivitymethod cannot determine the orientation of the individual fracturing and cannotdelineate the fractures. It provides a resistivity with depth profile that can beinterpreted in terms of the fractured zone and the resistivity of the fractured zone.The resistivity of the fracture zone will provide information on the frequency offracturing. The multi-electrode resistivity method can also be applied toinvestigate deeper fracture zones. It should be noted that these rocks are mostlycovered by unconsolidated sediments and geophysical surveying must extend togreater depths to be able to target any fractured rocks.

Fractured The geological model for sedimentary sequences will be fractured sandstone




Sedimentaryterrains:

interbedded with shale, siltstone, conglomerate, quartzite, etc. Exploration inmost layered sequences will involve the delineation of the fractured sandstonelayers using the resistivity sounding method. The resistivity changes with depthat a specific location can be mapped. These changes can then be related tochanges in lithology with depth. The fractured sandstone layers will, in mostcases, have a lower resistivity than other sedimentary rocks, because of theirhigher porosity and greater water content. The areas selected for groundwaterexploration should be covered by a number of soundings along a grid to determinethe spatial distribution of the sandstone layers. The targets for groundwaterexploitation will be the areas where the thickness of the fractured sandstone ismore extensive and well developed. Well-developed fracture zones with a higherfrequency of fracturing will have lower resistivity. The multi-electrode resistivitymethod is highly applicable to layered sequences. The continuous resistivitypseudo-section along a profile can be interpreted in terms of the thickness andfracturing of the different layers.

Carbonateterrains:

The geological model is a highly variable rock with huge water-filled voids.These voids will be the target for groundwater exploration. The gravity andresistivity sounding method can be used for exploration of carbonate rocks.Water-filled voids in the carbonate rock will change the local gravity field due toa decrease in the density of the rock. Negative gravity anomalies will thus beassociated with water-filled voids. The gravity data cannot be quantitativelyinterpreted in terms of the size and depth of the void, because of the inherentambiguity in interpretation. A small void at a greater depth can produce the sameanomaly as a larger void at a shallower depth. Resistivity will also decrease as aresult of the water-filled voids due to their higher porosity and water saturation.Resistivity sounding data acquired on a grid will provide a spatial distribution ofresistivity. The lower resistivity areas are associated with the water-filled voidsand should be targeted for groundwater exploration. The void size and depth canbe delineated with a certain degree of accuracy by altering the spacing of thesoundings, thereby constraining the expected resistivity.

Extrusives: The geological model for this terrain is a horizontal contact between an extrusiveand the host rock (which might also be an extrusive in the case of the Ventersdorplava) that is extensively fractured. This contact zone should be targeted forgroundwater exploitation. The resistivity sounding method can be used forgroundwater exploration in this terrain. The magnetic method can also be used todelineate intrusions – this method might not be so effective for exploration of theVentersdorp lavas where all the rocks are extrusive. According to Kearey andBrooks: “Magnetic anomalies can be attributed to dykes, faulted, folded ortruncated sills and lava flows, massive basic intrusions, metamorphic basementrocks and magnetite ore bodies.” The magnetic method is qualitative in natureand can only delineate the occurrence of a magmatic body. Interpretations interms of depth and size are very limited. The resistivity sounding method canprovide quantitative depth information and delineate the dolerite intrusion andmost probably the fractured contact zone surrounding the intrusion. The soundingmethod will have to be used on a grid to delineate drilling targets. The multi-electrode resistivity method will also be invaluable in this situation.

Karoo dykesand sills:

The dolerite dykes are vertical structures that result in lateral changes in rockproperties. Lateral profiling methods like the magnetic, resistivity andelectromagnetic profiling methods can be used to delineate the position of thesedykes. These methods are mostly qualitative and can be effectively used todelineate the dyke position, but dyke geometry is more difficult to define. In




certain instances the magnetic method can be used to determine the strike and dipof dykes. The resistivity sounding method can be used at selected sites todelineate from the profiling results quantitative information about the structure ofthe dyke. The multi-electrode resistivity method can be used to good effect todelineate both lateral and vertical changes around the dyke.

Youngergranites,gneisses andBushveldIgneousComplex:

The geological model for this geological terrain will be similar to the basementcomplex. Weathered granite will grade into fractured granite underlain by solidgranite and overlain by a thin layer of unconsolidated sediments. The fracturedgranite will be the target of investigation and the thickness of the weatheredgranite will have to be determined. The resistivity sounding method can beapplied to delineate the fractured zone, as the resistivity of the groundwatersaturated fracture zone will be significantly lower than the dry bedrock andsignificantly higher than the weathered granite. The thickness of the fracture zonecan also be determined by using this method. By comparing this data withsounding data acquired in the same area, areas with the greater frequency offracturing can be delineated for drilling targets, as the resistivity of a highlyfractured zone will be lower due to a higher water content and porosity.

Table 4.5: Summary of targeting tools for hydrogeological terrains of SA

Terrain Targets Geophysical Tools Other Tools

SurficialThicker saturated zonesof high permeability

Resistivity sounding andprofiling

EM

Geomorphology

Relative thickness ofvegetation

BasementFracture zones andsaturated thick weatheredzone

Resistivity soundingGeomorphologicalmapping

Fractured Sedimentary‘Open’, connectedfracture zones

Resistivity sounding andprofiling

Resistivity sounding &

Aerial photos

CarbonateDissolution fissures andvoids

Gravity

Resistivity sounding

Topographic maps of sinkholes

Extrusives Contact zone

Magnetic

Resistivity

Sounding

Geological maps

Younger igneousThicker weathered zoneand fractures

Resistivity

Sounding

Geomorphologicalmapping

Karoo Contact zone

Magnetic

Resistivity

EM

Resistivity sounding

Exploration Drilling

Most groundwater investigations involve the drilling of one or more exploration wells before theconstruction of production wells is begun. The information gained from exploration wells can be usedfor the following general purposes:




The drilling programme may be part of a regional groundwater assessment study;

Or it may be preliminary to the design and construction of one or more production wells at aparticular site.

Where no targets are intersected or the yield is below expectation, the exploration borehole maybe preserved for monitoring purposes.

During water resources studies features of likely groundwater occurrence are defined and locationswith a high probability of intersecting the target water body are pinpointed for drilling. The presenceof water and/or the quality of that occurrence can only established through the drilling of a borehole.

It is important for the results of the exploration drilling to be carefully recorded by a qualifiedgeotechnician or geologist. Among the more important things to note is the geology and mineralogy ofthe formations being drilled. This is done by requesting the drilling contractor to lay out drill samplesat regular intervals (usually at 1 meter depth intervals). Any water strikes, the size of the water strikeand its quality should also be noted. If an air drilling system is being used, the blow yield should benoted. Variations in water quality can provide valuable information on the interaction and mixing ofdifferent structures or formations. The information gathered during the drilling is usually summarisedin a lithological log Appendix F). Consultation with the drilling operator will help in compiling anaccurate and informative log. The drilling action and penetration rate provide valuable information onthe formation being drilled. As a supplement to the lithological log a drilling-time log that records theamount of time required to drill a certain distance may also be compiled. The drilling-time log isconstructed as a curve or diagram showing penetration for each length of drill rod, with significantchanges in drilling rate indicating that a different formation is being drilled (Appendix F).

This can then be used to correlate the results of the geophysics exploration programme. Each of drilllocations pinpointed during the exploration phase should be re-evaluated during the explorationdrilling study based on the results obtained during the exploration drilling phase.

A number of drilling techniques are available and in common use in South Africa. These are listedbelow.

Hollow stem auger Used in shallow, unconsolidated formations to depths of less than 30 m. Theauger looks like a large, untapered screw constructed around a hollow pipe.The rig rotates the auger, loosening the material as it is driven into the groundand the loose sediment is brought to the surface by the auger flights.

The shallow depths that are attainable limit the application of this method inexploration studies. The method is most appropriate for the drilling of softformations. This method is also ineffective in loose ground or when drillingbelow the water table (Driscoll, 1989).

Mud-rotary drilling Appropriate for drilling into unconsolidated sediments or bedrock to depths ofgreater than 30 m at a fairly rapid rate. A heavy drilling fluid or mud ispumped down inside the hollow drill rods and rises back to the surface in theannular space between the borehole wall and drill pipe, carrying the drillcuttings.

This is often the most practical method for many drilling sites, but may causeproblems with the mud sticking to the borehole walls, affecting the boreholechemistry and hydraulic properties.

Air rotary drilling Suitable for borehole construction in bedrock to great depths.




A casing must first be installed through any unconsolidated material (typicallyby mud rotary drilling) and the air rotary drill bit introduced through the casedsection.

Compressed air is blown down the inside of the drill pipe and the drill cuttingsforced back up the annular space.

The use of foaming agents for floating drilling chips to the surface should beavoided for contamination monitoring boreholes.

Reverse-rotarydrilling

This is similar to other rotary methods, but generally more expensive.

The circulating fluid drains down the annular space and is pulled up the centreof the drill stem by a suction pump.

Milder drilling fluids can be used, often clear water mixed with the drillcuttings instead of the heavier drilling muds.

Cable-tool drilling(Stamper rig)

An older, slower method of drilling. Cheaper drill rig costs are often offset byhigher labour costs due to the long drilling time.

A heavy bit is suspended from a cable and dropped repeatedly to hammer theconsolidated rock or loosen sediments. Steel casing is driven into the groundbehind the bit. Drill cuttings are removed using a bottom-loading bailer,adding water to the dry cuttings above the water table. No drilling fluids areused, so contamination is avoided.

Jetting Used in shallow, unconsolidated formations to depths of less than 20 meters.A high pressure hose is used to jet water to loosen sediments so that screenedcasing (usually PVC) can be driven into the loosened ground. This method isappropriate for monitoring or low delivery wellpoints, as the bore diameter isrelatively narrow (usually between 63 – 110 mm).

Core drilling Core drilling is relatively expensive, but may be used to gain a betterunderstanding of permeability variations and relationships in hard-rocksecondary aquifers. The drilling rigs are relatively small, and do not require anadditional air compressor to assist with the drilling. The slow drilling andresultant high cost of the method means that it is mostly applied toconstruction or mining projects where a detailed understanding of rockstrength, the location of fractures and mineral variations are required.

The drilling method selected will depend on the geological terrain in which the exploration study istaking place, the depth of the target and the budget available. It is common to use more than onetechnique where the target structure or formation occurs at depth. For example the mud rotary drillingmethod may be used to drill through unconsolidated or badly weathered deposits, and the air rotarymethod to drill through the harder formations.

A large number of drilling operators are active in South Africa. Most of them are affiliated to eitherthe Borehole Water Association of South Africa or the Drilling Contractors Association of SouthAfrica.

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