geology of allahabad (india) and assessment of recharge for
TRANSCRIPT
Proceedings of Indian Geotechnical Conference December 15-17,2011, Kochi (Paper No. R-014)
GEOLOGY OF ALLAHABAD (INDIA) AND ASSESSMENT OF RECHARGE FOR
SUSTAINABILITY
Saumya Singh, Ph.D. Scholar, Motilal Nehru National Institute of Technology, Allahabad, [email protected]
R.K.Srivastava, Professor, CED, Motilal Nehru National Institute of Technology, Allahabad, [email protected]
ABSTRACT: In this study geology of Allahabad has been studied and recharge has been found out. Geological structures
have great importance in ground water investigation since they influence and control ground- water recharge and
accumulation. Recharge has been computed using Water level fluctuation and rainfall infiltration factor method for each of
the 80 municipal ward of the study area i.e. Allahabad city. Spatial distribution of recharge has been mapped using GIS.
Correction factor has to be applied if the percentage of difference between Rwlf (recharge by Water level fluctuation
method) and Rrif (recharge by rainfall infiltration factor method) is found to vary from more than +20% to less than –20%
in the municipal wards. Strata chart has been studied for finding out the suitable depth for recharging.
Keywords: Ground water recharge, Recharge potential, Geology, Aquifers.
INTRODUCTION
Ground water scenario in India is not very encouraging due
to the imbalance between recharge and groundwater
exploitation. In India average rainfall is 1100 mm and even
then there is scarcity of water, which results in water crisis
[1]. The rate of drawing water from the water table is in
excess to the rate at which the water table gets recharged by
natural means due to exponential population growth and
industrialization. As a result of this, the water table is
decreasing drastically. It has been reported that in many
parts of India, the water table is declining at the rate of 1-2
m/year [2]. Over extraction of groundwater beyond natural
recharge rates is widespread in many parts of the Arabian
Peninsula, China, India, Mexico, Russia, and the United
States.
Groundwater, which was formerly a free resource, is
increasingly being used unsustainably, and the costs of
groundwater use are rising as both quality and groundwater
levels decline [3]. Exploitation of subsurface water from
deep aquifers also depletes resources, that have taken
decades or centuries to accumulate and on which the
current annual rainfall has no immediate effect. In order to
prevent the aquifer from fast depletion, there is urgent need
to recharge the aquifers.
Assessment of groundwater recharge is influenced by
various parameters like geology, geomorphology, soil,
landuse, drainage, slope etc. in different degrees depending
upon physiography of the concerned terrain. A multitude of
methods have been used worldwide to estimate
groundwater recharge. Techniques based on ground-water
levels are among the most widely applied methods for
estimating recharge. This is likely due to abundance of
available water-table data and the simplicity of estimating
recharge rates from temporal fluctuations or spatial patterns
of groundwater levels. The (Water Table Fluctuation) WTF
method has been used in various studies [4] and is
described in detail in [5]. The method is best applied over
short time periods in regions having shallow water tables
that display sharp rises and declines in water levels. Among
different indirect techniques, the water balance method is
widely used for assessing recharge.
The most commonly used methods for estimation of natural
ground water recharge in India include empirical methods,
ground water level fluctuation method and the ground water
balance method. While estimating natural ground water
recharge, it is essential to have a good idea of the different
recharge mechanisms and their importance in the study
area. Choice of methods should also be guided by the
objectives of the study, available data and the possibilities
to get supplementary data. Economy too is an important
factor.
GIS has been found to be effective for ground water studies
and for analyzing various parameters controlling ground
water such as geology, lithology, geomorphology, structure
and recharge conditions. Remote sensing and GIS are being
widely used for demarcation of ground-water potential
zones [6,7,8,9,10,11,12] and have gained popularity for
availability of real world data and integrated approach.
METHODOLOGY
Geological structures have great importance in ground
water investigation since they influence and control ground-
water recharge and accumulation. Detailed geology map by
GSI of 1:250,000 scale has been used to digitize various
geologic units of the region (Fig. 1). Attributes such as
geologic unit have been added such as channel alluvium,
terrace alluvium and Varanasi alluvium.
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Saumya Singh & R.K.Srivastava
Fig. 1 Geology map of Allahabad
Geology map of Allahabad city shows the existence of
Terrace Alluvium and Varanasi Alluvium as geologic unit.
The Varanasi Older Alluvium, a polycyclic sequence of
yellowish brown clay- silt and fine sand with dissemination
of kankar, is exposed in the north of Yamuna. The Newer
Alluvium is characterized by unoxidised khaki and grey
coloured sediments which consists predominantly of
micaceous sand, silt and clay. It has been divided into
Terrace and Channel Alluvium. The Terrace Alluvium of
both Ganga and Yamuna are developed on a cut and eroded
platform of Varanasi Older Alluvium. The Terrace
Alluvium of Ganga consists of multiple fill sequence of
grayish sand, silt and clay while that of Yamuna consists of
medium to fine grained quartzo- feldspathic and grey
micaceous sand. Geology of the area is characterized by a
thick pile of quaternary Alluvium consisting of sand, gravel
and clay with occasional presence of thin to thick kankar
intercalation.
Geo-hydrological characteristics of the area have been
studied from the available drilling records of the tube wells
located in various parts of the town. The bore log charts of
these tube wells provide important information regarding
subsoil structure of the town. The bore log chart shows that
the soil cover constituted of generally a 3.0 m thick layer of
surface clay at the top followed by layer of sand or sandy
clay of 3 to 25 m thickness. A layer of kankar occasionally
mixed with sand or clay is encountered next. A next layer
of about 20 to 40 m thickness is found of medium coarse
sand and stone. A reliable water bearing strata are generally
available beyond a depth of 95 m. Basic requirement for
recharging of ground water is to know the strata of the
given site. Strata chart has been shown in Fig. 2 and Fig. 3.
Fig. 2 Strata chart of Allahabad – (a)
Fig. 3 Strata chart of Allahabad – (b)
In order to find the degree of influence of geological
parameter on ground-water recharge water level fluctuation
under different geologic units of Allahabad city has been
plotted against relative percentage of wells (boreholes).
Water table fluctuation measures the changes in ground-
water levels before and after the monsoon season due to
monsoon rainfall. Fluctuation represents the recharge by
monsoon- rainfall. Wells associated with terrace alluvium
shows higher water level fluctuation which means higher
recharge (Fig. 4). Varanasi alluvium shows more variation
in water level fluctuation in the wells. Five wells in this
geologic unit shows very high water level fluctuation while
others have comparatively lesser fluctuation.
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Geology of Allahabad (India), and Assessment of Recharge for Water Sustainability
0
1
2
3
4
5
6
7
8
9
0% 20% 40% 60% 80% 100%Water Level Fluctuation (m)
Relative % of W ells
Geology
Varanasi Alluvium
Fig. 4 Water Level fluctuation in wells under different
geologic unit
Strata chart has been studied for finding out the suitable
depth for recharging. The map (Fig. 5) of recharge depth
has been prepared with the help of strata chart provided by
CGWB, Allahabad which shows that more then 65% of
total city area including Naini have depth below 20 m. This
depth level is mostly present in central, southern and
eastern parts of Allahabd. In fact central parts of Allahabad
have recharging depth as low as 10-15 m in some places.
The recharge depth is high in Jhunsi where the range is 20-
25 m in northern parts and 25-30 m in southern parts. In
Allahabad the recharge depth is found to be high in Norther
Allahabad and some parts of southern and western
Allahabad where both 20-25 m and 25-30 m ranges have
strong presence.
Fig. 5 Recharge depth identified for Allahabad city
Recharge potential of Allahabad city has been found out
and is presented in Fig. 6. Recharge potential map has been
prepared by integrating recharge controlling parameters
represented in five thematic maps (geology,
geomorphology, soil map, slope map and landuse map).
These maps were overlaid in GIS platform.
Fig. 6 Recharge potential map of Allahabad city
Based upon the analysis of the results obtained in the map
above, it can be stated that very high and high ground water
recharge potential zones make only 15-20 % of the total
city area and is restricted to eastern and northern parts.
These parts are nearer to the river course and hence have
high recharge potential. Even in northern Naini region, the
recharge potential is moderate to high due to proximity to
river Yamuna. It is observed that high potential zones are
areas having flood plains towards north and east and the
areas occupied by the alluvial plains in the north of
Yamuna river. Moderate class of ground water recharge
potential zone is scattered in patches in almost all parts of
the city, especially in the areas where urbanization is
comparatively less. This class also forms around 20%
percent of the total area. But the major concern is that
almost 60% of the total study area falls under low and very
low classes of the ground water recharge potential zones.
Also, it should be noted that very low recharge potential
zones form around 15 % of the total city area. Some parts
of Jhunsi and Naini are also under very low recharge
potential zone. The areas where there is high urbanization
and built up land is high, are the least recharge potential
zones. This reflects that the groundwater recharge potential
of the study area is poor. Though, there are patches of high
and very high recharge potential zones, major portion of the
city falls between moderate to very low recharge potential
zones. These zones fall in that part of city, which has more
built up area with high density of residential/commercial
units that inhibit natural recharging of the aquifers through
precipitation. In these parts ground water recharge is
difficult, as water cannot percolate through cemented
portions and flows off. Therefore, the objective should be to
catch rainwater where it falls and no drop of water should
be allowed to waste. Artificial seasonal recharge through
injection wells might be an effective way of managing these
aquifers.
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CONCLUSIONS
Groundwater recharge potential of the study area is poor.
Though, there are patches of high and very high recharge
potential zones, major portion of the city falls between
moderate to very low recharge potential zones. These zones
fall in that part of city, which has more built up area with
high density of residential/commercial units that inhibit
natural recharging of the aquifers through precipitation. In
these parts ground water recharge is difficult, as water
cannot percolate through cemented portions and flows off.
Various kinds of recharge structures are possible which can
ensure that rainwater percolates in the ground instead of
draining away from the surface. Some structures promote
the percolation of water through soil strata at shallower
depth (e.g. recharge trenches, permeable pavements), other
conducts water to greater depths from where it joins the
groundwater (e.g. recharge wells). At many locations,
existing features like wells, pits and tanks can be modified
to be used as recharge structures.
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