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, saumyasingh17@yahoo.co.in

R.K.Srivastava, Professor, CED, Motilal Nehru National Institute of Technology, Allahabad, rksciv@yahoo.com

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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