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CHAPTER 5

GROUNDWATER

5.1 INTRODUCTION

a hydrologic water cycle, groundwater comes from surface waters (precipitation, lake,

reservoir, river, sea, etc.) and percolates into the ground beneath the water table.

Groundwater is a significant part of the hydrologic cycle, containing 21%

freshwater.

As we know groundwater is an important source of water supply all

throughout the world, its uses in irrigation, industry, municipality and domestic areas

continue to increase. Therefore, greater emphasis is being laid for a planned and

optimal utilisation of water resources. Owing to uneven distribution of rainfall both in

time and space, the surface water resources are unevenly distributed. This has resulted

in increased emphasis on development of groundwater resources. The simultaneous

development of groundwater especially through dug wells and shallow tube wells will

decline water table. In such a situation, a severe problem is created resulting in drying

of shallow wells and increase in pumping head for deeper wells and tube wells. Thus,

planned and optimal development of water resources is necessary. An appropriate

strategy is to develop for water resources with planning based on conjunctive use of

surface water and groundwater. For this the first task would be to make a realistic

assessment of the surface water and groundwater resources and then plan their use in

such a way that full crop water requirements are met excessive declining of

groundwater table. It is necessary to maintain the groundwater reservoir in a state of

dynamic equilibrium over a period of time and the water level fluctuations have to be

kept within a particular range over the monsoon and nonmonsoon seasons.

5.2 GROUNDWATER OCCURRENCE

Groundwater occurs both in weathered zone overlaying the hard rock and in

fractures, fissures and in jointed hard rocks. The thickness of the weathered zone

varies from place to place. It depends on the topography and the extent of weathering.

The occurrence, movement and availability of groundwater in the subsurface horizon

depends on the geology, structure, disposition and geometry which vary from one

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rock formation to other, place to place and at different depths. The quantitative

parameters responsible in controlling groundwater are surface topography, landforms,

drainage, recharge structures, subsurface geological and geohydrological settings and

hydrometeorological conditions. Major part of the study area is made up of granitic

gneisses of various origin, granites, laterites, dykes, quartz gravels and pegmatite

intrusions. In this region, tropical or subtropical climate prevails the groundwater

containing in weathered zones of these rocks forming the source of water for

domestic, agriculture and industries.

5.2.1 Source of Groundwater

Rainfall is the only source of groundwater. Rainfall that falls on the ground, a

part of which evaporates, some as surface runoff and part of it percolates into the soil

and further flows downward to recharge the groundwater storage in the weathered and

fractured zones of rocks. The climate, which allows only a little part of the rainfall to

contribute to the groundwater recharge and the geological environment are the two

limiting and controlling factors for the occurrence of groundwater and its movement.

Groundwater occurs both in the water table condition in the weathered zone and

probably in semiconfined or unconfined condition in the fractured zone.

5.2.2 Movement of Groundwater

Groundwater in its natural state is invariably moving. Water moves from high

energy level to low energy level by the principles of hydraulic conductivity. Water

from rainfall percolates down to recharge the groundwater storage. The porosity,

permeability, fractures and lineaments play a significant role in the movement of

groundwater and its accumulation.

5.3 WATER-BEARING PROPERTIES OF DIFFERENT ROCK

TYPES

The occurrence and movement of groundwater is generally complicated in

hard rock regions. The occurrence and distribution of groundwater differ with rock

type, structures, landforms, lithology and recharge conditions. Occurrence of

groundwater in weathered and fractured granitic layers are highly localised and they

may not have continuity (Davis and Dewiest, 1966). Joints fracture and other

structural features are very important for groundwater circulation. The rock types in

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the study area are generally weathered up to a depth of about 30 m. Below the

weathered zone, lies the fractured rock underlying massive bed rock. The

groundwater occurs in weathered and fractured zones under unconfined and

semiconfined situation. The groundwater occurrence and behavior of hydrological

parameters in the study area are very complex and they may vary at a short distance

depending on the hydrological environment. The weathered granitic layers and

fractured layers are two distinct aquifers and no connection exists with each other

(Radhakrishna, 1970). Groundwater movement is controlled by lineaments, dolerite

dykes and quartz reefs. The lineaments serve as conduits for movement of

groundwater. Dykes and quartz reefs obstruct movement of groundwater. The yield of

wells in this terrain depends on weathering intensity and spacing of joints.

5.4 BEHAVIOUR OF GROUNDWATER LEVEL

It is controlled by physiography, lithology and precipitation. Behaviour of

groundwater level analysed by based on the monitoring of groundwater level at

representative network hydrograph stations by Central Ground Water Board (CGWB)

in the study area. The data of 16 observatory wells in the study area, which is

maintained by CGWB as well as Department of Mines Geology, were utilised. In

normal conditions in a normal year, the deepest groundwater level is generally

observed during April to May and shallow water level observed during October to

November. In general, groundwater level shows recession from November to May.

5.4.1 Groundwater Level in Premonsoon and Postmonsoon During

2012

Groundwater level in premonsoon ranges from 1.8 to 57.5 m bgl in the study

area. The highest fluctuation found at Nandi Hills and the lowest found at Adugodi.

Groundwater level in postmonsoon ranges from 1.2 to 63.31 m bgl again the highest

fluctuation observed in Nandi Hills and the lowest in Adugodi. A groundwater level

may fluctuate in elevation viz, Nandi Hills. Water levels in an aquifer are the

important parameter in groundwater hydrology and a careful and detailed analysis of

its spatio-temporal variation reveals useful information on the aquifer system. Various

causes such as rainfall recharge, groundwater withdrawals, evapotranspiration,

interaction with surface water bodies, etc. are affecting the groundwater levels.

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5.5 GROUNDWATER LEVEL FLUCTUATIONS

The groundwater level fluctuation is mainly due to anthropogenic, it is well

known that the groundwater withdrawal from an aquifer induces water level decline

creating a cone of depression depending among other parameters on the aquifer

hydrodynamic parameters and geometry. Ocean tides are also known to affect the

groundwater fluctuation in the coastal aquifers (Marechal et al., 2002).

The average water level since 2001 to 2012 in the study area fluctuates over

20.02 m bgl for premonsoon and 19.75 m bgl for postmonsoon, respectively (Table

5.1). There are 16 observatory wells in the study area the same are maintained by

CGWB as well as Department of Mines and Geology. These wells are monitored four

times in a year during May (premonsoon), August, November (postmonsoon) and

January. The trend lines in the hydrographs show fall in water level for premonsoon

and rise in water level for postmonsoon (Fig. 5.1). For both the seasons, the average

water level fluctuations are higher in the northern (Nandi Hills) and southern

(Kannamagala) parts of the study area; and lower in southern (Adugodi, Jayanagar,

etc.) and central (Devanahalli) parts of the study area (Maps 5.1a and b). The

groundwater flow pattern follows the topographic slopes.

5.6 CONCEPT OF GROUNDWATER BALANCE

Water balance techniques have been extensively used to make quantitative

estimates of water resources and the impact of human activities on hydrologic cycle.

The study of water balance is defined as the systematic presentation of data on the

supply and use of water within a geographic region for a specified period. With water

balance approach, it is possible to evaluate individual contribution of water sources in

the system over different time, periods and to establish the degree of variation in

water regime due to changes in components of the system. The basic concept of water

balance is

Input to the system Outflow from the system = Change in storage of the system

(over a period of time).

5.7 GROUNDWATER ESTIMATION OF THE STUDY AREA

The main source of groundwater in the study area is infiltration of rainwater.

In the absence of data relating to loss by surface runoff, it is difficult to estimate

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precisely the quantity of water recharged to the groundwater body annually. The

estimation of groundwater resources in the present study is based on the methodology

recommended by the Groundwater Estimation Committee (GWREM, 1997) Ministry

of Water Resources, Govt. of India.

5.7.1 Groundwater Resource Estimation Methodology

The two approaches recommended by GEC (1984), namely groundwater level

fluctuation method and rainfall infiltration method can still form the basis for

groundwater assessment. However, several improvements are made in the basic

approaches in GEC (1997), distinctions such as hard rock areas and alluvial areas,

canal command areas and noncommand areas and recharge in monsoon season and

nonmonsoon season is to be estimated by groundwater level fluctuation method. If

adequate data are not available, rainfall infiltration factor methods may be used. In the

present study, the rainfall infiltration method recommended by GEC (1997) is

followed.

5.7.2 Unit for Groundwater Recharge Assessment

An appropriate hydrological unit for groundwater resource estimation is a

watershed with well-defined hydrogeological boundaries. In hard rock areas, the

hydrogeological and hydrological units normally coincide which may not be the case

in alluvial areas where the aquifer traverses the basin boundaries. In hard rock area,

assessing the groundwater of watershed as a unit is desirable, which is adopted for the

present study. The availability of data required for the computation of groundwater on

the basis of watershed as a unit is also the same unit for groundwater estimation.

5.7.3 Groundwater Recharge

In general, groundwater recharge is defined as the downward flow of water

reaching the water table, forming an addition to the groundwater reservoir (De Vires

and Simmers, 2002). In study area, the recharge to groundwater table is considered

mainly infiltration of rainfall, field irrigation and tanks.

5.7.4 Recharge from Rainfall

The part of the rain water, that falls on the ground, is infiltrated into the soil.

This infiltrated water is utilised partly in filling the soil moisture deficiency and part

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of it is percolated down reaching the water table. The process of reaching the water

table is known as the recharge from rainfall to the aquifer. Recharge due to rainfall

depends on various hydrometeorological and topographic factors, soil characteristics

and depth to water table. The methods for estimation of rainfall recharge involve the

empirical relationships established between recharge and rainfall developed for

different regions, that is, groundwater estimation committee norms, water balance

approach and soil moisture data-based methods.

According to GEC, recharge from rainfall is estimated considering the

geographical area of the watershed as a unit, normal rainfall and percentage of

infiltration. This is estimated as a product of the geographical area, normal rainfall

and infiltration factor. Taking note of the climatic water balance, soil characteristic,

fluctuation in water table, land use and landfill, etc., an infiltration factor of 11% have

been considered (GEC, 1997). The amount of gross rainfall recharge in the study area

is estimated to be 99.46 Mm3 (Table 5.2).

5.7.5 Recharge from Field Irrigation

Water requirements of crops are met, in parts, by rainfall, contribution of

moisture from the soil profile, type of crops and applied irrigation water. A part of the

water applied to irrigated fields for growing crops is lost in consumptive use and the

balance infiltrates to recharge the groundwater. The process of re-entry of a part of the

water used for irrigation is called return seepage. Percolation from applied irrigation

water, derived from both surface water and groundwater sources which constitutes

one of the major components of groundwater recharge. The method of estimation

comprises application of the water balance equation involving input and output of

water in experimental fields.

The main crops in the study area are vegetables, ragi, paddy, groundnut,

maize, mulberry, etc., the water requirement is of the order of 0.6 m/year and about

20% of the irrigation water is expected to percolate again to recharge the groundwater

zone. The quantity of water recharged by irrigation in the study area is estimated to be

12.67 Mm3 (Table 5.2).

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5.7.6 Recharge from Tanks

The seepage from the tanks may be taken as 44 to 60 cm/year over the total

water spread, even the agro-climatic conditions in the area taken into account. The

seepage from percolation tanks is higher and may be taken as 50% of its gross

storage.

The recharge from tanks in the study area is estimated as a product of the

water spread area, seepage factor and number of water filled days in a year. A seepage

factor of 0.52 m/year, over a total spread of 120 days is assumed for computing the

amount of water recharged from the tanks. The amount of water recharged from tanks

is estimated to be 11.94 Mm3. The recharge from other sources such as flood prone

areas, shallow water table areas and canals are considered negligible in the study area.

Therefore, the gross groundwater recharge in the study area from all sources, that is,

rainfall, irrigation and tank recharge is estimated as 158.87 Mm3 and the net recharge

is 143.95 Mm3 (Table 5.3).

5.7.7 Groundwater Draft

The groundwater draft is the quantity of groundwater withdrawn artificially or

naturally from the aquifers in a study area, during certain period. The out flow from

the system is considered mainly through the groundwater draft.

The draft of groundwater is mainly through pumping from a number of bore

wells for the purpose of agricultural, domestic and industrial use. Annual draft for

irrigation purposes is estimated based on the total number of bore wells and average

annual unit draft. Unit draft is calculated based on the amount of water pumped from

wells, number of pumping hours and total number of pumping days in a year. The unit

draft of different categories of wells are 1.7 ha m for bore wells, 1.4 ha m for dug cum

bore wells, 0.9 ha m for dug wells with pump set and 0.2 ha m for a dug well

operating on the other modes (GWS No. 286). The draft of the dug wells are

considered negligible in the study area. By using the above estimation, the annual

gross draft is estimated as 122.09 Mm3 and the net groundwater utilisation is

85.46 Mm3 (Table 5.3).

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5.8 GROUNDWATER BALANCE

It is the most important factor to determine the dynamic groundwater reserve

of the study area for future development. The basic hydrological principal states that a

balance must exist among the quantity of water supplied to the basin (inputs) and the

amount leaving from the basin (outputs) and the change in groundwater storage

(Karanth, 1987). The groundwater balance is computed as follows:

GWB = (Net recharge to groundwater) (Net draft of groundwater)

= 143.95 85.46 Mm3 = 58.49 Mm3

The total groundwater resources, thus computed would be available for

utilisation to domestic, irrigation and industrial uses.

5.9 STAGE OF GROUNDWATER DEVELOPMENT

The level of groundwater development in an area is computed as the ratio of

net yearly draft to total utilisable groundwater resource for irrigation (GWS No. 286).

It can be expressed as

By using the above method, an attempt has been made to categorise the areas

of the watersheds into four categories viz, safe, semicritical, critical and overexploited

areas (Table 5.3). When net exploitation in 5 years is less than 70% of total utilisable

groundwater resource for irrigation made on the basis of watershed wise, such areas

are classified as safe area. If the projected net exploitation in an area is between 70%

and 90% of the total utilisable groundwater resource for irrigation, such areas are

categorised as semicritical areas. If the projected net exploitation of an area is more

than 90% and less than 100% of the total utilisable groundwater resource, the area is

categorised as critical. If projected exploitation is more than 100% the area

categorised as overexploited. Based on this classification as well as average decade

water level trend (Fig. 5.1), the watersheds NW3, SW3 and SEW fall in overexploited

area; watershed NW2 falls in critical; watersheds NW1, SW1 and SW2 fall in

semicritical and the only watershed EW falls in safe area.

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5.10 OVEREXPLOITATION OF GROUNDWATER AND

REMEDIAL MEASURES

It is well known that overexploitation of groundwater results in various

undesirable effects. Decrease in groundwater level reduces transmissivity in water

table aquifers, yield and specific yield of wells. This problem is more in thin

heterogeneous aquifers and fractured rock aquifers (Custodio, 1991). Lowering of

water table may cause the decrease in groundwater quality by naturally and man-made

activities. The use of low quality groundwater may seriously damage agricultural soil,

vegetation, industrial production and in some cases may be the cause of health

problems. In addition to this, substantial decrease in water table means exploitation

becomes more expensive and new investments are needed for well deepening or

substitution.

The groundwater is overexploited in watersheds NW3, SW3 and SEW, which

is also confirmed from the fall in decadal water level trend. Therefore, intensive

monitoring, evaluation and future groundwater development be linked with water

conservation measures. More widespread adoption of water conservation measures

based on watershed management technique would be beneficial in critical and

semicritical watersheds like NW2, NW1, SW1 and SW2. In addition to this

groundwater abstraction in this area needs to be controlled to avoid overexploitation

by bringing in proper legislative measure for efficient management of the

groundwater resource in both quantitative and quality wise. Artificial recharge

structures such as contour bunding at appropriate location, check dams, subsurface

barrier and rain water harvesting will be recommended.

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Figure 5.1 Hydrographs of the Different Observation Wells in the Study Area

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