GROUND WATER AND HYDROGEOLOGY

Nadheeka P and Angelin Jeneta BEnvironmental EngineeringGovernment College of TechnologyCoimbatore – 641 013, Tamil Nadu, IndiaE-mail: nadheeka@rediffmail.com Jenitans@yahoo.com

Water is a precious natural resource for sustaining life and environment.

Development of groundwater has already reached a critical stage in 102

blocks of the state of Tamil Nadu. Over exploitation of groundwater in

these areas has resulted in declining groundwater levels, shortage in water

supply, increased pumping lifts and consequent increase in power

consumption. Indiscriminate development of groundwater often results in

water quality deterioration and seawater intrusion in coastal zones. These are

primarily due to the lack of awareness amongst the society regarding the

importance of rainwater harvesting. Hence, there is an urgent need for

artificial recharge of groundwater and rainwater harvesting structures.

Water harvesting forms an important component for the development and

management of water resources for domestic, agricultural and municipal and

industrial needs. The area to be recharged and the type of recharge to be

adopted depend upon the geomorphology, geological and hydro geological

features present in the particular area. The various rainwater - harvesting

structures adopted in the field depend upon the terrain conditions and the

availability of space for storing the rainwater. Commonly adopted rainwater

harvesting structures are farm ponds, percolation ponds, check dams,

recharge pit, recharge shaft etc. So, these rainwater harvesting measures will

conserve water by replenishing groundwater table and improves

groundwater quality.

Introduction

Ground water is a very vital natural resource for the survival of

mankind in this world. Ground water is the water occupying all the voids in

the geological spectrum below the earth surface. Nowadays the role of

groundwater in public water supply schemes, industries and for irrigation

purposes has increased due to the following reasons:

1. Convenient availability near the point of use.

2. Excellent quality (which requires little or no treatment)

3. Relatively low cost of development

4. Uniform temperature and chemical composition, soluble mineral

content.

5. Free from turbidity and bacterial pollution

6. It can control water logging and soil salinity

7. Eco friendly means of development of irrigation.

In Tamil Nadu nearly 95% of surface water and 85% of groundwater

resources have been harnessed. Agricultural sector is the major consumer of

water. There are 1.43 lakh tube wells and 16.44 lakh open wells in the state.

In Tamil Nadu, the bulk of the annual precipitation is received during

Northeast monsoon and very small quantity during Southwest monsoon. Out

of 384 blocks, 89 blocks are classified as dark, 86 as Grey blocks and 209 as

white blocks depending on groundwater table in the respective blocks.

Due to increased exploitation of ground water in Tamil Nadu the

following problems have cropped up.

i. Depletion of Groundwater table below the economic pumping

level

ii. Over exploitation of ground water leads to increase in grey and

black area blocks,

iii. Seawater intrusion into the inland aquifers

The dependability on groundwater has reached an all time high in

recent decades due to reasons such as unreliable supplies from surface water

due to vagaries of monsoon, increase in demand for domestic, agricultural

and industrial purposes. This has resulted in over exploitation all over the

country and in certain places it has reached critical levels like drying up of

aquifers, warranting immediate attention from engineers and planners alike,

for formulating plans on a war footing augmenting the groundwater storage.

Artificial recharge is one such technique fulfilling the above objective. As a

large volume of surface runoff discharges into the sea, it can be judiciously

harnessed to recharge and replenish the under ground storage. ‘Rainwater

Harvesting’ is a method to collect the surface runoff from various locations

like roof tops of houses and buildings and convey it for recharging by

appropriate technique. The above technique is significant especially in urban

areas, where the availability of surface areas of natural soil is limited due to

paving thereby leading to a large surface runoff.

Artificial recharge:

Artificial recharge may be defined as augmenting the natural

movement of surface water into underground formations by some method of

construction, by spreading of water or by artificially changing the lateral

conditions. The choice of particular method is governed by the results of

hydrogeological studies, chemical studies and geophysical studies.

Hydrogeology:

Hydrogeological investigations principally relate to the occurrence,

storage and movement of groundwater within a water bearing formation.

Knowledge of hydrogeology is necessary for choosing the method of

artificial recharge for the required region and hydrogelogical data can be

collected using GIS and Remote Sensing.

Benefits of artificial recharge:

This system has the following advantages,

Reduces soil erosion

Increases the quality of water

Reduces floods

Replenishes ground water

Mitigates the effect of drought.

Maintains a safe ground water balance

Prevents seawater intrusion

Reduces the power consumption as 1 m rise in water level

results in saving of 0.4 kwh of electricity.

Artificial Recharge methods for augmentation of groundwater

resources:

The quantity of seepage, in one way or the other, depends upon the soil

conditions. The percolated water joins aquifers and this percolation

contributes enormously to the building of the ground water table. Artificial

recharge is a process of augmenting the underground water table by artificial

infiltration of rainwater and surface runoff. The rainwater harvesting can be

significantly done by artificial recharge methods.

Rainwater Harvesting:

The term ‘Rainwater Harvesting’ implies conservation of rainwater.

Catching rainwater when and where it falls for use during non-monsoon

months is called rainwater harvesting. This can be done in two ways: as

surface water by diverting the rainwater into tanks, ponds etc. or as ground

water by percolating it into the soil.

Need For Rainwater Harvesting And Recharging:

Following are some of the reasons for rainwater harvesting and recharging

of ground water:

Rain water is a source of fresh water on earth and if not harvested,

runoff gets wasted without augmenting the surface/subsurface

storage

Failure to harvest rainwater will flood the low lying areas and

cause lot of inconvenience

It is the solution for water problems where there is inadequate

ground water supply or insignificant surface resources or where

rainfall is very intense only over short periods (say 3-4 months in

a year)

Artificial Recharge Methods:

1) ROOF TOP RAINWATER HARVESTING:

Rooftops of houses serve as excellent and economical form of

collection centers for rainwater. If properly diverted and used for artificial

recharge it will augment the ground water table to a sufficient extent. The

initial rain water has to be flushed off to clean all impurities. In case dug

well, existing well, or bore well or sump available within the compound all

the down take pipes could be connected and led into it.

i) Percolation pit method:

To make more water to percolate down the soil, percolation pits are made,

when there is a paved pathway and are covered with perforated concrete

slabs wherever necessary. These are pits of 2-3 m deep, filled with broken

bricks or pebbles with river sand on top. The rainwater gets filtered in the

process of flowing through layers and reaches the underground aquifers and

recharges it. In case of clayey soils, percolation pit with bore well can be

used.

ARTIFICIAL RECHARGE

Roof top rainwater Harvesting Percolation Pit methodPercolation pit with Bore well method Existing Well method Existing Bore Well methodStorage Tank method

Surface runoff harvesting

PercolationPond

Check Dam

Farm ponds

Injector

wells

Spreading

methods

Flooding Basin Channel Pits and shafts

MAAS

1m

Figure:1.Percolation pit method

ii) Percolation pit with Bore method:

Whenever the depth of clay soil is more, recharge through percolation pit

with bore is preferable. The bores are deep pits of about 20’ to 25’ deep

depending on the soil condition. This bore can be at the centre of the square

pit and is filled with pebbles or brick jelly and the top portion with river

sand and covered with perforated concrete slab.

1.5m

G.L

PEBBLES

RIVER SAND

CONCRETE SLAB

Figure: 2 Percolation pit with bore well

iii) Existing Open Well method:

In case of a dug well available within the house/flat complex, all the

downward pipes could be connected and led into it either through a filter or

not, depending on the quality and quantity of water available in the said

well.

iv) Existing Bore Well method:

In case of a bore well available within the house/flat complex, all the

downward pipes could be connected and led into a filtering unit of size

21/2’x 21/2’x21/2’ and the filtered water can be allowed into the existing

bore well.

v) Storage tank method:

If a sump or storage structure is available within the site, then one or more

down pipes could be directed into it through a sand filter (containing a layer

of sand sandwiched between two layers of pebbles/blue metal) for

immediate use. Overflow water can be led into the well and in the absence of

a dug well within the site, the down take pipes could be led into either a bore

pit or a percolation well depending on the volume of rooftop water.

House-Top Floor

House-Ground

Sump

Pump

Rainwater

Tan

Filter

Pip

Figure:3.Storage tank method

Figure:4.Rooftop Rainwater harvesting for an Individual House

2) SURFACE RUNOFF WATER HARVESTING

i) Percolation Pond:

Percolation ponds are small water storage structures constructed

across natural streams and watercourses to harvest the run - off from the

catchments and impounded for longer time to facilitate percolation of water

into the soil substrata both vertically and laterally, thereby recharging

ground water.

ii) Check Dams:

Check dams are small and moderate barriers constructed across a

gully or stream to slow down the velocity of water and to make the

sediments to settle down. Though check dams are constructed as gully

control measures, they serve as ground water recharging structures.

iii) Farm Ponds:

Dug out ponds or farm ponds can be constructed in a relatively flat

terrain. The water is stored in the surface pond and is allowed to for surface

withdrawal for utilisation. The low point of a natural depression is after a

good location for an excavated pond or farm pond.

iv) Injection Wells:

Here the groundwater storage of a confined aquifer is done by

"pumping in" treated surface water under pressure. Because of low

permeability in the confining aquifer, it cannot get natural replenishment

from the surface and needs direct injection through recharge wells. Injection

wells are also used in coastal regions to arrest the ingress of seawater and to

combat the problems of land subsidence in areas where confined aquifers are

heavily pumped.

v) Spreading Methods:

This method comprises of increase in surface area to induce more

infiltration. Spreading methods are suitable where large area for basin is

available. Various spreading methods are as under.

i. Flooding method:

Flooding method is suitable for relatively flat area where the

topography is not spotted with large gullies and ridges. The water is spread

as a thin sheet over the undisturbed native vegetation and soil to obtain

higher rate of filtration. It is the cheapest method of spreading

ii. Basin Method:

In this method water is spread in a series of basins formed by a

network of small banks, check dams and sub-surface dykes. The basins are

arranged so that the entire area is submerged during spreading operations.

This is the most common method of artificial recharge, suitable for areas

where the ground surface is irregular and has shallow gullies and ridges.

iii. Channel Method:

It consists of a system of closely spaced flat bottomed channels to

expose the maximum infiltrating area. This is suitable for irregular areas and

where slopes are too steep for basin construction.

iv. Pits and Shafts:

In this method recharge is done through pits or shaft dug to puncture

the impervious layer encountered at shallow depth between the surface and

water table in the area, a condition that renders recharge by spreading

methods unfeasible in the area. Pits are also practiced where sand and

grand are encountered

MAAS (MINI ARTIFICIAL AQUIFER SYSTEM):

MAAS is a unique artificial recharge structure, which is ideally

suitable for open area particularly low - lying area. Besides, this structure is

suitable for junctions of roads, street corners, park, stadium, play ground,

bus terminus, theatres, open area of public buildings, schools, colleges, etc.

In open areas, the topsoil and clayey portion of sub surface should be

excavated and the excavated portion may be filled with locally available

boulders of various sizes of ascending order form the top. The top portion

may be filled with coarse river sand. Two or three recharge shafts may be

constructed at the bottom of the excavated portion. The necessary casing

should also be provided to prevent clogging. The recharge shafts are

essential wherever the sub soil conditions do not permit easy percolation.

Figure: 5. MINI ARTIFICIAL AQUIFER SYSTEM (MAAS)

The major advantages of MAAS

Ideally suited for deep water levels (more than 10m)

Impervious thick clay beds overlie aquifer.

Surface water gets filtered through sand media before percolation.

Less evaporation

Water stagnation can be checked.

It does not require acquisition of large piece of land like in the case of

percolation.

Design of Rainwater harvesting structures:

The design of the system depends on the following factors:

Availability of rainfall and its distribution over the year

The size of the catchment area

Availability of space required to implement the system

Depth of water table

Lithology of the area

Roof top catchment area = A

Total average rainfall = D

Runoff coefficient = f

The volume of rainfall collected can be computed by multiplying the area of

roof and average annual rainfall depth with the runoff coefficient (f), which

is taken as 0.8-0.9.

Total volume of rainfall collected (Q)= f x A x D

DESIGN OF FILTRATION UNIT

THE FILTRATION UNIT CONSISTS OF TWO LAYERS, NAMELY

1.Broken brick stones of size 25 mm

2.Coarse gravel of size 10 mm

Fig.6.Filtering unit

The filtration rate of clean gravel is 407.2 m3/day/m2(i.e. 17 m/hr)

Where,

Area is plan area in m2

DISCHARGE IN CUMECS

FILTRATION RATE IN M/SEC

Note:

The freeboard should be kept as 0.3 m

A concrete slab of thickness 0.05 m should be placed on the top of the

filtering unit

A cost benefit-working sheet is furnished below:

Area of rooftop surface =100 sq.m

Annual rainfall =1100 mm or 1.1m

Assuming 80%of rainfall can be saved and a percolation ratio of 70%

Total volume of recharge water in a year = 100x1.1x0.8x0.7

= 61.6m3

` = 61,600 litres

Cost of rainwater scheme = Rs.5000/-

Annual drinking water requirement for a

family of 5 adults at 15 litres per day = 5x15x365

= 27,375 litres

Cost of purchase of drinking water at Rs.500/- for a lorry load of 5000 litres

Value of water saved in one year = 61,600 x 500

5000

= Rs.6160/-

Cost of purchase of drinking water = 27375 x 500

5000

= Rs.2727/-

The water saved is more than double the annual requirement and the entire

cost is reimbursed by way of savings with in the year of installation as

against the assured life of 5 years for the rainwater harvesting system

Conclusion:

As our country is in a situation where efficient management of water

resources has become a necessity, different methods of artificial recharge of

ground water has come to lime light. All plans and schemes by the

Government would be efficient only if the public is aware of the importance

of recharging.

The outflow of water balance to "unproductive" should be minimised,

"maximum" utilisation of water resources should be stopped and "optimum"

utilisation should be the guiding principle. We will have to realise that most

cities in our country are water starved but not rain starved, thus implying

that if harvested efficiently the amount of rain that we get can make us self

sufficient in terms of all our requirements. Once this is realized, there will

be no need for considering other mega projects like linking of rivers, using

seawater through desalination etc.

The public should be made aware of the fact that

"A drop of water saved now is a drop of water used in future"