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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"
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