Chennai terrain guide - Rainwater Harvesting

the major groundwater extraction structures almost vanished from madras
landscape, drilling of deeper borewells and frequent borewell failures were the
indicators of constant water table depletion. The depletion of water table in coastal
areas has resulted in seawater intrusion causing irreparable damage to the coastal
aquifers. Borewells drilled in Triplicane, Royapettah, Besant nagar areas has already
started yielding brackish water with very high TDS(3000 – 4000 ppm).
In order to get an idea of the change water table in Chennai city, water table maps
were prepared using the observation well data for 1991 and 2002 (Refer water
level maps of 1991 and 2002). The map prepared for the year 2002 shows the
extension of red polygon (ie., depth to water table between 7m and 10m ) extending
in all directions with more predominance in north Chennai and coastal areas.
GROUNDWATER QUALITY
Because of the heterogeneous nature of the formations, quality of groundwater
changes both seasonally and depth-wise. This may be due to the depositional
environment, pollution and over development of groundwater. Analysis of shallow
aquifers in an around Chennai indicates that the groundwater is alkaline in general
with PH values ranging from 7.8 to 9.0. Many pockets have high chloride and
sulphate making the water unsuitable for drinking purposes. Groundwater quality is
good in select pockets like Besant Nagar, Kotturpuram, Annanagar, Kilpauk and parts
of Nungambakkam. Excess iron is noticed in a number of pockets like Adyar, Shastri
Nagar, K.K. Nagar, Ashok Nagar, Raja Annamalaipuram and Mylapore.
The Buckingham canal constructed at sea level for navigation has resulted in
seawater intrusion in the freshwater sediments. The intrusion of seawater due to
over development of alluvial aquifer system is noticed in north Chennai. Floating
lenses of freshwater and brackish water zones were observed in the Thiruvanmiyur –
Covalam belt, which are the potential freshwater aquifers in dunes. The same is
harnessed extensively resulting in local groundwater mining in Besant Nagar,
Thiruvanmuyur, Kottivakkam, Neelangarai area and there is a danger of seawater
intrusion locally.
The presences of number of industrial units in Guindy and Ambattur area and in
north Chennai area add to the pollution of groundwater. Generally dilution of
groundwater by artificial recharge will help in improving the water quality by
reducing the concentration of salts.