Wednesday, 3 August 2011

NGRI study debunks age-old theory of seawater intrusion in Krishna and Godavari delta

By Syed Akbar
Hyderabad, Aug 1: Scientists at the city-based National Geophysical Research Institute have debunked the age-old theory that the sea water is intruding into the Godavari-Krishna delta through ground water channels and turning it saline.
There has been a hue and cry by farmers and environmentalists over reports of intrusion of sea water into the delta regions of the Godavari and the Krishna rivers. Godavari and Krishna deltas are agriculturally important as they together make up the rice bowl of South India.

But now NGRI scientists point out that there's no intrusion of sea water through ground water channels. NGRI experiments have shown that the presence of salinity in ground water in the delta regions is due to "in situ" factors i.e the area was originally part of the sea millions of years ago.

"The coastal area between Vijayawada and Visakhapatnam from the sea coast to the railway line (Chennai-Kolkata) all along was once part of the Bay of Bengal. The sea receded millions of years ago exposing the land up to the coastal ridge, which incidentally is now the railway line. The salt content that's present in the ground water was deposited when sea was originally present," said Dr VVS Gurunadha Rao, deputy director, Ecology and Environment Group, NGRI.

The NGRI team conducted "delta oxygen 18" tests to rule out that the salinity in the ground water is due to sea water intrusion. "If water samples are enrichment with oxygen 18 molecules, it shows that the source is the sea. But we found the tests negative. Oxygen 16 molecules were present in abundant number, indicating that the salinity is due to in situ factors, and not due to intrusion of sea landwards," Dr Gurunadha Rao pointed out.

The salinity in the delta regions was originally 10,000 mg per litre but because of the network of irrigation canals and the two major rivers flowing, the salinity has come down to 6000 mg per litre. The salinity levels in ground water in the delta will further down over a period of time, he added.

The NGRI scientists also encountered thick marine clays up to thickness of 20-25 metres from the surface. There may be some encroachment of sea water into freshwater zones and infiltration during tidal fluctuation through mainly  Pikaleru drain, and to some extent rarely through Kannvaram and Vasalatippa drains in the downstream area.

The study was taken up to clear the misunderstanding about the sea water intrusion as the presence of sea water in delta leads to impairment of the quality of the freshwater aquifers. The study area in Godavari delta covered about 250 square km. A network of 32 observation wells was established in the area for monitoring water level and water quality assessment. 

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