A STUDY OF SALT WATER ENCROACHMENT IN THE COASTAL AQUIFER AT DIGHA,MIDNAPORE DISTRICT,WEST BENGAL,INDIA

Synopsis

Between 1963 and 1965, sinking of anger boreholes and testing of quality of groundwater at different depths along Digha coast provided the principal basis for defining the distribution and movement of the saline and fresh groundwater within a phreatic aquifer (7–9 metres in thickness) consisting of fine to coarse grained sand with occasional soft clay lenses of Recent age. The auger holes were sunk along 10 selected lines in an area of about 25 sq.km. extending from 3 km east of Digha on the east to the confluence of the Subarnarekha river with the Bay of Bengal on the west.

The results of the study revealed the existence of a fresh water wedge (chloride content varying from 20 to 300 ppm) separated by two saline water zones, one near the top and the other at the bottom of the aquifer. Isochlor of 500 ppm and 2.3 chloride-bicarbonate ratio delineated the saline groundwater body. The 300 and 500 ppm isochlors defined the zone of diffusion which had a variable shape and thickness depending on several factors.

Minor movements of the interface occur due to tidal and groundwater recharge variations. The interface was found to occur at much shallower depth than that calculated on the basis of the Ghyben-Herzberg principle. The reasons for such deviation have been discussed.