DELINEATION OF SUBMARINE GROUNDWATER DISCHARGE ZONES IN A MICRO-TIDAL COAST OF THE BAY OF BENGAL BY THERMAL IMAGING AND HYDROGEOCHEMISTRY

Understanding groundwater-seawater interactions may provide estimates of groundwater discharges to the sea. The present study is carried out to delineate the groundwater discharge zones in the inter-tidal sea bed of an aquifer adjoining the Bay of Bengal in eastern coast of India (Chandipur), by using groundwater temperature as a physical tracer. Following several river/lake water-groundwater interaction studies, it has been assumed that discharging groundwater would reflect the mean annual air temperature, which would be reasonably insulated from the diurnal and seasonal variable sea water temperature. Temperature mapping (seabed and pore water) by use of thermal sensor and digital thermometer, along with measurement of salinity, total dissolve solid and conductivity profiles of the multidepth wells were used to delineate groundwater discharge zones along multiple (max. n =11) transects within an area with dimension of 165 m × 110 m for post-monsoon and pre-monsoon seasons of 2012-2013. Highly variable mean hyporheic water temperatures ranging from 24.1°C to 32.2°C at the seepage points were observed in the post monsoon sampling season. Temperature of pore water interpolated in 2D profiles and 3D voxel models indicate dominant groundwater discharge zones at a vertical depth of 0.45m to 1.05m, extending from high tide line upto 110 m offshore, horizontally. Multidepth profiles of conductivity, salinity and TDS are showing that there is a prominent presence of freshwater groundwater plume in the seabed, upto 40m offshore, at a vertical extent of 0.2 m to 1m depth, during the post-monsoon season. The seabed was observed under continuous discharge condition while there is a spatial variation of the discharge throughout the transect. Results indicate that majority of the pore water samples collected from the multidepth wells are composed of freshwater mixed with circulated brackish water. The thermal profile suggests that even there is a positive groundwater discharge observed for the vertical depth 0.1m to 0.45m, this discharging water may also contain the circulated sea water, which recently infiltrated the seawater-groundwater mixing zone.