Stable isotope signature of groundwater-seawater interaction in a micro-tidal coast of the Bay of Bengal

Abstract:

Stable isotopes of oxygen (δ¹⁸O) and hydrogen (δD) are widely used for finger printing the sources of water. In the present study, at a micro-tidal coast of the Bay of Bengal, δ¹⁸O and δD have been used to delineate the seasonal patterns of discharging terrestrial-sourced groundwater to the sea. Further, the stable isotopes are also being used to qualify and quantify the groundwater-seawater mixing and interactions. Seasonal, multi-depth pore water samples (0 to 105 cm) were collected from a transect that is perpendicular to the high tide line (HTL), and runs for 110 m towards the offshore. Stable isotope composition of the pore water demonstrate significant seasonal variability in the source of the discharging groundwater ( δ¹⁸O ranges: -8.98‰ and +1.82‰, δD ranges: -68‰ and +10.8‰). Seawater during the same period showed a δ¹⁸O range of -1.1‰ and +1.92‰, Consistent patterns of relatively isotopically-depleted pore waters (δ¹⁸O <-4.0‰) were observed at certain parts of the transect, suggesting preferential groundwater discharge zones in those areas that may be dictated by the hydraulic gradients, flow paths and aquifer properties. The gradient of the linear, best-fit trend line of the δ¹⁸O-δD bivariate plot for all of the pore water samples were found to be lower than the local or global meteoric water lines. Such a low slope may indicate discharge and/or mixing of terrestrially-sourced groundwater that have not been directly recharged from precipitation water along local flow paths. Most of the relatively δD depleted groundwater were found up to 40 m distance offshore from the HTL, probably suggesting the extent of terrestrially-sourced discharging groundwater that are undergoing mixing with the re-circulated marine-sourced pore water.