Influence of tidal variation and wave forcing on shallow groundwater discharge to the sea adjoining the Bay of Bengal, India

Abstract:

Tidal fluctuation and wave pumping control the groundwater discharge and solute transport from the coastal aquifers to the sea. This discharged groundwater and solute flux could cause significant geochemical evolution at the groundwater-seawater (GW-SW) interaction zone, and have a potential impact on marine ecosystems. In this study we have tried to trace these tidal influences on discharging groundwater flow path and its quality at the Bay of Bengal, India by using numerical modelling. We have done multi-season time-series sea-bed porewater sampling from nested wells throughout a tidal cycle, along with groundwater sampling from the tube-wells in the vicinity, aquifer parameter estimation and beach geomorphology, to delineate the variations of solute chemistry within a tidal cycle. Numerical modelled data suggest that the tidal pumping at the study area leads seawater intrusion in the backshore aquifers. The local geology and low beach slope (<1°) also accentuate this phenomenon. The salinity of porewater at a specific well was found to vary temporally along tidal cycle, from being highest at the start of high tide and lowest during a low tide period. Time series analysis of chemical characteristics of the samples depicts that tidal fluctuation on a diurnal-scale significantly affects the ionic composition of the discharged groundwater along with piezometric level for unconfined aquifer. Graphical geochemical plots suggest that ionic exchanges at GW-SW interaction zone along with redox cycles are likely to be the main processes responsible for water quality changes. These findings highlight the significant influence of tidal fluctuations and wave pumping on discharged groundwater quality and groundwater-seawater hydrodynamics in the coastal areas.