Seasonal variation of tidal-scale patterns of groundwater-seawater interactions in a micro-tidal coast adjoining Bay of Bengal, India

The chemical dynamics of solute fate and transport through discharging groundwater from a coastal aquifer is closely dependent on the temporal groundwater flux and its periodicity. Thus, the groundwater discharge rates and patterns have a strong influence on the nutrient supply to the coastal ecosystem. In the present study, 30 sea-bed porewater observation wells and 3 groundwater wells were sampled for solute chemistry for two complete lunar tidal cycles for pre-monsoon and post-monsoon season at study site in the eastern coast of India, adjoining the Bay of Bengal. The study objective was to understand the influence of tidal height, wave force, variability in tidal amplitude and seasonal variation on discharging groundwater composition and hydrodynamics in the study site, and implications to the Bay of Bengal. High-resolution groundwater level data collected from a groundwater well installed in the back-shore beach suggested groundwater level fluctuation of 8-15cm for tidal cycles in both season. The measured salinity of the sampled porewater during post-monsoon suggested freshwater dominates the near-surface sea bed up to a distance of 40m offshore, beyond which, a column of brackish porewater overlay the fresh groundwater. During pre-monsoon, the extent of the freshwater porewater lens reduced to only up to 20m offshore. Diurnal porewater salinity variations at individual observations wells were found to be minimal. However, significant variations were observed for lunar tidal cycle period and across the seasons. For both study seasons, minimum salinity was observed typically on the 8^th day of the tidal cycle. The solute fluxes of Na⁺, Ca²⁺, Mg²⁺ Cl^-, SO₄^2-, NO₃^- were found to follow a similar pattern. Hence, it is expected that the highly transient nature of chemical dynamics over tidal-to-seasonal cycles in the discharge zones would have a strong influence on the nutrient discharge and geochemical reactions in the coastal groundwater-seawater interaction zone.