Shallow and deep submarine groundwater discharge to a tropical sea: Implications to coastal hydrodynamics and aquifer vulnerability

Abstract

The groundwater of the coastal aquifers, the primary source of drinking water for the community around it, is under stress due to changing climatic conditions, extreme climate events, over-exploitation, and natural/anthropogenic interferences. To sustain coastal groundwater, it is essential to understand the hydrogeological control on the distribution of salinity and submarine groundwater discharge (SGD) in various spatial and temporal scales at different depths. This study is focused to delineate the SGD and salinity distribution for shallow surficial and deep aquifers by application of the hydrogeochemical and geophysical approach in an intertidal zone. The resistivity [Electrical Resistivity Tomography (ERT), Vertical Electrical Sounding (VES)] and Ground Penetrating Radar (GPR) data show surficial aquifers have (up to 1.5 m below ground level (m bgl)) have higher freshwater discharge at the post-monsoon season and recirculation of infiltrated brackish water with increasing offshore distances. The re-circulated seawater in distant offshore was also predominant for intermediate (>1.5 up to 100 m bgl) (brackish water-saturated) and deeper depths (>100 m bgl) (fresh groundwater), which is observed from the distinct resistivity contrasts from VES. The interaction process for all the depths with seawater has been induced by the coastal hydrodynamics, meteoric recharge, and fluctuation of groundwater levels. However, variations of resistivity and sounding data delineate that the groundwater resources underneath the shallow depth brackish water are physically vertically disconnected from the surficial aquifer, but spatial variation in sounding data shows the hydraulic connectivity, which would lead to a serious concern for fresh groundwater resources, where density-dependent transportation of solute is plausible, both vertically and horizontally.