Large submarine groundwater discharges to the Arabian Sea from tropical southwestern Indian Coast: Measurements from seepage meters deployed during the low tide

Abstract

Submarine Groundwater Discharge (SGD) is the flow of fresh groundwater and recirculated seawater to the sea. SGD plays a crucial role in the transport of solute-rich terrestrial groundwater as well as recirculated seawater. We conducted seasonal investigations for two years on a tropical, high-rainfall occurring coastline in southwestern India to quantify the SGD from the nearshore. A combination of subsurface seepage meters and porewater samplers was used for an entire tidal cycle to estimate the seepage rates and to understand the processes controlling the discharge. The estimated seepage rates from this region are one of the highest reported in the world. This could be due to the large span of the highly porous coastal aquifer and the high annual rainfall (over 450 cm) in this region. The seepage rates are 754 cm/day, 572 cm/day and 296 cm/day for the pre-monsoon (February 2020), post-monsoon (December 2020) and pre-monsoon (February 2021) seasons, respectively and showed high spatial and temporal variability. The end-member concentrations of groundwater and seawater were used to delineate the fresh and recirculated SGD from the total seepage. The recirculated SGD (rSGD) dominates during all the seasons taking up to 99 % of the total SGD during the pre-monsoon seasons (February) and 70 % during the post-monsoon (December) season. The fresh SGD (fSGD) is low (∼1 %) during the pre-monsoon season and increases up to 30 % during the post-monsoon. A considerable amount (0.1–10 % of the total SGD) of salinity enrichment was observed at the upper saline plume. We suspect this could be due to the evaporation of recirculated seawater due to ambient weather conditions. The fSGD in the study area is mainly controlled by the high inland hydraulic head, and the rSGD is regulated by the tides. The subsurface seepage meters used in this study eliminate the instabilities in seepage measurements and can be replicated in less-studied tropical coastal zones to quantify the volume of SGD entering the world oceans. The inferences reported can benefit the public and decision-makers in managing coastal groundwater resources and are interesting to the researchers working on delineating the hydrological and geochemical processes in the nearshore.