Enhanced submarine groundwater discharge and freshening of a subterranean estuary from rain

Subterranean estuaries (STEs), the seawater-groundwater mixing zone in coastal areas, are important for water quality and ecosystem health of coastal areas. How STEs respond to heavy tropical rain is poorly known, and thus any associated hydrologic and biogeochemical effects on coastal waters are also poorly understood. The response of a STE to a period of heavy rain was studied at a beach in the Philippines. The beach water table rose rapidly with rain events and then gradually receded over the course of several days. Groundwater hydraulic head measurements, intertidal zone sediment subsurface temperature profiles, electrical resistivity geophysical surveys, and seawater ²²²Rn concentration trends indicated an increase in submarine groundwater discharge (SGD) during and after the rainy period for a few days. A broad reduction in porewater salinity at the intertidal zone STE was observed within 24 h of the storm. Recharge from heavy rainfall caused seaward head gradients to rapidly increase, which affected the salinity and temperature of the STE. Such sudden hydrologic changes within the STE and increases in SGD can potentially affect geochemical processes and deliver nontrivial amounts of solutes such as nutrients to coastal waters, similar to surface water floods. Such events could become more common as climate change amplifies the magnitude and frequency of extreme rain events.