The hydrological connection between fresh submarine groundwater discharge and coastal groundwater: an isotopic and a decadal hydrochemistry approach in an alluvial fan, central Japan

As a part of the coastal groundwater system, fresh submarine groundwater discharge (SGD) is an important source of freshwater discharge into the ocean. Since SGD is inherently difficult to obtain, understanding the hydrological connection between SGD and coastal groundwater is a useful and effective method to clarify the dynamic changes of fresh SGD. We selected a well-studied coastal alluvial fan in central Japan, whereas the hydrochemical composition of fresh SGDs was determined at the two sites in 2002. The geochemistry, hydrogeology, stable isotopes, and a 10-year hydrochemical monitoring data of groundwater were used to reveal the hydrological evolution and quality of fresh SGD. The fresh SGD and the coastal shallow groundwater are the same water type with intensively hydrological connection. The fresh SGDs were dominated by the groundwater on top of the clay layer in the ancient river courses aquifer. However, the SGD at 22 m under sea level was generally a combination of fresh groundwater and re-circulated seawater which was controlled by hydraulic gradient in the adjacent aquifer. The water quality of groundwater and fresh SGDs is good and stable. The confined groundwater beneath the clay layer has high salinity as a result of freshwater–seawater mixing. A better understanding of SGD and coastal groundwater is urgent and important to clarify the global environmental changes.