Evolution of the groundwater system in the northern continental shelf of South China Sea since Late Pleistocene

The South China Sea (SCS) is a marginal sea of the Western Pacific Ocean with a broad continental shelf exposed since the last glacial maximum. Five enormous subaqueous deltas were developed in the then river deltaic estuaries and adjacent continental shelves of the SCS where buried paleochannel systems are widely distributed. Saline groundwater with salinity up to 25 g/L has been observed in the terrestrial aquifer system in the Pearl River Delta but freshened groundwater with salinity <1 g/L observed in the offshore part of the aquifer system in the subaqueous delta. Such a co-existence of both saline groundwater inland and freshened groundwater offshore in the same aquifer system is widely observed in other large-river deltaic estuaries and their adjacent shelves, but the mechanism for such a phenomenon is not much addressed in literature. Using the Pearl River Delta and its adjacent continental shelf in the northern margin of the SCS as an example, a sophisticated paleo-hydrogeologic model considering sea-level change, sedimentation processes, and precipitation variation in the past 50 ka is conducted to simulate the evolution of the groundwater system and is further calibrated with present porewater geochemistry data and stable isotopes. The results indicate that the offshore freshened groundwater was formed during the low-stands since the late Pleistocene, whereas the onshore saline groundwater was generated by paleo-seawater intrusion during the Holocene transgression and that the intrusion disconnected the onshore freshwater and offshore freshened groundwater bodies near coastlines. The response of the groundwater system to the paleoclimatic changes was delayed by about 7-8 ka, thus the paleoclimatic forcings still have a dominant influence on the present-day distribution of the groundwater salinity.