Hydrochemical characteristics and origin of offshore freshened groundwater in the Pearl River Estuary and its adjacent shelf

The existence of offshore freshened groundwater (OFG) has been well recognised over the globe but studies on the chemistry of OFG are extremely limited due to the scarcity of dedicated drilling investigations. In this study, we integrate offshore hydrogeology, geochemical and isotopic tracers, and transport modelling to quantitatively evaluate the hydrochemical characteristics and persistence of an OFG system in the Pearl River Estuary and its adjacent shelf. Offshore drilling suggests the OFG system comprises a vast low-salinity groundwater body extending up to 180 km offshore, with a chloride concentration as low as only ~2.5% of that in seawater. The integrated analysis of porewater isotopic signals ¹⁸O and ²H with sediment radioisotope pair ²²⁶Ra/²³⁰Th indicates that the OFG in the Pearl River Estuary and its adjacent shelf is fossil groundwater, sequestered since the late Pleistocene during periods of low sea level. Geochemical modelling of conservative tracers further corroborates the system’s persistence, estimating its residence time at approximately 69–82 kyr. The significant reduction of major ions, along with the isolated status and long residence time of porewater in the estuarine-shelf sediment system, suggests distinctive redox conditions in the OFG systems compared to OFG-free sediments. The study underscores the profound role of OFG in influencing sub-seafloor biogeochemical cycles and ecosystems on local and global scales over extended timescales.