Occurrence of offshore freshened groundwater in the Pearl River Estuary and adjacent continental shelf

The ~100 ka Milankovitch cycles during the Quaternary generated a significant sea level fluctuation with the lowest sea level of 120 m below the present level at the Last Glacial Maximum in the coastal areas. The large-river deltaic estuaries (LDEs), due to the proximity to sea environments, therefore, may archive the periodical transgression and regression information. During delta-front progradation, the sedimentation process is usually dominated by coarse-grained fluvial deposits, and the river networks extend further to the sea, whereas during marine transgressions, fine-grained marine sediments dominated by clay and silt are deposited. From a hydrogeological perspective, this geologic scenario leads to the formation of multi-layered aquifer-aquitard systems in current continental shelves. Therefore, we hypothesize that the offshore freshened groundwater (OFG) may be widely distributed in the LDEs and their adjacent continental shelves.

Pearl River is the second largest river in China in terms of water discharge, and the accompanying subaqueous paleo delta extends to the slope at the northern margin of the South China Sea with an offshore distance of 200 km. To address the key scientific issues raised in OFG of the LDEs and their adjacent shelves, we have studied the offshore hydrogeology, marine seismic profiles, and porewater hydrogeochemistry in the subaqueous paleo-delta and adjacent shelf of the Pearl River. A total of 31 offshore boreholes with high-resolution porewater geochemistry profiles have been obtained in this area. These boreholes have led to an identification of a large and unexpected OFG with a volume of ~523.3×10⁹ m³, with the freshwater (salinity < 1 PSU) extending as far as 55 km offshore. The total OFG volume is twice of the annual discharge of the Pearl River. The distribution of the OFG is closely related to the morphology of the subaqueous paleo-delta of the Pearl River, where the buried paleochannel system is widely distributed. The values of δ²H and δ¹⁸O together with the chlorinity of the OFG in the Pearl River Estuary and adjacent shelf clearly reveal its meteoric origins. Besides, the systematic analysis of water quality indices including major ions, nutrients, heavy metals, and trace elements indicates that the OFG can be used as potable water with minor treatment or raw water source for effective desalination. Hotspots of OFG in the LDEs and their adjacent shelves, likely a global phenomenon, have a great potential for useful water resources for highly urbanized coastal areas suffering from water shortage.