Offshore freshened groundwater in the subaqueous paleo-delta of the Pearl River in the northern South China Sea

Abstract

Offshore freshened groundwater (OFG) refers to water stored in geological formations beneath the seabed, which has significantly lower salinity compared to the seawater above it. While it has been reported that OFG is widely distributed throughout the global continental shelf system, the study of OFG is still in its early stages, particularly in the South China Sea. This lack of understanding is primarily stems from the limited knowledge about the mechanisms and timing of OFG generation and evolution. In light of this, the objective of this thesis is to investigate the occurrence, dynamics, hydrochemical characteristics, and evolution of the offshore groundwater system using the subaqueous paleo delta of the Pearl River as a case study. To achieve this objective, a combination of various methods is employed, including offshore borehole drilling, geophysical surveying, biogeochemical analysis, sophisticated numerical modeling, and high-resolution isotope geochemical fingerprinting. This thesis focuses on addressing several key scientific questions that are crucial to advancing our understanding of OFG. Firstly, the study aims to explore the connectivity between groundwater in the subaqueous paleo delta and the terrestrial aquifer system, shedding light on the sources and pathways of this offshore groundwater. Secondly, it seeks to determine the extent and static volume of OFG, providing insights into its potential as exploitable water resources. Thirdly, the thesis investigates the state of OFG circulation, examining the movement and flow patterns of this groundwater system. Fourthly, it systematically describes the hydrochemical characteristics of OFG and assesses it modulates on the subseafloor biogeochemical processes. Finally, the study evaluates the impact of nearshore land reclamation projects on the offshore groundwater system, elucidating the interactions between these two groundwater systems. By leveraging a comprehensive geological database and paleo-hydrogeological modeling, the analysis revealed that the OFG is widely distributed throughout the Pearl River subaqueous paleo-delta. The static volume of the OFG reserves is estimated to be 575.6 ± 44.9 km3, with the offshore extent extending more than 180 km from the coastline. The OFG within the subaqueous paleo-delta formation originated during periods of low sea level since the late Pleistocene, with the groundwater ages ranging between 20-40 ka. OFG exhibits depleted stable isotopes and major ions relative to OFG-free sediment and its persistence time is over millennia. Furthermore, the findings from Hong Kong reclaimed airport indicate that nearshore land reclamation activities will impact the connectivity of the submarine aquifer system, thereby altering the distribution and characteristics of the OFG. In summary, the knowledge gained from this study can be applied not only to the Pearl River Estuary but also to other similar offshore regions, fostering a broader understanding and facilitating practical applications in the context of large-river deltaic estuaries worldwide.