Occurrence of CO2 and CH4 and the behavior of inorganic carbon in the groundwater of the Pearl River Delta

Large river delta-front estuaries are critical contributors to greenhouse gases (GHGs) emissions and serve as substantial carbon reservoirs. However, the presence and origins of CO2 and CH4, as well as the sources of dissolved inorganic carbon (DIC) in groundwater within deltaic systems, remain insufficiently understood. This gap stems from the limited exploration of biogeochemical processes in groundwater and the impact of sedimentary environment shaped by Holocene transgression and regression. In this study, we examined the spatial variations of GHGs, DIC, and δ13C-DIC in groundwater within the deltaic aquifer-aquitard system of the Pearl River Delta. By integrating hydrogeological and hydrogeochemical data, we evaluated the behaviors of GHGs in groundwater and proposed hypotheses regarding the evolutions of GHGs and DIC. Our findings revealed that paleo saltwater intrusion, reflected in groundwater salinity, significantly influenced CO2 generation and methanogenesis. Elevated CH4 levels in the Holocene and Pleistocene aquitards highlighted the importance of marine sediments, while groundwater CO2 mainly resulted from biogeochemical processes. DIC in the Holocene aquitard primarily originated from marine deposited sediments, whereas biogenic indicators dominated DIC sources in older layers due to active biogeochemical environments. This study provides a novel assessment of the impact of marine-deposited environments and biogeochemical processes on groundwater CO2, CH4, and DIC dynamics in the Pearl River Delta. These findings are crucial for understanding the GHG sources and sinks in deltaic aquifer-aquitard systems and for improving GHG predictions. © 2024 Elsevier B.V.