752 / 2024-09-19 11:17:51

Spatiotemporal dynamics of groundwater metal elements and DOC in response to natural and human intervention in the Yellow River wetland, China

redox dynamics, groundwater, trace metals, DOC, sediment-water regulation, Yellow River wetland

The Yellow River wetland (YRW) is one of the most well-studied and important coastal wetlands in China. Our current understanding of groundwater dynamics in the unconfined aquifer system remains an understudied, yet fundamental knowledge gap for explaining hydrogeochemical budgets in response to annual dam releases targeted to mitigate human interventions including; sediment starvation, saltwater intrusion, and land subsidence. A sediment-water regulation scheme (SWRS) for the Yellow River has been in place since 2002, resulting in drastic coastline landward erosion at the old river channel and seaward expansion at the new river channel. Previous studies have investigated how the SWRS impacts variations in biogeochemical cycles over parts of the YRW. However, due to difficulty in groundwater sampling at high vertical resolution, it is unclear how the spatiotemporal dynamics over groundwater redox conditions and metal element geochemistry (e.g., metal element speciation, transformation, and tranfer) across the YRW may response to the SWRS. In this study, we present 3.5 m-long sediment cores collected at three geomorphic land types (riparian wetlands, salt marshes, and tidal flats near the old river channel) before, during and after the SWRS event in 2024 that allow measurements of concentrations of groundwater major and minor element and DOC, and corresponding solid phase element content in high-vertical resolution. Our preliminary data show concurrent variations in porewater Fe(II) and DOC concentrations, and solid phase Fe/Al and Mn/Al ratios. Multiple iron redox zones are identified at the marsh and tidal flat sites far from the river. Flooding during the SWRS significantly impacts groundwater redox biogeochemistry, showing weaker and more localized Fe reduction over a broader depth range at the tidal flat site. The localized Fe(II) variation depths at this site are accompanied by changes in solid phase Fe/Al, Mn/Al, Co/Al, Rh/Al, Fe/Ti, and magnetic susceptibility, implying an important control of bedrock lithology on groundwater geochemistry. These results highlighted that the Yellow River SWRS program considerably impacts the groundwater redox zonation and metal speciation during sediment-groundwater interaction at the YRW. Findings in our study will contribute to a better understanding of the evolution and hydrogeochemical budgets of the YRW coastal ecosystems in response to annual dam releases, and a better estimate of trace metal budgets at land-ocean boundaries.