Controls on alkalinity biogeochemistry in a coastal aquifer-aquitard system

Coastal carbon cycle and budgets are significant drivers of regional climate change, generating widespread global attention. This study explored the spatial and seasonal variability of alkalinity biogeochemistry within a coastal aquifer-aquitard system in the Pearl River Delta, China. We measured physicochemical parameters including salinity, temperature, and pH, total alkalinity (TA), stable isotopes, cations, and anions of groundwater samples, which were collected every season using permanent multilevel groundwater sampling systems installed at three field sites of PRD. Results revealed that the elevated production of total alkalinity and dissolved inorganic carbon in the deltaic aquifer-aquitard system stemmed from sedimentary organic matter due to the presence of the aquitard formed during the Holocene marine transgressive event. Cluster analysis, incorporating various components of inorganic carbon and physical-chemical features, classified sources of TA in groundwater samples into four categories: modern weathering dominated, Holocene transgression dominated, late Pleistocene weathering dominated, and early Pleistocene weathering dominated. The study suggests that Holocene marine sediments act as dynamic biogeochemical reactors, supplying organic matters and influencing carbon cycles amidst complex hydrogeological and biogeochemical conditions. • Marine deposited aquitard increased production of total alkalinity. • Denitrification mainly controlled biogeochemical processes in freshwater. • Sulfate reduction dominated the source of dissolved inorganic carbon in saltwater.