Groundwater contributions to the carbon budget of the Greater Bay Area, China

Total organic carbon (TOC) in surface water significantly impacts the global carbon cycle, ecosystem productivity, and potable water quality. Although physically based watershed models, such as the Terrestrial-Aquatic Sciences Convergence (TASC) model, can simulate carbon cycling in surface waters, challenges persist in representing groundwater contributions due to limitations in TASC's groundwater module. Additionally, like its foundation, the Soil and Water Assessment Tool (SWAT), the TASC model struggles to accurately represent distributary stream systems. In this study, we modified the TASC model to examine the carbon budget of the Guangdong-Hong Kong-Macao Greater Bay Area (GBA), a densely populated coastal delta of the Pearl River. The modified model successfully simulated the carbon dynamics of the region’s distributary stream system, estimating a dissolved organic carbon (DOC) flux of 1.05 × 10⁹ g/day across eight watershed outlets. TOC in the western outlets was primarily influenced by suspended sediments, while TOC in the eastern outlets originated mainly from agricultural runoff and domestic sewage. Furthermore, seasonal variations revealed important patterns in hydrological contributions. Groundwater flow contributed significantly to river discharge during the winter months (November to January), occasionally surpassing overland runoff. Remarkably, groundwater DOC fluxes dominated riverine DOC throughout the year, accounting for 41% to 62% of the total DOC contribution. This study highlights groundwater as a vital pathway for the transport and release of dissolved carbon into rivers, representing a potentially significant carbon loss within watersheds.