Nutrient input through submarine groundwater discharge in two major Chinese estuaries: the Pearl River Estuary and the Changjiang River Estuary

In this study, we used a ²²⁴Ra mass balance model to evaluate the importance of submarine groundwater discharge (SGD) for the budgets of biogenic elements in two major Chinese estuaries: the Pearl River Estuary (PRE) and the Changjiang River Estuary (CRE). The apparent water age in the PRE was estimated to be 4.8 ± 1.1 days in the dry season and 1.8 ± 0.6 days in the wet season using a physical model based on the tidal prism. In the dry season, the water age in the CRE was estimated to be 11.7 ± 3.0 days using the ²²⁴Ra/²²³Ra activities ratios apparent age model. By applying the ²²⁴Ra mass balance model, we obtained calculations of the SGD flow in the PRE of (4.5–10) × 10⁸ m³ d⁻¹ (0.23–0.50 m³ m⁻² d⁻¹) and (1.2–2.7) × 10⁸ m³ d⁻¹ (0.06–0.14 m³ m⁻² d⁻¹) in the dry season and wet season, respectively, and the estimated SGD flux was (4.6–11) × 10⁹ m³ d⁻¹ (0.18–0.45 m³ m⁻² d⁻¹) in the dry season of the CRE. In comparison with the nutrient fluxes from the rivers, the SGD-derived nutrient fluxes may play a vital role in controlling the nutrient budgets and stoichiometry in the study areas. The large amount of dissolved inorganic nitrogen and phosphorus fluxes together with high N: P ratios into the PRE and CRE would potentially contribute to eutrophication and the occurrence of red tides along the adjacent waters.