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 Ra-224 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 R-224/Ra-223 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) x 10(8) m(3) d(-1) (0.23-0.50 m(3) m(-2) d(-1)) and (1.2-2.7) x 10(8) m(3) d(-1) (0.06-0.14 m(3) m(-2) d(-1)) in the dry season and wet season, respectively, and the estimated SGD flux was (4.6-11) x 10(9) m(3) d(-1) (0.18-0.45 m(3) 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. (C) 2018 Elsevier Ltd. All rights reserved.