Spatial distribution and export of nutrients and metal elements in the subterranean estuary of Daya Bay

Subterranean estuaries (STE) are important seawater-groundwater mixing zones with complex biogeochemical processes, which play a vital role in the migration and transformation of dissolved materials. In this study, we first investigated the spatial distributions of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorous (DIP), dissolved inorganic silicon (DSi) and metal elements (As, Ba, Cr, Cu, Fe, Mn, Ni, Pb, and Zn) in STE including upper intertidal, seepage face and subtidal zones. We then estimated submarine groundwater discharge (SGD) and associated nutrient and metal element fluxes. From the generalized Darcy’s law method, SGD was estimated to be 30.13 cm/d, which was about 7 times larger than the inflow (4.16 cm/d). The nutrient and metal fluxes from SGD were estimated to be (5.33 ± 4.99) mmol/(m 2 ·d) for DIN, (0.22 ± 0.03) mmol/(m 2 ·d) for DIP, (16.20 ± 2.05) mmol/(m 2 ·d) for DSi, (1 325.06 ± 99.10) μmol/(m 2 ·d) for Fe, (143.41 ± 25.13) μmol/(m 2 ·d) for Mn, (304.06 ± 81.07) μmol/(m 2 ·d) for Zn, (140.21 ± 13.33) μmol/(m 2 ·d) for Cu, (84.49 ± 2.94) μmol/(m 2 ·d) for Pb, (37.38 ± 5.51) μmol/(m 2 ·d) for Ba, (27.88 ± 3.89) μmol/(m 2 ·d) for Cr, (10.10 ± 6.33) μmol/(m 2 ·d) for Ni, and (6.25 ± 3.45) μmol/(m 2 ·d) for As. The nutrient and metal fluxes from SGD were relatively higher than those from the inflow, suggesting that nearshore groundwater acted as the sources of nutrients and metal elements discharging into the sea. The environmental potential pollution of coastal seawater was evaluated by pollution factor index ( P i ), comprehensive water quality index (CWQI), and ecological risk index (ERI). Pb mainly caused potential danger of nearshore environment with considerable contamination ( P i = 5.78 ± 0.19), heavy pollution (CWQI = 4.09) and high ecological risk (ERI = 18.00). This study contributed to better understanding the behavior of nutrients and metal elements and improving the sustainable management of STE under the pressure of anthropogenic activities and climate change.