Groundwater-surface water exchange associated metals at two intertidal transects, Dan'ao Estuary, Daya Bay, China

In the evaluation of aquatic environments in estuarine waters, the groundwater-surface water exchanges and associated metal fluxes are difficult to quantify and often ignored. This study made such an attempt based on field data at two intertidal transects in the estuarine wetland of Dan'ao River, the largest river flowing into Daya Bay, China. The results of groundwater-surface water exchange associated six common metals (Cu, Zn, As, Cd, Pb, and Cr) indicate the following: the cumulative concentrations of the six metals are ranked as seawater > groundwater > river water. Among all estuarine groundwaters, the sum of metal concentrations in rhizospheric groundwater at the upstream transect is the highest, owing to the enriching effects of mangrove rhizospheres on metals. The net fluxes of Cu, Zn, As, Cd, and Cr at the upstream transect are 4.8-13.8, 11.9-16.7, 147.8-190.2, 4.1-4.9, and 10.0-12.6 times higher than those at the downstream one. The much higher groundwater discharge rates at the upstream mangrove transect plays a vital role in increasing metal fluxes from groundwater to surface water in this estuary.