Metals in coastal groundwater systems under anthropogenic pressure: a synthesis of behavior, drivers, and emerging threats

Submarine groundwater discharge (SGD) dynamically links land- and ocean-derived chemical constituents, such as metals, in the coastal ocean. While many metals are sediment-bound, changing environmental conditions, particularly along the coast, may lead to increased release of metals to their dissolved and more bioavailable form. Here, we review metal behavior, speciation, and drivers of mobilization in the coastal environment under anthropogenic influence. We also model global metal contamination risk to the coastal ocean via SGD considering anthropogenic and hydrogeologic pressures, where tropical regions with high population density, SGD, and acid sulfate soils (4% of the global coast) present the highest risk. Although most SGD studies focus on other analytes, such as nutrients, this review demonstrates the importance of considering SGD as a critical pathway for metals to reach the coastal ocean under rapidly changing environmental conditions.