Submarine groundwater discharge as a main source of rare earth elements in coastal waters

To evaluate the role of submarine groundwater discharge (SGD) as a source of rare earth elements (REEs) in the coastal ocean, we estimated the SGD associated discharge of REEs into two semi-enclosed coastal bays in the southern coast of Korean peninsula. The mass balances of REEs demonstrated that the REE fluxes through SGD were two to three orders of magnitude higher than those through the other sources, such as diffusion from bottom sediments and atmospheric dust fallout. The Nd inputs through the total SGD from the two small coastal bays, Gamak Bay (148 km2) and Hampyeong Bay (85 km2), were estimated to be 0.7 × 104–1.3 × 104 mol y− 1, which is 0.06–0.3% of the total Nd fluxes from global rivers. In this region, coastal seawater was observed to have a substantially higher middle REE (MREE), which appears to be due to a large discharge of groundwater that is highly enriched with MREE. High MREE bearing waters were observed over the entire shore of the southern coast of Korea, implying that such high SGD-driven REE fluxes are common in coastal area. Our results suggest that the SGD-driven REE fluxes may contribute considerably to the global budget of REEs in the ocean.