Submarine groundwater discharge is an important source of REEs to the coastal ocean

Rare earth element (REE) concentrations of submarine groundwater discharge (SGD) were measured in three subterranean estuaries (i.e., Indian River Lagoon, Florida; Pettaquamscutt estuary, Rhode Island; Kona Coast, Hawaii). Using site-specific SGD estimates previously obtained by a variety of techniques (e.g., seepage meters, Ra, and Rn), we estimated SGD-derived fluxes of REEs to the coastal ocean using simple, one-dimensional modeling techniques. Our results indicate that the SGD fluxes of REEs are either of the same magnitude as riverine REE fluxes (Indian River Lagoon; Pettaquamscutt estuary), or far exceed surface runoff sources of REEs to the coastal ocean (Kona Coast). At each site important biogeochemical reactions occurring in the subterranean estuary, such as redox reactions, sediment bioirrigation, mineral dissolution and re-precipitation, and salt-induced mobilization from “nano-colloids”, appear to facilitate release of REEs into solution, which are then advected to the coastal ocean via SGD. Neodymium isotope analysis of SGD and aquifer sediment are consistent with sediment diagenesis and redox transformations of Fe(III) oxides/oxyhydroxides, as well as preferential weathering of REE-bearing minerals like apatite, as being important sources of REEs to coastal seawater. Our investigations demonstrate that geochemical reactions occurring in the studied subterranean estuaries represent a net source of light and middle REEs to coastal seawater, whereas the heavy REEs appear to be sequestered in the subterranean estuary sediment.