Submarine groundwater discharge of rare earth elements to a tidally-mixed estuary in Southern Rhode Island

- Weathering of apatite is the most likely source of the MREE-enrichment in groundwater.
- Precipitation of secondary LREE-phosphates is responsible for HREE-enriched surface water.
- The Nd SGD flux is roughly equivalent to the local river flux of Nd.

Rare earth element (REE) concentrations were analyzed in surface water and submarine groundwater within the Pettaquamscutt Estuary, located on the western edge of Narragansett Bay in Rhode Island. These water samples were collected along the salinity gradient of the estuary. Rare earth element concentrations in the majority of the groundwater samples are substantially higher than their concentrations in the surface waters. In particular, Nd concentrations in groundwater range from 0.43 nmol kg− 1 up to 198 nmol kg− 1 (mean ± SD = 42.1 ± 87.2 nmol kg− 1), whereas Nd concentrations range between 259 pmol kg− 1 and 649 pmol kg− 1 (mean ± SD = 421 ± 149 pmol kg− 1) in surface waters from the estuary, which is, on average, 100 fold lower than Nd in the groundwaters. Groundwater samples all exhibit broadly similar middle REE (MREE) enriched shale-normalized REE patterns, despite the wide variation in pH of these natural waters (4.87 ≤ pH ≤ 8.13). The similarity of the shale-normalized REE patterns across the observed pH range suggests that weathering of accessory minerals, such as apatite, and/or precipitation of LREE enriched secondary phosphate minerals controls groundwater REE concentrations and fractionation patterns. More specifically, geochemical mixing models suggest that the REE fractionation patterns of the surface waters may be controlled by REE phosphate mineral precipitation during the mixing of groundwater and stream water with incoming water from the Rhode Island Sound. The estimated SGD (Submarine Groundwater Discharge) of Nd to the Pettaquamscutt Estuary is 26 ± 11 mmol Nd day− 1, which is in reasonable agreement with the Nd flux of the primary surface water source to the estuary, the Gilbert Stuart Stream (i.e., 36 mmol day− 1), and of the same order of magnitude for a site in Florida.