The Mobility of Rare Earth Elements and Redox Sensitive Elements in the Groundwater/Seawater Mixing Zone of a Shallow Coastal Aquifer

The concentrations of Rare Earth Elements (REE) and Redox Sensitive Elements (RSE) were measured in groundwaters along a transect of the forest-marsh interface of a surficial aquifer system in North Inlet, SC. The well transect extended from a forest recharge area across the marsh and tidal creek to a tidal recharge area of beach ridge. The concentrations of the RSE (Fe, Mn, and U) were consistent with reducing conditions through the transect. Fe was present at concentrations ranging from a few micromolar to greater than one hundred micromolar in most wells. U was depleted with respect to salinity predicted concentrations, indicating removal with respect to the seawater endmember. Dissolved Mn concentrations were generally low in all wells, indicating no significant solid source of Mn (as MnOx) in this system. When extrapolated to a global scale, estimates of U removal during seawater exchange with the aquifer solids equaled 10–20% of the total riverine dissolved U input flux. REE concentrations in the forest recharge area were high in shallow wells, and showed a light enriched fractionation pattern, characteristic of soil leaching by Natural Organic Matter (NOM) rich waters. A decrease in REE concentration with depth in the forest wells coupled with a trend towards Heavy REE (HREE) enriched fractionation pattern indicated removal of the REE coincident with NOM and Dissolved Organic Carbon (DOC) removal. The saline waters of the beach ridge wells show a Light REE (LREE) enriched fractionation pattern and have the highest overall concentrations of the REE, indicating a significant REE source to the seawater endmember waters. The concentration gradients along the beach ridge flow path indicate a large source in the deep wells, and net export of dissolved REE to the tidal creek system and the coastal ocean. Ultrafiltration experiments indicate a transition from a colloidal dominated reservoir for the REE in the forest wells to a colloidal and dissolved reservoir in the beach ridge wells. The ultrafiltration data coupled with a correlation with Dissolved Inorganic Carbon (DIC) release suggest that there is diagenetic mobilization of an REE rich organic carbon phase in the saline endmember wells. We suggest here that degradation of this relic terrestrial organic carbon and REE rich phase results in the export of dissolved REE equal to or exceeding river inputs in this region.