RARE EARTH ELEMENT (REE) BEHAVIOR DURING MIXING IN SUBTERRANEAN ESTUARIES ALONG THE KONA COAST OF HAWAII

Submarine groundwater discharge (SGD) is the primary flux of terrestrial waters to the ocean along the arid Kona Coast of Hawaii. We collected groundwaters from wells along the Kona Coast to investigate REE behavior in local subterranean estuaries. Groundwater and coastal seawater samples were sequentially filtered through 0.45 or 0.02 μm nominal pore-size filters to study the colloidal and more truly dissolved fractions of the REEs in these waters. Moreover, laboratory experiments using a groundwater sample collected immediately down gradient from a wastewater treatment plant, and another sample from a more “pristine” setting, were mixed with local seawater. The mixtures were also filtered through 0.45 or 0.02 μm filters to examine the potential role of colloids on REEs in the mixing experiments. Dissolved concentrations were nearly identical between the differently filtered aliquots, indicating that REEs are either transported as ionic species in solution or associated with colloids that are smaller than 0.02 μm (i.e., < 20 kDa). Because REEs were released to solution at low salinity (i.e., < 15 psu) during the mixing experiments, we suggest that REEs are associated with very fine colloids in Kona groundwaters, and are subsequently released to solution in the subterranean estuary via complexation and ionic exchange reactions. Numerical simulations are consistent with the notion that increases in the proportion of each REE complexed with carbonate ions (LnCO₃⁺ + Ln(CO₃)₂^-) that accompanies increasing salinity and pH in the subterranean estuary, lowers the activity of the free metal ion, Ln³⁺, which promotes the release of REEs to solution. In contrast, anthropogenic Gd exhibits conservative behavior across the salinity gradient of the subterranean estuary, suggesting that Gd anomalies may provide a useful tracer for investigating anthropogenic inputs to the coastal ocean. Kona Coast seawater is enriched by 7- to 29-fold in the REEs over open-ocean North Pacific seawater, and exhibits shale-normalized fractionation patterns that are identical to local groundwaters, suggesting that SGD is the chief source of REEs to these coastal waters.