Rare earth element fractionation and concentration variations along a groundwater flow path within a shallow, basin-fill aquifer, southern Nevada, USA

摘要

Rare earth element (REE) concentrations were measured in 5 well water samples and 3 springs located along a groundwater flow path in a shallow, tuffaceous alluvial aquifer from southern Nevada, USA. The REE concentrations in these groundwaters decrease in the direction of groundwater flow. A previous investigation demonstrated that REE solid-liquid phase partitioning coefficients (i.e., K_d's) for groundwaters from tuffaceous alluvial aquifers in southern Nevada are relatively high (mean K_d=10~(2.6)). Our groundwater REE data, in conjunction with these K_d's, support strong sorption of aqueous REEs to aquifer surface sites as the primary removal mechanism of REEs from these groundwaters. In addition, relatively high aqueous REE concentrations occur at distinct locations along the groundwater flow path. The elevated REE concentrations are explained by addition of deeper groundwaters, influx of geothermal waters from a hot spring system, differences in solution complexation, and/or mixtures of regional and local; recharge sources. Solution complexation modelling of REEs in the groundwaters indicate that carbonate complexes account for more than 99% of each REEs in solution. Moreover, groundwater Yb/Nd ratios (a measure of REE fractionation) are associated with alkalinity (HCO_3~-+CO_3~(2-); r=0.71). The data and speciation model results indicate that REE fractionation (i.e., the observed heavy REE, HREE, enrichments compared to rock-sources) is controlled by formation of progressively stronger carbonate complexes in solution with increasing atomic number, which inhibits HREE sorption compared to light REEs (LREE); and a greater affinity for the LREEs to sorb to surface sites in the local tuffaceous alluvial aquifers compared to the HREEs.