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

Rare earth element (REE) concentrations were measured in 5 well water sampl es and 3 springs located along a groundwater flow path in a shallow, tuffac eous alluvial aquifer from southern Nevada, USA. The REE concentrations in these groundwaters decrease in the direction of groundwater flow. A previou s investigation demonstrated that REE solid-liquid phase partitioning coeff icients (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 aqueo us REEs to aquifer surface sites as the primary removal mechanism of REEs f rom these groundwaters. In addition, relatively high aqueous REE concentrat ions occur at distinct locations along the groundwater flow path. The eleva ted REE concentrations are explained by addition of deeper groundwaters, in flux of geothermal waters from a hot spring system, differences in solution complexation, and/or mixtures of regional and local recharge sources. Solu tion complexation modelling of REEs in the groundwaters indicate that carbo nate 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 (HCO3- + CO32-; r = 0.71). The data and speciation model re sults indicate that REE fractionation (i.e., the observed heavy REE, HREE, enrichments compared to rock-sources) is controlled by formation of progres sively stronger carbonate complexes in solution with increasing atomic numb er, which inhibits HREE sorption compared to light REEs (LREE); and a great er affinity for the LREEs to sorb to surface sites in the local tuffaceous alluvial aquifers compared to the HREEs. Copyright (C) 1999 Elsevier Scienc e Ltd.