Rare earth elements as geochemical tracers of regional groundwater mixing

The rare earth elements (REE) were analyzed in a groundwater system from south-central Nevada (i.e., Ash Meadows National Wildlife Refuge, the Spring Mountains: Pahranagat National Wildlife Refuge, and the Nevada Test Site) in order to investigate their potential use as tracers of regional groundwater flow. Previous investigations using conservative tracers (e.g., deuterium and uranium isotopes) identified recharge in local mountains as the primary source (60-70% of total discharge) for the springs in the regional discharge zone (i.e., Ash Meadows) with the remaining contribution being interbasin flow from the northeast. Initial mixing calculations for these groundwaters using shale-normalized REE patterns agreed well with the previous studies; however, because the REEs are not expected to behave conservatively in natural waters, the effect of both solution complexation, which acts to enhance the stability of the REEs in solution, as well as surface complexation, responsible for the particle reactive behavior of the REEs, were examined in subsequent mixing calculations. In order to assess the roles of solution and surface complexation, relative partitioning coefficients were estimated for each REE in each groundwater by evaluating the ratio of the ionic strength corrected co(3) beta(1)REE, co(3) beta(2)REE, and [CO32-](F) to the first hydrolysis binding constants for the REEs. The relative partitioning coefficients were then used to calculate REE patterns expected to develop and persist in solution as a consequence of solution and surface complexation. The calculated REE values closely resembled the actual measured REE concentrations, suggesting that the REEs are, in fact, controlled by solution and surface complexation in these groundwaters. The calculated REE concentrations were subsequently used to determine mixing ratios, the results of which coincided with the initial calculations as well as the previous studies. The results of this study suggest that solution complexation of the REEs is sufficient to overcome. to a certain degree, the affinity of:he REEs to be adsorbed onto surface sites in the aquifers such that distinctive REE signatures develop and persist in solution in groundwaters from different aquifers. The ability of solution complexation to overcome surface complexation is likely related to the formation of the negatively charged dicarbonato complex [i.e., Ln(CO3)(2)(-), where Ln is any REE], which accounts for significant fractions of each REE in these groundwaters. Copyright (C) 1997 Elsevier Science Ltd.