RARE-EARTH ELEMENTS AS GEOCHEMICAL TRACERS OF REGIONAL GROUNDWATER MIXING

The rare earth elements (REE) were analyzed in a groundwater system fr om south-central Nevada (i.e., Ash Meadows National Wildlife Refuge, t he Spring Mountains: Pahranagat National Wildlife Refuge, and the Neva da Test Site) in order to investigate their potential use as tracers o f regional groundwater flow. Previous investigations using conservativ e tracers (e.g., deuterium and uranium isotopes) identified recharge i n local mountains as the primary source (60-70% of total discharge) fo r the springs in the regional discharge zone (i.e., Ash Meadows) with the remaining contribution being interbasin flow from the northeast. I nitial mixing calculations for these groundwaters using shale-normaliz ed REE patterns agreed well with the previous studies; however, becaus e 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, re sponsible for the particle reactive behavior of the REEs, were examine d in subsequent mixing calculations. In order to assess the roles of s olution and surface complexation, relative partitioning coefficients w ere 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, a nd [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 close ly resembled the actual measured REE concentrations, suggesting that t he REEs are, in fact, controlled by solution and surface complexation in these groundwaters. The calculated REE concentrations were subseque ntly used to determine mixing ratios, the results of which coincided w ith the initial calculations as well as the previous studies. The resu lts of this study suggest that solution complexation of the REEs is su fficient to overcome. to a certain degree, the affinity of:he REEs to be adsorbed onto surface sites in the aquifers such that distinctive R EE signatures develop and persist in solution in groundwaters from dif ferent aquifers. The ability of solution complexation to overcome surf ace 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 ground waters. Copyright (C) 1997 Elsevier Science Ltd.