Rhenium, molybdenum, and uranium in groundwater from the southern Great Basin, USA: Evidence for conservative behavior

Abstract Dissolved Re, Mo, and U concentrations, and the concentrations of the major cations and anions, were measured in groundwaters from twenty-three springs in the southern Great Basin, USA, from June 1992 through March 1994. Rhenium concentrations ranged from 5 ± 1 pmol/kg to 190 ± 20 pmol/kg. The highest Re values were observed in the saline groundwater of Saratoga Spring in Death Valley and Cold Spring in Ash Meadows, whereas the lowest Re concentrations were found in Nevares Spring in Death Valley. The mean Re concentration for all of the spring waters sampled is 44 ± 33 pmol/kg. The concentrations of Re differs dramatically for groundwaters that discharge from the regional Paleozoic carbonate aquifer and for local groundwaters from felsic volcanic rocks. The mean Re concentration for groundwaters issuing from the carbonate aquifer is 37 ± 14 pmol/kg (which is similar to the average Re value reported for seawater, i.e., 39.8 ± 0.2 pmol/kg), whereas for the spring waters that discharge from the felsic volcanic aquifers, the mean Re concentration is 18 ± 3.5 pmol/kg. The overall mean concentrations of U and Mo in these groundwaters are 18 ± 12 nmol/kg and 120 ± 60 nmol/kg, respectively. Differences in the U and Mo concentrations for groundwaters from the carbonate aquifer and those from the volcanic aquifer are also observed such that the mean U and Mo concentrations in the carbonate groundwaters are 13 ± 5 nmol/kg and 106 ± 40 nmol/kg, respectively, and the mean U and Mo concentrations in the volcanic spring waters are 31.5 ± 4 nmol/kg and 94 ± 12 nmol/kg, respectively. By comparison, U and Mo concentrations reported for seawater are 13.9 nmol/kg and 107 nmol/kg, respectively. In the oxygenated groundwaters of the Great Basin, Re, Mo, and U appear to be acting conservatively, as in seawater, as evidenced by the covariance observed between Re, Mo, and U with chloride, sodium, and sulfate. Groundwaters discharging from the regional carbonate aquifer in Ash Meadows typically exhibit a U Re ratio of about 300 while groundwaters that discharge in Death Valley from local felsic volcanic sources have ratios of about 1700. Waters in the remainder of the Death Valley springs are intermediate and may result from the mixing of the carbonate and volcanic derived waters. The Re, Mo, and U concentrations in addition to the major solute chemistry, indicates that dissolution of carbonate rocks is of primary importance in the groundwaters from Ash Meadows, whereas the weathering of felsic volcanic rocks contributes substantially to the chemical signatures of the Death Valley groundwaters. The differences between U Re ratios and different aquifer materials suggest that there is a relationship between the dissolved U Re ratios and the aquifer materials. Similar Re/Mo/U ratios for the groundwaters from the carbonate aquifer and seawater are strongly suggestive of quantitative uptake of these trace elements from seawater by carbonate mineral precipitation and subsequent quantitative release of these trace elements during dissolution of the carbonate rocks by groundwater. The substantially larger salinity normalized Re, Mo, and U concentrations in the groundwaters compared to seawater, rule out the possibility that these groundwaters represent simple mixing of dilute terrestrial waters and seawaters or reconstitution of evaporite deposits by groundwater dissolution.