RHENIUM, MOLYBDENUM, AND URANIUM IN GROUNDWATER FROM THE SOUTHERN GREAT-BASIN, USA - EVIDENCE FOR CONSERVATIVE BEHAVIOR

Dissolved Re, Mo, and U concentrations, and the concentrations of the major cations and anions, were measured in groundwaters from twenty-th ree springs in the southern Great Basin, USA, from June 1992 through M arch 1994. Rhenium concentrations ranged from 5 +/- 1 pmol/kg to 190 /- 20 pmol/kg. The highest Re values were observed in the saline groun dwater of Saratoga Spring in Death Valley and Cold Spring in Ash Meado ws, whereas the lowest Re concentrations were found in Nevares Spring in Death Valley. The mean Re concentration for all of the spring water s sampled is 44 +/- 33 pmol/kg. The concentrations of Re differs drama tically for groundwaters that discharge from the regional Paleozoic ca rbonate 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 valu e reported for seawater, i.e., 39.8 +/- 0.2 pmol/kg), whereas for the spring waters that discharge from the felsic volcanic aquifers, the me an Re concentration is 18 +/- 3.5 pmol/kg. The overall mean concentrat ions of U and Mo in these groundwaters are 18 +/- 12 nmol/kg and 120 /- 60 nmol/kg, respectively. Differences in the U and Mo concentration s for groundwaters from the carbonate aquifer and those from the volca nic aquifer are also observed such that the mean U and Mo concentratio ns in the carbonate groundwaters are 13 +/- 5 nmol/kg and 106 +/- 40 n mol/kg, respectively, and the mean U and Mo concentrations in the volc anic spring waters are 31.5 +/- 4 nmol/kg and 94 +/- 12 nmol/kg, respe ctively. By comparison, U and Mo concentrations reported for seawater are 13.9 nmol/kg and 107 nmol/kg, respectively. In the oxygenated grou ndwaters of the Great Basin, Re, Mo, and U appear to be acting conserv atively, as in seawater, as evidenced by the covariance observed betwe en Re, Mo, and U with chloride, sodium, and sulfate. Groundwaters disc harging from the regional carbonate aquifer in Ash Meadows typically e xhibit a U/Re ratio of about 300 while groundwaters that discharge in Death Valley from local felsic volcanic sources have ratios of about 1 700. Waters in the remainder of the Death Valley springs are intermedi ate and may result from the mixing of the carbonate and volcanic deriv ed waters. The Re, Mo, and U concentrations in addition to the major s olute chemistry, indicates that dissolution of carbonate rocks is of p rimary importance in the groundwaters from Ash Meadows, whereas the we athering of felsic volcanic rocks contributes substantially to the che mical signatures of the Death Valley groundwaters. The differences bet ween U/Re ratios and different aquifer materials suggest that there is a relationship between the dissolved U/Re ratios and the aquifer mate rials. 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 di ssolution of the carbonate rocks by groundwater. The substantially lar ger salinity normalized Re, Mo, and U concentrations in the groundwate rs compared to seawater, rule out the possibility that these groundwat ers represent simple mixing of dilute terrestrial waters and seawaters or reconstitution of evaporite deposits by groundwater dissolution.