Multielement chemical and statistical analyses from a uranium hydrogeochemical and stream-sediment survey in and near the Elkhorn Mountains, Jefferson County, Montana. Part I: Surface water

Abstract

Fifty-two surface-water samples, collected from an area south of Helena, Jefferson County, were analyzed for 51 chemical species. Of these variables, 35 showed detectable variation over the area, and 29 were utilized in a correlation analysis. Two populations are distinguished in the collected samples and are especially evident in the plot of Ca versus U. Samples separated on the basis of U versus Ca proved to represent drainage areas of two differing lithologies. One group was from waters that drain the Boulder batholith, the other from those that drain the Elkhorn Mountains volcanic rocks. These two groups of samples, in general, proved to have parallel but different linear trends between U and other elements. Therefore, the two groups of samples were treated separately in the statistical analyses. Over the area that drains the Boulder batholith, U concentrations in water ranged from 0.37 to 13.0 μ g/l , with a mean of 1.9 μ g/l. The samples from streams draining volcanic areas ranged from 0.04 to 1.5 μ g/l, with a mean of 0.42 μ g/l. The highest U values (12 and 13 μ g/l) occur along Badger Creek, Rawhide Creek, Little Buffalo Gulch, and an unnamed tributary to Clancy Creek. Conductivity, hardness, Ba, Ca, CI, K, Mg, Na and Sr are significantly correlated with U at or better than the 95 percent confidence limit in both populations. For water draining the Boulder batholith, uranium correlates significantly with akalinity, pH, bicarbonate, Li, Mo, NO2+NO3, P04, SiO2, SO4, F, and inorganic carbon. These correlations are similar to those found in a previous study of water samples in north-central New Mexico (Wenrich-Verbeek, 1977b). Uranium in water from the volcanic terrane does not show correlations with any of the above constituents, but does correlate well with V. This relationship with V is absent within the Boulder batholith samples.