RADIONUCLIDES IN GROUNDWATER OF JEFFERSON COUNTY AND SURROUNDING AREAS, SOUTHWESTERN MONTANA, 2007 THROUGH 2010

The U.S. Geological Survey collected groundwater samples from 128 domestic wells from 6 generalized geologic units in southwestern Montana from 2007 through 2010 to evaluate the occurrence of naturally-occurring radioactive constituents and to identify geologic settings and environmental conditions in which elevated concentrations occur. Samples were analyzed within a few days after collection; therefore, data closely represent the quality of water consumed by area residents. Initial water-quality analyses included uranium, radon, gross alpha-particle activity, and gross beta-particle activity. Wells with elevated concentrations were sampled a second time and analyzed for additional constituents including polonium-210 and isotopes of radium and uranium.

Nearly 41 percent of the sampled wells from five of the six geologic units assessed had at least one radioactive constituent that exceeded U.S. Environmental Protection Agency drinking-water standards or screening levels. Water from the Late Cretaceous Boulder batholith and other similar intrusive rocks exceeded standards or levels most frequently (38 of the 62 wells). Uranium and radon concentrations exceeded established or proposed standards in 14 and 27 percent of the wells, respectively. Adjusted gross alpha-particle activity and combined radium (radium-226 and radium-228) both exceeded standards in about 24 percent of the samples. Gross alpha-particle activities may be a potential indicator for elevated radioactive constituents in the area as all radioactive constituents whose concentrations exceeded drinking-water standards or screening levels had gross alpha-particle activities of 7 pCi/L or more.

Nonparametric Spearman correlation coefficients (rho) were computed to measure the strength of the relation among various constituents and field parameters and some relations were statistically significant (p-value < 0.05). Gross beta-particle activity was moderately correlated (rho = 0.72 to 0.82) with potassium, most likely because one potassium isotope (potassium-40) is a beta-particle emitter. Three radium isotopes were moderately to strongly correlated (rho = 0.78 to 0.92) to one another. Gross alpha-particle activities were moderately to strongly correlated (rho = 0.77 to 0.91) with gross beta-particle activities.