Assessment of selected ground-water-quality data in Montana

Ground-water-quality data for Montana in the U.S. Geological Survey 's computer data file WATSTORE were evaluated for nine geohydrologic units in part of the State east of the Rocky Mountains and for two geohydrologic units in the western mountainous part of the stated region. The availability of data for inorganic, trace, and organic constituents for each grouping of units was assessed. Median dissolved-solids concentrations for the groupings of units range from about 100 to 5,000 milligrams per liter. However, the number and distribution of data sites for some groupings of units wee inadequate to be representative of the aquifer as a whole. Concentrations of most trace constituents do not exceed Federal primary drinking-water standards, although exceptions occur. Few data were available for organic constituents. (USGS)