Ground-water quality in the western Snake River basin, Swan Falls to Glenns Ferry, Idaho

Water-quality data were collected from 92 wells in the western Snake River basin, Swan Falls to Glenns Ferry, Idaho. Current data were compiled with pre-1980 data from 116 wells to define water-quality conditions in major aquifers. Factors affecting water quality are composition of aquifer materials, water temperature, and source of recharge. Mixing of water by interaquifer flow, from confined, hot water aquifers (40 degrees Celsius or greater) with water from cold water aquifers (less than 20 degrees Celsius) occurs along regional complex fault systems, and through partially cased boreholes. Cold water generally contains calcium, magnesium, and bicarbonate plus carbonate ions; hot water generally contains sodium, potassium, and bicarbonate plus carbonate ions. Warm water (between 20 degrees and 40 degrees Celsius) has an intermediate chemical composition resulting from mixing. Ground-water quality is acceptable for most uses, although it locally contains chemical constituents or physical properties that may restrict its use. Effects of thermal water used for irrigation on quality of shallow ground water are inconclusive. Long-term increase in concentrations of several constituents in parts of the study area may be due to effects of land- and water-use activities, such as infiltration of septic-tank effluent. (USGS)