Reconnaissance of ground-water quality in the North Platte Natural Resources District, western Nebraska, June-July 1991

One-hundred twenty wells completed in unconfined Quaternary alluvial, Ogallala, Arikaree, Brule fractured, sand and confined Chadron and undifferentiated Cretaceous water-bearing units were sampled in June and July 1991 to characterize the quality of ground water in the study area. More than 75 percent of the water samples had nitrate and nitrite as nitrogen concentrations equal to or less than 6.0 milligrams per liter. Samples from six wells completed in Quaternary alluvial and Brule fractured water-bearing units exceeded the U.S. Environmental Protection Agency Maximum Contaminant Level of 10 milligrams per liter nitrate and nitrite as nitrogen. Water from several wells completed in Quaternary alluvial and the Brule water-bearing units had detectable concentrations of alachlor, atrazine, deethylatrazine, or prometon. Major element concentrations in water from 44 wells indicated that the water-bearing units had distinct chemistry. Water from unconfined water- bearing units generally was a calcium bicarbonate type and water from the confined water-bearing units generally was a sodium bicarbonate type. Measurements of pH and concentrations of dissolved solids, sulfate, chloride, fluoride, arsenic, beryllium, manganese, adjusted gross alpha activities, radon, and uranium in ground water exceeded final or proposed U.S. Environmental Protection Agency Maximum Contaminant Levels or Secondary Maximum Contaminant Levels.