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Ground water is the source of drinking water for the residents of Pumpkin Creek Valley, western Nebraska. In this largely agricultural area, shallow aquifers potentially are susceptible to nitrate contamination. During the last 10 years, ground-water levels in the North Platte Natural Resources District have declined and contamination has become a major problem for the district. In 2000, the U.S. Geological Survey and the North Platte Natural Resources District began a cooperative study to determine the age and quality of the ground water and the sources of nitrogen in the aquifers in Pumpkin Creek Valley. Water samples were collected from 8 surface-water sites, 2 springs, and 88 ground-water sites during May, July, and August, 2000. These samples were analyzed for physical properties, nutrients or nitrate, and hydrogen and oxygen isotopes. In addition, a subset of samples was analyzed for any combination of chlorofluorocarbons, tritium, tritium/helium, sulfur-hexafluoride, carbon-14, and nitrogen-15. The apparent age of ground water in the alluvial aquifer typically varied from about 1980 to modern, whereas ground water in the fractured Brule Formation had a median value in the 1970s. The Brule Formation typically contained ground water that ranged from the 1940s to the 1990s, but low-yield wells had apparent ages of 5,000 to 10,000 years before present. Data for oxygen-18 and deuterium indicated that lake-water samples showed the greatest effects from evaporation. Ground-water data showed no substantial evaporative effects and some ground water became isotopically heavier as the water moved downgradient. In addition, the physical and chemical ground-water data indicate that Pumpkin Creek is a gaining stream because little, if any, of its water is lost to the ground-water system. The water-quality type changed from a sodium calcium bicarbonate type near Pumpkin Creek’s headwaters to a calcium sodium bicarbonate type near its mouth. Nitrate concentrations were large

Ground water is the source of drinking water for all residents of the North Platte Natural Resources District (NPNRD). Most of the drinking water in the NPNRD comes from unconfined aquifers that are part of the regional High Plains aquifer. The water tables of these local aquifers lie close to the land surface. As a result, the aquifers can be affected by infiltration of irrigation waters that are prone to contain contaminants. Once in the aquifer, contaminants can adversely affect drinking-water supplies. In 2002, the U.S. Geological Survey (USGS) and the NPNRD began a cooperative study to determine whether or not 112 pesticides and selected degradation products occur in ground water of the NPNRD. Forty-one monitoring wells at 23 sites (fig. 1) throughout the NPNRD were selected for sampling to determine the areal occurrence of pesticides. This fact sheet summarizes the results of the 2002 sampling.