GROUND-WATER QUALITY AND AGE IN PUMPKIN CREEK VALLEY, WESTERN NEBRASKA

Ground water is the source of drinking water for residents of the North Platte Natural Resources District, western Nebraska. The water tables for these aquifers typically are within 10s of feet to the land surface and the aquifers locally are affected by infiltration of water containing agricultural chemicals. Historical water-quality data indicate the presence of high nitrate concentrations in ground water in Pumpkin Creek Valley. To determine the ground-water quality and age of water in Pumpkin Creek Valley the U.S. Geological Survey, in cooperation with the North Platte Natural Resources District and the University of Nebraska, began a study in 2000. Water from 86 monitoring wells was analyzed for physical characteristics, nutrients, major ions, and ²H and ¹⁸O. In addition, 36 of these sites were sampled for ¹⁵N/¹⁴N to determine the source of the nitrate. For age dating, 60 of these sites were sampled for CFC-12, CFC-11, CFC-113, 43 sites were sampled for ³H/³He, and 8 sites were sampled for either SF₆ or ¹⁴C.

The largest nitrate concentration in water samples (41 mg/L as N) occurred in the siltstone of the Brule Formation of Tertiary age. Median nitrate concentrations were largest in the alluvial aquifer (5.0 mg/L) and smallest in the Brule Formation (3.2 mg/L). Sources of nitrogen in the ground water include commercial fertilizer and animal waste. The d¹⁵N values in the ground-water samples typically were 2 to 6 per mil. The isotope data indicate that the source of most of the nitrate in water samples that had more than 10 mg/L as N was commercial fertilizers. Nitrate concentrations above 10 mg/L as N, with d¹⁵N values near 10 per mil, could not be identified exclusively as coming from either fertilizer or animal waste.

Preliminary results indicate that the ground water varies from a sodium calcium bicarbonate type in the western part of the study area to a calcium sodium bicarbonate type in the eastern part. CFC data generally yield apparent model ages in the alluvial aquifer from 1980 to recent, but ground water in the fractured Brule Formation had a median age in the 1970s. Recharge dates determined using the ³H/³He method generally were concordant with CFC results. Carbon-14 dating of water from the Brule Formation that was not fractured yielded ages on the order of thousands of years or less.