N-15 IDENTIFICATION OF NONPOINT SOURCES OF NITRATE CONTAMINATION BENEATH CROPLAND IN THE NEBRASKA PANHANDLE - 2 CASE-STUDIES

Monitoring of municipal wells near the town of Sidney and domestic wells near Oshkosh in Nebraska's Panhandle indicated the nitrate-nitrogen (NO3-N) levels were increasing and exceeded the maximum contaminant level of 10 mg/l NO3-N in several wells. Both areas are located in narrow stream valleys that are characterized by well-drained soils, highly permeable intermediate vadose zones, shallow depths to groundwater, and intensive irrigated corn production. Both areas also have a large confined cattle feeding operation near the suspected contamination and potentially could be contaminated by more than one nitrate source. At Sidney NO3-N concentrations were measured in 13 monitoring wells installed along an east-west transect in the direction of groundwater flow, 26 private wells, and eight municipal wells. Nitrate-nitrogen concentrations were homogeneous beneath a 5 km by 1.2 km area and averaged 11.3 +/- 1.8 mg/l NO3-N. The delta N-15-NO3 values in the monitoring and municipal wells had a narrow range from +5.8 to +8.8 parts per thousand. The isotopic ratios are indicative of a mixed source of nitrate contamination, which originates from agronomic (commercial fertilizer N and mineralized N) N and animal waste. Both commercial fertilizer N and animal wastes are applied to the irrigated fields. Nitrate-nitrogen concentrations in two multilevel samplers installed downgradient from irrigated cornfields at the Oshkosh site averaged 20.1 +/- 13.3 mg/l NO3-N and 37.3 +/- 8.2 mg/l NO3-N. The delta N-15-NO3 values spanned a narrow range from +3.5 to +5.9 parts per thousand and averaged +4.0 +/- 0.5 parts per thousand and +5.0 +/- 0.6 parts per thousand. These low values are indicative of leachates from commercial fertilizer applied to the irrigated fields.