N-15 Identification of nonpoint sources of nitrate contamination beneath cropland in the Nebraska Panhandle: two case studies

• Published in: Applied geochemistry 1994, Vol.9 (1), p.73-81
• Main Authors: Exner, M.E., Spalding, R.F.
• Format: Article
• Language: English
• Subjects: Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Geochemistry; Hydrogeology; Hydrology. Hydrogeology; Mineralogy; Pollution, environment geology; Silicates; Water geochemistry
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/0883-2927(94)90054-X

Monitoring of municipal wells near the town of Sidney and domestic wells near Oshkosh in Nebraska's Panhandle indicated the nitrate-nitrogen (NO 3-N) levels were increasing and exceeded the maximum contaminant level of 10 mg/l NO 3-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 on nitrate source. At Sidney NO 3-N concentrations were measured in 13 monitoring wells installed along an east-west transect im 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 NO 3-N. The δ 15N-NO 3 values in the monitoring and municipal wells had a narrow range from +5.8 to +8.8%. 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 NO 3-N and 37.3 ± 8.2 mg/l NO 3-N. The δ 15N-NO 3 values spanned a narrow range from +3.5 to +5.9% and averaged +4.0 ± 0.5% and +5.0 ± 0.6%. These low values are indicative of leachates from commercial fertilizer applied to the irrigated fields.