Me. Exner et Rf. Spalding, N-15 IDENTIFICATION OF NONPOINT SOURCES OF NITRATE CONTAMINATION BENEATH CROPLAND IN THE NEBRASKA PANHANDLE - 2 CASE-STUDIES, Applied geochemistry, 9(1), 1994, pp. 73-81
Monitoring of municipal wells near the town of Sidney and domestic wel
ls near Oshkosh in Nebraska's Panhandle indicated the nitrate-nitrogen
(NO3-N) levels were increasing and exceeded the maximum contaminant l
evel of 10 mg/l NO3-N in several wells. Both areas are located in narr
ow stream valleys that are characterized by well-drained soils, highly
permeable intermediate vadose zones, shallow depths to groundwater, a
nd intensive irrigated corn production. Both areas also have a large c
onfined 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 insta
lled along an east-west transect in the direction of groundwater flow,
26 private wells, and eight municipal wells. Nitrate-nitrogen concent
rations 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 thousa
nd. The isotopic ratios are indicative of a mixed source of nitrate co
ntamination, which originates from agronomic (commercial fertilizer N
and mineralized N) N and animal waste. Both commercial fertilizer N an
d 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 N
O3-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 lo
w values are indicative of leachates from commercial fertilizer applie
d to the irrigated fields.