Nitrate concentrations in excess of national drinking-water standards 10 mg/l as N) are present in certain sand-plain aquifers in central Minnesota. To investigate nitrate sources in the aquifers, nitrogen-isotope values of nitrate (??15NNO3) were measured in shallow ground water from 51 wells in five land-use settings. The land-use settings and corresponding average nitrate concentrations (as N) and ??15NNO3 values are: livestock feedlots, 12.7 mg/l, 21.3 per mil; cultivated-irrigated fields, 13 mg/l, 7.4 per mil; residential areas with septic systems, 8.3 mg/l, 6.0 per mil; cultivated-nonirrigated fields, 15.5 mg/l, 3.4 per mil and natural, undeveloped areas, 3.8 mg/l, 3.1 per mil. Values of ??15NNO3 less than 2 per mil suggest that nitrogen from commercial inorganic fertilizers exists in ground water beneath all settings except the feedlots. Values of ??15NNO3 greater than 10 per mil suggest that nitrogen from animal waste is present in ground water beneath certain feedlots, cultivated-irrigated fields that are fertilized with manure, and residential areas with septic systems. Values of ??15NNO3 between 22 and 43 per mil in ground water beneath the feedlots probably result from denitrification. Values of ??15NNO3 increase with depth in many locations in the sand-plain aquifers. These increases may be caused by progressive denitrification with depth or by changes with depth in the proportions of nitrate from different sources. Similarly, variations of ??15NNO3 values from 1986 to 1987 in certain locations may be due to temporal variations in the amounts of denitrification or to changes in the proportions of nitrate from different sources. Ambiguities in the interpretation of changes in ??15NNO3 values could be eliminated by increasing the spatial and temporal frequency of sampling.
Additional publication details
Nitrogen isotopes as indicators of nitrate sources in Minnesota sand-plain aquifers