NITROGEN ISOTOPES AS INDICATORS OF NITRATE SOURCES IN MINNESOTA SAND-PLAIN AQUIFERS

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 (delta(N(NO3))-N-15) 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 delta(N(NO3)-N-15 values are: livestock feedlots, 12.7 mg/l, 21.3 parts per thousand cultivated-irrigated fields, 13 mg/l, 7.4 parts per thousand; residential areas with septic systems, 8.3 mg/l, 6.0 parts per thousand; cultivated-nonirrigated fields, 15.5 mg/l, 3.4 parts per thousand; and natural, undeveloped areas, 3.8 mg/l, 3.1 parts per thousand. Values of delta(N(NO3))-N-15 less than 2 parts per thousand suggest that nitrogen from commercial inorganic fertilizers exists in ground water beneath all settings except the feedlots. Values of delta(N(NO3))-N-15 greater than 10 parts per thousand 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 delta(N(NO3))N-15 between 22 and 43 parts per thousand in ground water beneath the feedlots probably result from denitrification. Values of delta(N(NO3))-N-15 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 delta(N(NO3))-N-15 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 delta(N(NO3))N-15 values could be eliminated by increasing the spatial and temporal frequency of sampling.