Nitrogen isotopes as indicators of nitrate sources in Minnesota sand-plain aquifers

Nitrate concentrations in excess of national drinking-water standards (10 mg/1 as N) are present in certain sand-plain aquifers in central Minnesota. To investigate nitrate sources in the aquifers, nitrogen-isotope values of nitrate (δ¹⁵N^NO3No3) 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 δ¹⁵N^NO3 values are: livestock feedlots, 12.7 mg/1, 21.3%₀; cultivated-irrigated fields, 13 mg/1, 7.4%₀; residential areas with septic systems, 8.3 mg/1, 6.0%₀; cultivated-nonirrigated fields, 15.5 mg/1, 3.4%₀; and natural, undeveloped areas, 3.8 mg/1, 3.1%₀. Values of δ¹⁵N^NO3 less than 2%₀ suggest that nitrogen from commercial inorganic fertilizers exists in ground water beneath all settings except the feedlots. Values of δ¹⁵N^NO3 greater than 10%₀ 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 δ¹⁵N^NO3 between 22 and 43%₀ in ground water beneath the feedlots probably result from denitrification. Values of δ¹⁵N^NO3 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 δ¹⁵N^NO3 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 δ¹⁵N^NO3 values could be eliminated by increasing the spatial and temporal frequency of sampling.