Regional patterns in the isotopic composition of natural and anthropogenic nitrate in groundwater, High Plains, U.S.A.

Mobilization of natural nitrate (NO₃^-) deposits in the subsoil by irrigation water in arid and semiarid regions has the potential to produce large groundwater NO₃^- concentrations. The use of isotopes to distinguish between natural and anthropogenic NO₃^- sources in these settings could be complicated by the wide range in δ¹⁵N values of natural NO₃^-. An ∼10 000 year record of paleorecharge from the regionally extensive High Plains aquifer indicates that δ¹⁵N values for NO₃^- derived from natural sources ranged from 1.3 to 12.3‰ and increased systematically from the northern to the southern High Plains. This collective range in δ¹⁵N values spans the range that might be interpreted as evidence for fertilizer and animal-waste sources of NO₃^-; however, the δ¹⁵N values for NO₃^- in modern recharge ( less than 50 years) under irrigated fields were, for the most part, distinctly different from those of paleorecharge when viewed in the overall regional context. An inverse relation was observed between the δ¹⁵N[NO₃^-] values and the NO₃^-/Cl^- ratios in paleorecharge that is qualitatively consistent with fractionating losses of N increasing from north to south in the High Plains. N and O isotope data for NO₃^- are consistent with both NH₃ volatilization and denitrification, having contributed to fractionating losses of N prior to recharge. The relative importance of different isotope fractionating processes may be influenced by regional climate patterns as well as by local variation in soils, vegetation, topography, and moisture conditions.