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 delta15N values of natural NO3-. An approximately 10 000 year record of paleorecharge from the regionally extensive High Plains aquifer indicates that delta15N values for NO3- derived from natural sources ranged from 1.3 to 12.3 per thousand and increased systematically from the northern to the southern High Plains. This collective range in delta15N values spans the range that might be interpreted as evidence for fertilizer and animal-waste sources of NO3-; however, the delta15N values for NO3- in modern recharge (< 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 delta15N[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.