Regional Patterns in the Isotopic Composition of Natural and Anthropogenic Nitrate in Groundwater,High Plains,U.S.A.

摘要

Mobilization of natural nitrate (NO_3~-)deposits in the subsoil by irrigation water in arid and semiarid regions has the potential to produce large groundwater NO_3~-concentrations.The use of isotopes to distinguish between natural and anthropogenic NO_3~-sources in these settings could be complicated by the wide range in (delta~(15)N values of natural NO_3~-.An approx 10 000 year record of paleorecharge from the regionally extensive High Plains aquifer indicates that delta~(15)N values for NO_3~-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 delta~(15)N values spans the range that might be interpreted as evidence for fertilizer and animal-waste sources of NO_3~-;however,the delta~(15)N values for NO_3~-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 delta~(15)N[NO_3~-] values and the NO_3~-/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 NO_3~-are consistent with both NH_3 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.