Received for review November 7, 2005

Revised manuscript received February 28, 2006

Accepted March 9, 2006

Abstract:

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 ¹⁵N values of natural NO₃^-. An ~10 000 year record of paleorecharge from the regionally extensive High Plains aquifer indicates that ¹⁵N 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 ¹⁵N values spans the range that might be interpreted as evidence for fertilizer and animal-waste sources of NO₃^-; however, the ¹⁵N 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 ¹⁵N[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.

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