Spatial Variation in Nitrogen Isotope Values Beneath Nitrate Contamination Sources

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Abstract
Nitrogen isotope ratios of soil water and ground water nitrate have been used to identify or implicate sources; however, lack of data on δ15N in thick (>12 m) vadose zones beneath the source and potential effects of denitrification on δ15N signatures have raised questions about the meaning of δ15N values measured in ground water. In this study, nitrogen isotope ratios (δ15N) were measured on nitrate extracted from 218 core samples removed from the surface to the water table below natural (soil organic matter) fertilizer, onsite sewage disposal systems (septic tank effluent), and animal sources located in Salinas and Sacramento Valleys, California. Additionally, spatial variability of δ15N in the horizontal plane was measured beneath an agricultural field, and δ15N values of native geologic materials (organic‐rich shales) were determined. In general, δ15N values throughout the vadose zone were consistent with the literature and remained fairly constant with depth, indicating little denitrification during transport. The δ15N values from soil organic matter sources varied from about 0 to +4‰, and the mean was not significantly different from that of fertilizer sources. The δ15N values of animal sources varied from about +8 to +20‰ and were dependent on site and animal source. The δ15N of onsite sewage disposal sources varied from about +2 to +12‰, and the mean was significantly different from that of animal sources at a 90% confidence level. δ15N of organic‐rich shales of the Panoche and Moreno Formations (Cretaceous; San Joaquin Valley) and soils derived from these rocks were generally between +4 and +8‰. Thus, it may be difficult to distinguish a geologic‐N source from septic tank or commercial fertilizer sources using δ15N. Little spatial variation was found in δ15N beneath the agricultural field in the horizontal plane.