Spatial Variation in Nitrogen Isotope Values Beneath Nitrate Contamination Sources

Nitrogen isotope ratios of soil water and ground water nitrate have been used to identify or implicate sources; however, lack of data on δ 15 N in thick (>12 m) vadose zones beneath the source and potential effects of denitrification on δ 15 N signatures have raised questions about the meaning of δ 15 N values measured in ground water. In this study, nitrogen isotope ratios (δ 15 N) 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 δ 15 N in the horizontal plane was measured beneath an agricultural field, and δ 15 N values of native geologic materials (organic‐rich shales) were determined. In general, δ 15 N values throughout the vadose zone were consistent with the literature and remained fairly constant with depth, indicating little denitrification during transport. The δ 15 N 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 δ 15 N values of animal sources varied from about +8 to +20‰ and were dependent on site and animal source. The δ 15 N 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. δ 15 N 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 δ 15 N. Little spatial variation was found in δ 15 N beneath the agricultural field in the horizontal plane.