Nitrogen isotope ratios identify nitrate contamination sources

Abstract

Nitrate contamination of groundwater is becoming a widespread problem in California. To evaluate the utility of stable nitrogen isotopes for identifying sources of nitrate contamination, nitrogen isotope ratios (?15N) were measured on nitrate extracted from core samples taken below natural, fertilizer, on-site sewage disposal (septic) and animal sources in the Sacramento and Salinas valleys. The mean ?15N value from natural sources was not significantly different from that of fertilizer sources. The mean ?15N value from animal sources was significantly different from that of septic sources and natural and/or fertilizer sources. Nitrogen isotope ratios tend to be site specific and should be measured below suspected sources in the subsurface and in groundwater.