Stable Isotopes of Oxygen and Nitrogen in Source Identification of Nitrate from Septic Systems

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Abstract
Stable isotopes, 15N and 18O, have been used as tracers to differentiate a contaminant nitrate plume emanating from a single domestic septic system, in a ground‐water system characterized by high and similar nitrate content outside and inside of the contaminant plume. A good delineation of the nitrate plume of septic origin was obtained using 15 N analysis in nitrate. The 15N content ofthe nonplunie nitrate is in agreement with the sources of nitrate: solid cattle manure, synthetic fertilizer (NH4‐NO3), and soil organic nitrogen, at the study site. 18O analysis in nitrate did not provide enough isotopic contrast to permit separation of nitrate derived from the septic system and that in the surrounding ground water, derived from agricultural fertilizer sources. 18O data indicated that nitrification of ammonium is the main process responsible for formation of nitrate at the study site. 18O in ground water clearly delineated the ground‐water plume associated with the septic system and suggest that this tracer should be considered in studies related with contaminant plumes of different origin.