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

TLDR

Stable isotopes (15 N and 18 O) are employed to trace nitrate plumes from a single septic system in a groundwater setting with uniformly high nitrate levels. The study applied 15 N and 18 O isotope analysis to distinguish septic‑derived nitrate from background groundwater. 15 N analysis successfully delineated the septic‑derived nitrate plume and matched expected source signatures, whereas 18 O analysis did not distinguish septic from fertilizer nitrate but revealed nitrification as the main nitrate formation process and still delineated the plume, indicating its usefulness for future plume studies.

Abstract

Abstract Stable isotopes, 15 N and 18 O, 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 15 N content ofthe nonplunie nitrate is in agreement with the sources of nitrate: solid cattle manure, synthetic fertilizer (NH 4 ‐NO 3 ), and soil organic nitrogen, at the study site. 18 O 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. 18 O data indicated that nitrification of ammonium is the main process responsible for formation of nitrate at the study site. 18 O 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.