Assessment of stable nitrogen isotopes in fingerprinting surface water inorganic nitrogen sources

Abstract

In many coastal areas of Louisiana, surface water quality is deteriorating rapidly due to elevated nutrient input from agricultural, domestic and industrial sources. This study investigates the potential use of natural abundance variations in 15N/14N ratios for identification and tracing surface water inorganic N sources. Surface water samples were collected from streams and point sources in Louisiana and analyzed for NH4+-N, NO3−-N and associated 15N/14N (δ15N ‰) concentrations. Ammonium-N from domestic sewage and industrial discharge point sources was found to have distinct δ15N ranges. Domestic sewage discharge into a slow flowing stream was traced for about 30 km downstream using 15N/14N ratios. At the sewage point source NH4+-δ15N values averaged +43%o and increased linearly to +162‰ with distance from the discharge. In a larger stream with a greater flow velocity the NH4+-δ15N surface water signature of an industrial discharge source was identifiable for approximately 1 km from the point source. Surface water NO3−-°15N values generally ranged from +1 to +99‰ and no significant association was observed between δ15N values with distance from the domestic sewage and industrial point sources. The discrete NH4+-δ15N signatures of domestic sewage and industrial point sources compared to downstream surface water NH4+-δ15N values suggest that N isotopic ratios have the potential to be used as tracers in surface waters contaminated with inorganic N.