Use of Nitrogen-15 Natural Abundance Method to Identify Nitrate Sources in Kansas Groundwater

Nitrogen isotopes have been used as a tool for identifying sources of nitrate contamination in groundwater for almost 30 years. The method is used most successfully in areas with permeable vadose zones with a shallow groundwater table and in rock units such as fractured limestone, where travel times and/or distances from the source(s) are short. The hydrogeology of Kansas is variable – groundwater is present in aquifers ranging from shallow alluvial systems to deeper formations where the water table is hundreds of feet deep. Geochemical and biogeochemical processes that occur in the vadose zone can alter nitrate concentrations and nitrogen-15 signatures, complicating source identification. Known ranges of d15N in ground water are associated with different sources: fertilizer from -2 to +8 per mil (‰) and animal waste greater than +10‰ (Heaton, 1986). However several processes such as denitrification, nitrification, and volatilization can alter the d15N signature. Complete water chemistry analyses (major ions plus iron and manganese, dissolved oxygen, and dissolved organic carbon), soil-core descriptions, and site and land-use assessments may be necessary to apply the method in a useful manner. Case studies from Kansas are used to demonstrate the strengths, weaknesses, and caveats associated with the method.