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Abstract

The nitrogen-15 natural abundance method has been a tool for identifying sources of nitrate contamination in groundwater for almost 30 years. Different ranges of δ15N in groundwater are associated with different sources such as fertilizer and animal wastes. However processes such as denitrification, nitrification, and volatilization can alter the δ15N signature.

Use of this method in Kansas has had mixed results. One case study documented in this paper showed the utility of the method in an area with a sandy unsaturated zone and aquifer, a shallow water table, and an animal waste lagoon. The regional groundwater had an average nitrate-N concentration of 13 mg/L and a δ15N signature of +3‰, which indicates a fertilizer source. Effluent leaking from the lagoon (δ15N value of +18‰) facilitated denitrification, affecting the regional groundwater, and resulted in a lower nitrate-N concentration of 4 mg/L with a δ15N signature of +12 to +20‰ downgradient of the site. The second case study illustrated the utility of the method to identify a variety of sources and the effects of the unsaturated zone on the observed δ15N signature in areas with no observable animal waste sources. The presence of carbonate in a loess unsaturated zone resulted in enriched 15N, owing to possible volatilization and preferential loss of 14N.

10.4148/1090-7025.1029

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