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.

Keywords:

• nitrate,
• groundwater,
• nitrogen-15 isotope,
• sources,
• Kansas