Preliminary Identification of Ground-water Nitrate Sources Using Nitrogen and Carbon Stable Isotopes, Kansas

Abstract

Increasing nitrate-N in ground water is a problem in many areas with limited ground-water supplies, such as west-central Kansas. However, potential sources of nitrate-N are not known. Nitrate-N concentrations in ground water in the Hays study area in Ellis County, west-central Kansas, range from 0.9 to 26 mg/L. The δ¹⁵N signatures of the ground waters are more enriched (+16.8 to +28.7‰) than those of the soils (+8.4 to +13.7‰), strongly suggesting that nitrogen sources are not from mineralized and labile nitrogen present in the unsaturated zone. In this study, nitrate-N values greater than the U.S. EPA drinking water limit of 10 mg/L occur with δ¹⁵N values of greater than +10‰. This relationship between high nitrate-N concentrations and enriched δ¹⁵N values (greater than +10‰) in ground water has been observed in other studies in Kansas and is usually related to a human- and/or animal-waste source.

Soil cores collected near municipal wells had mean total nitrogen values of 1.2-15 mg/kg. Increased δ¹⁵N with depth in several of the cores suggests that microbial mineralization, denitrification, or volatilization processes caused the enriched δ¹⁵N signatures. Decreasing total nitrogen and nitrate-N values with depth also help support the idea of microbial processes.

Stable carbon isotopes provide supporting evidence that soils are not a major contributor to the observed nitrate-N concentration in the ground water. δ¹³C values of the dissolved organic carbon (DOC) in soils generally were more enriched (-11.6 to -18.8‰) while corresponding ground-water δ¹³C values were more depleted (-19.9 to -22.2‰), suggesting that the source of the DOC in ground water is not from the soils.