USE OF THE NITROGEN-15 ISOTOPE NATURAL ABUNDANCE METHOD AND CHEMICAL RATIOS TO DETERMINE SOURCES OF POTENTIAL NITRATE CONTAMINATION OF GROUND WATER, NORTHEASTERN KANSAS

The geology of the study area in northeastern Kansas consists of Pennsylvanian limestones and shales overlain by loess deposits developed from glacial deposits. The study area encompasses a buried valley that is used as a water source by a rural water district. Chemical analysis of the water shows increased nitrate-N concentration close to the USEPA drinking-water limit of 10 mg/L. Surrounding wells also show concentrations near or above the nitrate-N drinking-water limit. Comparisons of ground-water chemistry and geology of the sited wells indicates that the wells in the buried valley are more contaminated than wells in the Pennsylvanian bedrock. The nitrogen-15 (δ¹⁵N) isotope natural abundance method and chemical ratios of bromide/chloride, sulfate/chloride, bicarbonate/nitrate, and bicarbonate/chloride were used to determine potential sources of the observed high nitrate concentration.

Historical land use in the study area included a dairy, cattle pastureland, and a horse corral. Aerial photographs show the location of a now abandoned pond that was present when the dairy was active. Current land use includes cattle pastureland and row-crop farming near the well of interest. Enriched δ¹⁵N values in conjunction with elevated nitrate-N and chloride strongly support an animal-waste source for the nitrate. Bromide/chloride and sulfate/chloride ratios indicate that evapoconcentration of animal-waste is the likely source of the observed chloride content (and associated nitrate-N). These plots successfully indicate that septic-tank waste or dissolution of water-softener salt or road salt, are not sources of the observed chloride.

Plots of bicarbonate/chloride versus nitrate and bicarbonate/nitrate versus nitrate separate the buried valley samples from Pennsylvanian rock samples suggesting different sources and processes affecting the observed nitrate in the wells. A plot of nitrate-N versus chloride indicates that manure is the likely source for the observed nitrate values. Zones of higher sand in the loam-soil profile (13 m depth) showed higher nitrate-N values suggesting higher permeability in the soils overlying the buried valley. The potential evolution of cattle and horse manure sources through soil to ground water are shown by a plot of δ¹⁵N values versus percent nitrogen.