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.