Hydrogeochemical controls and spatial variability of groundwater contamination in private wells of the Great Bend Prairie aquifer, South-central Kansas

Abstract

Private well water quality is largely unregulated in the United States, making it a responsibility of the well owner for testing and mitigating contamination. However, previous studies show that testing is infrequent, leaving well owners exposed to potential health threats. To help fill this knowledge gap, this study examined the quality of groundwater from private wells completed within the Great Bend Prairie Aquifer in south-central Kansas, a region characterized by extensive agricultural activity and sandy soils and subsurface sediment, which promote high subsurface permeability and increase vulnerability to contamination from the surface. Between 2020 and 2025, we sampled 217 wells in the study area which included a combination of domestic, irrigation, and livestock wells. Among potential threats to water quality, nitrate (NO₃⁻) contamination was most common, with 46% of the wells having NO3- concentration above the US EPA maximum contaminant level for public water systems (10 mg/L as N) and concentrations ranging as high as 55.4 mg/L as N. Stable isotope data for a subset of samples indicated that chemical fertilizer was the main source of NO₃⁻. Consistent with this result, our principal component analysis and random forest model indicated that NO3- concentrations were significantly controlled by hydrogeological, land use, geochemical, and climatic conditions, which are the main factors contributing to the transport of surface contaminants to the subsurface. Microbial NO₃⁻ reduction may also help limit NO3- accumulation, particularly along the northern margin of the study area where dissolved oxygen (O2(aq)) concentration was lower on average. Aside from NO3- contamination, trace element concentrations and concentrations of PFAS were generally low and below drinking water standards, but groundwater total dissolved solids (TDS) exceeded the US EPA secondary water quality standard (500 mg/L) in most samples and hardness was generally high. Nitrogen fertilizer use may contribute to elevated TDS and hardness through acid production by nitrification in overlying crop soil and indeed, NO3- correlated with calcium (Ca2+), magnesium (Mg2+), hardness, and TDS. Additionally, a mixing analysis and water stable isotope data suggest that brine contamination from oil and gas production, which occurs over much of the aquifer, may have also increased the TDS of some of the groundwater we sampled. The proportion of wells with NO3- concentration above the standard for this study is high relative to comparable studies in other regions, even those focused on agricultural regions, demonstrating the importance of private well water quality testing, particularly in areas that are highly vulnerable to contamination from the surface. Moreover, study results demonstrate the value of private wells as opportunities to assess regional variation in water quality and controls on contaminant distributions.