Abstract
<title>Abstract</title> <p>Private domestic wells in western Kansas, USA, supply drinking water to a rural population outside routine monitoring under the Safe Drinking Water Act, while the underlying High Plains Aquifer is exposed to anthropogenic nitrate loading and geogenic arsenic mobilization. Fifty-one wells and springs across Ellis, Norton, Russell, and Decatur counties were analyzed for major ions and trace elements by inductively coupled plasma–mass spectrometry and ion chromatography. Non-parametric statistics, principal component analysis, Hazard Quotient (HQ), Incremental Lifetime Cancer Risk (ILCR), and a composite Health Risk Water Quality Index (HQ-WQI) were applied. Nitrate ranged from 0.0 to 25.2 mg L⁻¹ (mean 5.41 mg L⁻¹; 9.8% above the U.S. Environmental Protection Agency Maximum Contaminant Level), and arsenic from below detection to 19.0 µg L⁻¹ (mean 3.88 µg L⁻¹; 2.0% above the Maximum Contaminant Level). Kruskal–Wallis tests detected no significant inter-county differences (As H = 3.19, p = 0.20; nitrate H = 2.17, p = 0.34), indicating contamination is structured at the well scale. Arsenic correlated with iron (r = + 0.41, p < 0.01), pH, and fluoride, consistent with reductive dissolution of iron oxyhydroxides; nitrate correlated with chloride, calcium, and total dissolved solids, consistent with diffuse agricultural loading. The HQ-WQI classified 84.3% of wells as moderate-to-very-high pediatric risk despite 90.2% regulatory compliance, and 25.5% of wells exceeded the unacceptable adult ILCR threshold (1 × 10⁻⁴). Prospective 5 mg L⁻¹ nitrate and 5 µg L⁻¹ arsenic thresholds reclassified 52.9% of wells as non-compliant, supporting risk-informed private well governance.</p>