<p>Increasing sulfate (SO₄²⁻) concentrations in groundwater systems are a growing environmental concern due to their implications for drinking-water quality and biogeochemical cycling. While sulfate is commonly of geogenic origin, its behavior in agriculturally influenced glacial aquifers remains poorly constrained. This study investigates the distribution, depth dependence, and seasonal variability of sulfate concentrations in groundwater within a riparian system comprised of sediments deposited by glaciers in central Illinois, USA. The study specifically evaluates whether sulfate concentrations 1) vary with groundwater depth, 2) exhibit seasonal trends, and 3) reflect contributions from agricultural tile drainage waters. Groundwater samples collected from 35 monitoring wells over a nine-year period yielded approximately 2,000 sulfate observations. Cumulative probability analysis identified two statistically distinct sulfate populations separated by a threshold concentration of approximately 20&#xa0;mg/L. The two populations represented deep groundwater derived from unweathered diamicton and shallow groundwater from within weathered diamicton. With no consistent seasonal variability in sulfate concentrations across the sources, the elevated sulfate concentrations in deeper groundwater are interpreted to reflect depth-related geogenic controls, including prolonged residence time and enhanced water–rock interactions, whereas shallow groundwater and tile drainage waters exhibited comparatively low sulfate concentrations consistent with recently infiltrated waters. Overall, sulfate concentrations are depth-controlled and geogenic, with minimal agricultural influence.</p>