SULFATE SOURCES IN GROUNDWATER FROM AN AGRICULTURAL AREA CENTRAL ILLINOIS

Increasing sulfate (SO₄^2-) concentrations in the water environment, corresponding with increases in urbanization and industrialization, are a rising global concern. In excess of 250 mg/L, sulfate threatens human health and ecosystems. Elevated concentrations influence carbonate rock weathering, which contributes to the evolution of the global carbon cycle. Knowledge of sulfate sources, whether natural or anthropogenic, is essential for understanding sulfate transport and fate in groundwater. This study investigates SO₄^2- origin and transport in groundwater in an area dominated by agricultural land use in Mclean County, Illinois. Specifically, we explored these questions: 1) Do sulfate concentrations change with groundwater depth? and 2) Are there seasonal differences in sulfate concentrations in groundwater? Water samples collected over 9 years (2015-2024) from 37 observation wells were analyzed for major anions, including SO₄^2-. Based on depths, wells screened at 4.6m (A) and 3.1m (B) were categorized as deep groundwater, while wells screened at 2.3m (C) and 1.5m (D) were categorized as shallow groundwater. Seasons were subdivided into spring/planting (April - June), summer/growing (July - September), fall/harvest (October - December), and winter/fallow (January - March), corresponding with agricultural practices. A cumulative probability plot and a one-way ANOVA revealed two SO₄^2- populations, with deep groundwater being statistically different from shallow groundwater, indicating depth-related changes in SO₄^2-. A two-way ANOVA showed no seasonal differences in SO₄^2- concentrations. Depth differences in SO₄^2- are due to geogenic rock-water interactions releasing more sulfate in deeper groundwater wells. In contrast, shallow groundwater wells have lower sulfate levels, influenced by infiltration and recharge. Groundwater flow dynamics also contribute, with deeper groundwater having longer residence time, allowing more rock-water interactions compared to shallow groundwater. The lack of seasonal variation in SO₄²⁻ concentrations suggest sulfate levels are unaffected by seasonal changes like precipitation, agricultural runoff, or temperature fluctuations. The lack of seasonal variation suggests a stable groundwater system, potentially minimizing the immediate impact of seasonal surficial activities on sulfate levels.