Changes in groundwater chemistry over two decades in a fen complex impacted by surface mining in northeastern Illinois, USA

A hydrogeochemical investigation of a fen complex in northeastern Illinois, USA, was conducted during 1998–2020 to assess the impacts of anthropogenic activities on groundwater conditions. Situated in an urban/suburban setting downgradient of various land uses, including surface sand and gravel mining and transportation corridors, the fen complex has experienced alterations in both water flow and chemistry. Long-term groundwater monitoring recorded changes in groundwater chemistry. Elevated chloride concentrations from anthropogenic sources were detected, despite the fen complex being protected as an Illinois nature preserve since 1990. Elevated concentrations of sulfate and nitrate were also observed. Although much of the area upgradient of the fen complex has remained unchanged since mining ceased, the reclamation of a quarry pit with native sand and gravel modified downgradient groundwater chemistry. Concentrations of bicarbonate and calcium, essential constituents for fen ecology, increased following reclamation and remained elevated for more than two decades. These findings provide field-based evidence that backfilling excavations with native materials of appropriate composition and texture can effectively and sustainably enhance groundwater quality. In addition, monitoring results suggest that historical surface mining activities upgradient of the fen may have reduced essential groundwater constituents and influenced fen ecological integrity. This study provides insights relevant to the management and restoration of calcareous fens and to the reclamation of surface sand and gravel mines.