ORIGIN AND TRANSPORT OF DISSOLVED CHEMICALS IN A KARST WATERSHED, SOUTHWESTERN ILLINOIS

An extensive base of water-quality information, emphasizing the effects of land use and hydrology, was obtained in the karstified Fountain Creek watershed of southwestern Illinois to help resolve local water-quality issues. Profiles of the concentrations and mass loads of chemical constituents along the major streams were established under various hydrologic conditions and were compared with detailed land-use maps to identify important sources of water-quality components. Agrichemicals dominate the loads of most components in the streams and shallow karstic groundwater. Only Ca, Mg, Al, and sulfate ions are predominantly derived from bedrock or soils, whereas agrichemicals contribute most of the Na, K, Cl, nitrate, F, P, and atrazine. Concentrations of individual ions correlate with discharge variations in karst springs and surface streams; highly soluble ions supplied by diffuse groundwater flow are diluted by high flows, whereas less soluble ions increase with flow as they are mobilized from the fields to karst conduits under storm conditions. Treated wastewater containing detergent residues dominates the boron load of streams and provides important subordinate loads of several other constituents, including atrazine derived from the Mississippi River via the public water supply. Average surface-water concentrations at the watershed outlet closely approximate a 92:8 mixture of karst groundwater and treated wastewater, demonstrating the dominance of groundwater contributions to streams. Therefore the karst aquifer and watershed streams form a single water-quality system that is also affected by wastewater effluent.