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Hydrogeologic sensitivity to contamination throughout Winona County in southeastern Minnesota was assessed using the recently developed Trojan-Perry rating method. Sensitivity varied across and within three analysis regions. The Prairie du Chien Aquifer, comprising Region I, showed a wide range of sensitivity, varying from moderate to extreme. Areas of greatest sensitivity were sites where the aquifer was unconfined and overlain by a thin layer of unconsolidated material and karst bedrock. Under these conditions water may rapidly infiltrate through the soil zone and highly dissolved bedrock and into underlying aquifers. The Ironton-Galesville Aquifer, comprising Region II, was protected from surface infiltration by the St. Lawrence siltstone and was not sensitive to contamination unless depth-to-water was less than 25 feet. An unconsolidated surficial aquifer along the Mississippi River comprises Region III, and is highly to extremely sensitive due to large water inputs from adjacent areas. Sites located on sand terraces in this region show the greatest susceptibility due to high permeability throughout the vertical profile.

The accuracy and scope of evaluations made with the Trojan-Perry rating method will improve and expand as data is collected and utilized in the methodology and computer capabilities are employed. The following primary data needs would improve the precision of hydrogeologic sensitivity analysis in Winona County: 1) soil textural classifications and thicknesses to aid in infiltration and depth-to-water analysis, particularly along the Mississippi floodplain; 2) knowledge of land management practices such as tillage and chemical application, which aid in infiltration and contaminant behavior analysis; 3) knowledge of land-use to improve infiltration, permeability, recharge, and contaminant behavior analysis; and 4) soil pH, organic matter contents, and mineralogy, which can be utilized to assess the behavior of specific contaminants or contaminant classes. Evaluating contamination sensitivity to atrazine, a widely used herbicide frequently found in county aquifers, illustrates the need for improved data.

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