The lower drift aquifer consists of discontinuous sand and gravel deposits overlying Platteville Formation bedrock and has a maximum thickness of 20 feet where it is overlain by the lower drift confining unit.
Model-computed vertical hydraulic conductivities for the upper and lower drift confining units ranged from 0.0002 to 5 feet per day. The lower drift confining unit is as much as 50 feet thick. The horizontal hydraulic conductivity of the middle drift aquifer ranges from about 50 to 500 feet per day. The saturated thickness of the middle drift aquifer generally is 20 to 30 feet in areas where the aquifer is both overlain and underlain by a confining unit. The upper drift confining unit generally is less than 20 feet thick, with a maximum thickness of 62 feet. Horizontal hydraulic conductivities of the upper drift aquifer range from less than 1 to about 25 feet per day in peat areas and from about 50 to 400 feet per day in sand and gravel areas. The upper drift aquifer has a maximum saturated thickness of about 25 feet. A contamination plume consisting of coal-tar derivatives exists in the drift aquifers and in the Platteville aquifer underlying the southern part of the plant site and areas to the south and east of the plant site. The hydrogeologic units of the drift, in descending order, are the upper drift aquifer, the upper drift confining unit, the middle drift aquifer, the lower drift confining unit.
Three aquifers and two confining units have been delineated within the drift underlying the area near the site of a former coal-tar distillation and wood-preserving plant in St.
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