The Prairie du Chien group overlies the Jordan Sandstone, which is a medium- to coarse grained, poorly cemented quartzose sandstone. Despite their differing lithologism, the Prairie du Chien Group and the Jordan Sandstone function as one aquifer, because there is no regional confining bed between them. Locally, however, small head differences may be caused by relatively impermeable beds of limited extent. This aquifer consists of a glauconitic, very fine to medium grained quartzose sandstone interbedded with shale and glauconitic to non-glauconitic dolomite. In Scott County the ability of surface contaminants to enter the bedrock formations that can serve as sources of water supply depends mainly on the permeability and thickness of the overlying unconsolidated materials and the depth to the water table.
Secondary permeability can exist through abandoned and poorly constructed wells.
Outwash and ice-contact deposits of sand and gravel are highly permeable because water moves readily through spaces between grains and pebbles. Silt and clay particles fill the spaces between coarser grains in glacial till, and till is therefore relatively compact and impermeable. The areas most susceptible to contamination are along the Minnesota River and the lower part of Sand Creek, where the water table is near the land surface and the unconsolidated materials are both thin and highly permeable. The area in Helena, Prior Lake, and Spring Lake Townships is moderately susceptible because the overlying unconsolidated materials are fairly permeable, although the water table there is generally about 70 to 100 feet below the surface. Visit Susceptibility to Ground Water Contamination Within Scott County to view a map showing the different zones of susceptibility within Scott County.