5.2. ERMUSR 02-12-2008 - Laserfiche WebLink

Home Browse Search

through the overlying glacial drift. This aquifer system is also in direct hydraulic connection with major rivers. All groundwater that is not pumped (with the exception of the Mt. Simon-Hinckley Aquifer) discharges into the major river systems in the Twin Cities. 6. The Basal St. Peter Aquitard The lower 50 feet of the St. Peter Sandstone is a shale that is considered a regional aquitard. This unit can yield domestic-quantities of water to wells in some locations. 7. The St. Peter Aquifer The upper 100 feet of the St. Peter Sandstone is a very uniform, clean sand. In most locations, it can yield domestic quantities of water but it is generally not permeable enough to be used as a domestic water supply. It is typically present only near Minneapolis-St. Paul and in isolated locations in Washington and Dakota Counties. 8. The Platteville-Glenwood-Decorah Aquitard The Platteville Formation is a relatively low permeability dolomitic Limestone that is underlain by the Glenwood Shale and in some locations it is overlain by the Decorah Shale. These units rarely produce usable quantities of water but the Platteville Formation is the site of many springs along the Mississippi River in Minneapolis. 9. Quaternary Buried and Water Table Aquifers and Aquitards The glacial sediments (drift) in the Twin Cities area consist of tills (clays) and outwash (sand and gravel). The outwash can be very permeable and can be used as an aquifer where it is present. The till layers act as aquitards. Glacial outwash deposits can be of irregular and unpredictable extent. Glacial stratigraphy is very complex, with tills interfingering with sands over very short distances. The water table is typically in these deposits. In some locations, a till layer may overlie a buried outwash deposit, resulting in a buried or "confined" glacial aquifer. Some communities produce substantial quantities of water from these outwash deposits (e.g., Maple Grove, Brooklyn Park). They can be subject to contamination (and have higher levels of nitrate) and they are more susceptible to drought than bedrock aquifers. In many locations, the glacial sediment (or "Quaternary") aquifers are directly connected to the bedrock aquifers. In 2007, pumping in the Prairie du Chien-Jordan Aquifer System in Chanhassen depleted the storage in an overlying buried glacial aquifer that was connected to the Prairie du Chien Group. P:\Mpls\23 MN\71\2371105 Water Supply Alternative Study\FinalDeliverables\Alternatives_Report_final.doc 14