5.2. ERMUSR 02-12-2008 - Laserfiche WebLink

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Geological Survey's County Well Index (CWI) data base, an estimate of the distribution of sandy areas and clayey areas can be made. The aerially averaged percentage of sand in a given area is shown on Figure 5. High percentages of sand and gravel are found along the Mississippi River and north, along Highway 10. 2.3 Regional Aquifer Systems Aquifers are geologic units (or unconsolidated sediment) of sufficiently high permeability that can transport useable quantities of water. Aquitards are geologic units (or unconsolidated sediment) of relatively low permeability that cannot transport usable quantities of water. Examples of aquifers are sandstones, fractured limestones, and sand. Examples of aquitards are shales (formed from mud) and glacial till (silts and clays). Aquifers and aquitards generally must be of large enough aerial extent that they can be mapped. Aquitards typically separated aquifers from one another. Groundwater typically flows horizontally in aquifers and vertically in aquitards. The vertical flow in aquitards represents leakage between aquifers. Groundwater always flows "downhill" -from areas of high pressure to low pressure. The major aquifers and aquitards in the Twin Cities area (from deepest to shallowest) are: 1. The Mt. Simon-Hinckley Aquifer. The Mt. Simon Sandstone and the Hinckley Sandstone together make up the Mt. Simon-Hinckley Aquifer. This is the deepest aquifer system in the Twin Cities area. It varies in thickness up to about 300 feet thick. The aquifer is underlain by lower permeability Precambrian rocks. The Mt. Simon- Hinckley Aquifer is recharged primarily by infiltrating precipitation and downward leakage from unconsolidated units where it "crops out" (i.e. is the uppermost bedrock aquifer) along the edges of the Hollandale Embayment. The recharge area for the Mt. Simon-Hinckley aquifer is shown on Figure 6. Elk River is in an area where the Mt. Simon-Hinckley Aquifer is recharged. Groundwater in the Mt. Simon-Hinckley Aquifer generally flows toward the Minneapolis area, where water leaks slowly upward through the overlying Eau Claire Aquitard into shallower aquifer systems. The Mt. Simon-Hinckley Aquifer is unique in the Twin Cities area because it generally has a poor hydraulic connection with the major river systems (i.e. the Mississippi, St. Croix, and Minnesota Rivers) and is not readily recharged by leakage from overlying aquifers. Groundwater pumping out of the Mt. Simon-Hinckley Aquifer in the Minneapolis area during the 1970's and 1980's was depleting storage in this aquifer. As a result, the Minnesota DNR generally does not issues P:\Mpls\23 MN\7l\2371 I05 Water Supply Alternative Study\FinalDeliverables\Alternatives_Report_final.doc 10