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where these units are the uppermost aquifer, the DNR has strongly encouraged its use as a municipal water supply -particularly in the northwest metro area. The Minnesota Geological Survey (MGS) has conducted extensive evaluation of the FIG Aquifer in the northwest metro area and has found that where the Franconia Formation or the Ironton-Galesville Sandstones are the uppermost bedrock unit, they are more highly fractured and thus, more permeable. Where these units are overlain by other bedrock units (e.g., the St. Lawrence Formation), the fracturing is much more poorly developed and the ability to produce usable quantities of water is substantially reduced. The fracturing appears to be associated with "lithostatic pressure relief' -the fractures develop where the weight of the overlying bedrock had been removed. 4. The St. Lawrence Aquitard The St. Lawrence Formation is afine-grained sandy shale of relatively low permeability. It is considered a regional aquitard that separates the underlying FIG Aquifer from the overlying Prairie du Chien -Jordan aquifer system. While of relatively low permeability, the St. Lawrence Aquitard does allow substantial leakage between the two adjoining aquifers and in some locations it can produce usable quantities of water. 5. The Prairie du Chien-Jordan Aquifer System The Jordan Sandstone, combined with the overlying Prairie du Chien Group, is the most important aquifer system in the Twin Cities. Many cities rely on this aquifer system to supply most or all of their water needs (e.g., Eden Prairie, Plymouth, Chanhassen, Minnetonka, St. Louis Park, Edina, Woodbury, Oakdale, Eagan, Burnsville, Apple Valley, Cottage Grove, etc.). The Prairie du Chien Group is divided up into two Formations: the basal Oneota Dolomite (which is not very permeable) and the Shakopee Formation (which is very permeable and transmits water in horizontal fracture systems -particularly near the contact with the Oneota Dolomite). At a local scale, the Shakopee Formation and the Jordan Sandstone are sometimes considered different aquifers but there is enough leakage through the Oneota Dolomite for them to act as a single aquifer system at a more regional scale. Many municipal wells are open to both units. Typical well capacities in this aquifer exceed 1,000 gallons per minute (gpm). The Jordan Sandstone is about 90 feet thick and the Prairie du Chien Group is about 150 feet thick (unless it has been eroded down). The Prairie du Chien-Jordan Aquifer System has proven to be very reliable because it typically the uppermost bedrock unit, where present, and is readily recharged by precipitation that infiltrates P:\Mpls\23 MN\7l\2371105 Water Supply Alternative Study\FinalDeliverables\Alternatives_Report_final.doc 13