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Figure 2 is a bedrock map of southern Anoka County and shows the distribution of bedrock units in the Centerville area. The major bedrock units in Anoka County, relative to groundwater resources, are the Mt. Simon (and Hinckley) Sandstone Aquifer system and the Franconia- Ironton-Galesville Aquifer system. Locally in southeastern Anoka County, the Prairie du Chien- Jordan Aquifer System, the most heavily used in the Twin Cities Metropolitan Area, is important. The geologic aspects of these aquifers and the units separating them were reviewed in the Ramsey-Anoka-Washington County Groundwater Flow Model (EOR and KDB Consulting, 2000) and were taken from MossIer (1972). Underlying Formations - The geologic formations underlying the lowest bedrock aquifer are principally comprised of red beds oflate Keweenawan age. Their thickness is estimated to be as much as 4000 feet. Because of their generally low permeability, they are regarded as the base of the aquifer systems in the Twin Cities basin. Hinckley Sandstone - The Hinckley Sandstone is a quartzose sandstone that directly overlies the Keweenawan red beds. It is lithologically similar to the overlying Mt. Simon Sandstone and appears to be differentiated in this area primarily by being involved in late Keweenawan faulting. The Hinckley is thickest in western Anoka County, where thicknesses on the order of 250 feet or more have been observed. Mt. Simon Sandstone - The Mt. Simon Sandstone directly overlies the Hinckley with no intervening aquitard. It is a mostly medium- to coarsely-grained, friable quartzose sandstone. It is generally about 250 feet thick along the western margin of the Twin Cities basin and thins to the east. Eau Claire Formation - The Mt. Simon conformably grades upward into the Eau Claire Formation, which is composed of glauconitic siltstones, very fine-grained sandstones and a green shale. The formation is approximately 100 to 125 feet thick in the Twin Cities basin. This formation is regarded as the separating aquitard between the underlying Mt. Simon- Hinckley Aquifer and the overlying Franconia-Ironton-Galesville Aquifer. The Eau Claire is largely intact, except in the vicinity of buried bedrock valleys, throughout all but northwestern Anoka County. Ironton and Galesville Sandstones - The Galesville Sandstone unconformably overlies the Eau Claire. Both the Galesville and lronton Sandstones are medium-grained quartzose sandstones. Their aggregate thickness is 35 to 65 feet in the Twin Cities basin. Because of their similar hydraulic properties and lack of separating layer, these sandstones are regarded as a single aquifer. Franconia Formation - The Ironton-Galesville is conformably overlain by the Franconia Formation, which is generally composed of very fine to fine-grained, glauconitic sandstones and siltstones. The upper portion of this formation is comprised of coarser, non-glauconitic sandstones representing near-shore facies, collectively termed the Mazomanie Member. These are interfingered with the glauconitic sandstones in the northern portion of the basin. The thickness of the Mazomanie varies from approximately 40 feet in the southern portion of Anoka County to approximately 125 feet at its northeastern comer; the Franconia as a whole correspondingly increases in thickness from approximately 140 feet to 180 feet (Runkle, 1995). Because the permeable Mazomanie Member comprises a majority of the Franconia and because a clear underlying aquitard 4