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within the lower Franconia is lacking, the Franconia is commonly included with the Ironton and Galesville Sandstones as part of a Franconia-Ironton-Galesville Aquifer complex. The St. Lawrence Formation - This formation conformably overlies the Franconia and is typically the uppermost bedrock aquitard in the Anoka County vicinity. It is mainly composed of dolomitic siltstone and very fine sandstone of the Lodi Member of the St. Lawrence. It has a maximum thickness of 65 feet (MossIer, 1972). The St. Lawrence has not been completely mapped by the Minnesota Geological Survey in the Anoka County vicinity (MossIer and Tipping, 2000). Existing mapping shows subcrops along the southern boundary of Anoka County that extend as far north as the cities of Anoka, Coon Rapids, Blaine, and Lino Lakes. Jordan Sandstone - The Jordan Sandstone lies above the St. Lawrence and is present only in the southern (principally the southeastern) part of the county. Several large outliers are present beyond its sub crop limits. Prairie du Chien Dolomite - The Prairie du Chien Dolomite lies above the Jordan. This group of units includes the Oneota Dolomite, the New Richmond Sandstone and the Shakopee Dolomite. The Oneota can be massive and less fractured than the overlying Shakopee and is documented to serve as a confining unit locally (Runkle, 1999). The Prairie du Chien in Anoka County is present only in the southeast although discontinuous outliers are documented as far north and west as Anoka. Unconsolidated deposits - Bedrock is overlain by a sequence of stratified and unstratified drift units. These units can be particularly thick over buried bedrock valleys (such as the Phalen Channel) and may hydraulically connect subcropping aquifers. Delineation of the Wellhead Protection Area The MDH delineated the wellhead protection area for the City of Centerville in response to a request from Anoka County and the Anoka County Municipal Wellhead Planning Group. MDH agreed to provide assistance prior to adopting a policy of providing this service to small-sized public water supply systems (serving 3300 or fewer persons) to address unfunded mandate concerns regarding implementation of the state wellhead protection rule. However, the public water supplier is responsible for adopting the delineated area and developing a wellhead protection plan for it. Delineation method - Delineation of the wellhead protection area was performed using an analytic element model termed MLAEM. A copy of the input files used to delineate the wellhead protection area can be obtained from MDH. These input files are modified slightly from the regional groundwater flow model prepared for the Prairie du Chien - Jordan Aquifer system by a consultant under contract to MDH (EOR and KDB Consulting, 2000). Aquifers represented in the Anoka County deep bedrock aquifer model (EOR and KDB Consulting, 2000) are assumed to be laterally continuous in the Centerville area and to respond to boundary conditions in the far field. Recharge is generally represented as leakage from adjacent units. Discharge from these aquifers is assumed to occur to the major river systems, especially in areas where the unit is in direct contact with the river. The relatively distributed nature of the recharge and the somewhat focused discharge together serve as the primary boundary conditions controlling groundwater flow. Wells in this aquifer system collectively have a large influence on 5