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COMMUNITY PROFILE Ramsey County Multijurisdictional Hazard Mitigation Plan | July 2012 2-15 Groundwater Hydrology Ramsey County is located over the Prairie Du Chien-Jordan Aquifer which is the major source of ground water for the Minneapolis & Saint Paul Twin Cities Metropolitan Area. Other Aquifers that supply Ground Water to Ramsey County are the St. Peter Aquifer, Franconia-Ironton-Galesville Aquifer and the Mt. Simon Aquifer. ST. PETER AQUIFER The St. Peter aquifer is not a major source of ground water. It is used locally for domestic supply and other low-capacity uses. The St. Peter aquifer underlies about three- fourths of the county. It is absent in northwestern Ramsey County and in bedrock valleys filled by glacial sediments. The upper part of the St. Peter Sandstone is composed of fine- to medium-grained sandstone with an average thickness of about 100 feet. The lower third of the formation (Plate 4) is composed of mudstone, siltstone, and shale beds having an average thickness of 50 to 60 feet. They act as confining units between the St. Peter aquifer and the underlying Prairie du Chien Group. Ground water in the St. Peter aquifer flows from areas with the highest hydraulic head in northern Ramsey County toward the Mississippi River. In the southern part of Ramsey County the aquifer is unconfined, because discharge into the Mississippi River lowers the pressure created farther north by the overlying confining units. PRAIRIE DU CHIEN-JORDAN AQUIFER The Prairie du Chien Group and Jordan Sandstone together form the most heavily used aquifer in the county. This aquifer is present throughout the county except in a narrow bedrock valley in the northwest (the Phalen channel), where the first bedrock is the St. Lawrence or Franconia Formation. The aquifer is overlain by the shaly basal part of the St. Peter Sandstone in much of the county. The Prairie du Chien Group is composed predominantly of dolostone; water flows mainly through fractures, joints, and solution cavities. Its total thickness is about t 120 to 130 feel. The Jordan Sandstone (70 to 100 feet thick) consists of highly permeable, fine- to coarse-grained quartzose sandstone, and most water movement is intergranular. Despite their difference in rock type, the Prairie du Chien Group and Jordan Sandstone function as a single aquifer because no regional confining bed separates them. Locally, however, small water-level differences may exist, owing to relatively impermeable beds of shale of limited extent. In general, the ground water flows from areas with the highest hydraulic head in northeastern Ramsey County toward the Mississippi River. This flow pattern indicates that the Prairie du Chien-Jordan aquifer discharges into the river. The flow pattern may be altered by localized pumping of high-capacity wells, especially during the summer when heavy demands are placed on it. The Prairie du Chien-Jordan aquifer is a confined aquifer except for the southeastern comer of the county. ST. LAWRENCE CONFINING UNIT The St. Lawrence Formation consists of dolomitic shale and siltstone and ranges in thickness from 30 to 60 feet. Regionally, it has low permeability and thus hydrologically separates the Prairie du Chien Jordan aquifer from the Franconia- Ironton-Galesville aquifer. FRANCONIA·IRONTON·GALESVILLE AQUIFER The Franconia-Ironton-Galesville aquifer underlies the entire county. The aquifer has three parts: (I) the upper part is the Franconia Formation, which consists of about 115 to 160 feet of feldspathic and glauconitic sandstone with some shale and dolomite; (2) the middle part is the 15- to 20-foot -thick Ironton Sandstone, which contains minor shale partings; and (3) the basal part is the 30- to 40-foot -thick Galesville Sandstone. All three bedrock units are hydrologically connected, although small hydraulic head differences may be found locally. Ground-water movement in this aquifer, like that in the overlying Prairie du Chien-Jordan aquifer, is from areas having the highest hydraulic head in northern Ramsey County toward the Mississippi River. The