Guidebook 8. Field Trip Guide Book for Hydrogeology of the Twin Cities Artesian Basin

Prepared for the 85th Annual Meeting of THE GEOLOGICAL SOCIETY OF AMERICA and ASSOCIATED SOCIETIES Minneapolis, Minnesota, 1972, 35 p.This field trip focuses on several of the more significant aspects of the hydrogeology of the Minneapolis-St. Paul (Twin Cities) area. Emphasis is placed on the principal bedrock aquifer and the role of glacial drift in the hydrology. Stops (fig. 1) include field examination of the Jordan-Prairie du Chien aquifer, a well-screen manufacturing plant and its research well field, views of surface expression of partly buried bedrock valleys, an artificial-recharge site where experiments were run on deep-well water injection into the Prairie du Chien Group (carbonate rock), and a site where ground water discharges as a large spring from the Prairie du Chien Group. Although much glacial geology is seen along the trip route, emphasis is placed on the drift-filled bedrock valleys. An understanding of the hydrogeology of the Twin Cities area is becoming more critical as urbanization progresses. The population of the metropolitan area is 1,874,000 (1970), an increase of 350,000 since 1960. Predictions of population for the year 2000 are in the neighborhood of 3,000,000. Accompanying the population increase are questions concerning the adequacy of water supply; the effect of surface and subsurface waste disposal, paving, sewering, and other land-use practices on the hydrologic system; and the reasons for lake-level declines and accelerated eutrophication. The Twin Cities supply themselves and 13 surrounding municipalities with water from the Mississippi River. Water from a large number of wells within the cities is used mainly for industry and air conditioning. In addition, most suburban communities use ground water to the extent that ground water now exceeds surface water as a source of supply. Total ground water use in 1970 in the metropolitan area was 194 mgd (million gallons per day); 48 mgd was for domestic purposes, 75 mgd for air conditioning (based on 137 days), 100 mgd for industrial and commercial purposes, and 38 mgd for irrigation (based on 90 days). According to Norvitch (unpublished data), the amount of ground water that can be developed in the metropolitan area on a sustained basis is about 845 mgd. Optimum development of the ground-water resources will be possible only through proper management based on a thorough understanding of the hydrologic system