Characterization of Ground-Water Discharge from Bedrock Aquifers to the Mississippi and Minnesota Rivers at Three Areas, Minneapolis-St. Paul Area, Minnesota

The hydrogeology at three areas along the Mississippi and Minnesota Rivers in the Minneapolis-St Paul area, Minnesota, were studied in cooperation with the Legislative Commission on Minnesota Resources and the Minnesota Department of Natural Resources. This report characterizes ground-water discharge from bedrock aquifers to the Mississippi and Minnesota Rivers. Along the Mississippi River between Fridley and Brooklyn Center, a buried valley underlying the Mississippi River cuts through the overlying terrace deposits and glacial-drift deposits into two underlying bedrock hydrogeologic units: the St. Peter aquifer, and a rubble zone between the St. Peter and Prairie du Chien-Jordan aquifers. Shallow ground-water flow in the near-surface gray and upper red tills and sand and gravel outwash aquifer discharges to springs along the edge of the river. Ground water flowing through the rubble zone and upper pan of the Prairie du Chien-Jordan aquifer probably discharges through alluvial deposits to the river. Along the Minnesota River between Eagan and Bloomington, almost 200 feet of post-glacial alluvium, glaciofluvial sand and gravel, Pleistocene lake deposits, and peat fill a bedrock valley under the present-day Minnesota River. As much as 40 feet of post-glacial peat, silty clay, clay, and muck lie near the river-valley walls. Confining units beneath the river channel impede the discharge of ground water from the underlying Prairie du Chien-Jordan aquifer to the river. Ground water discharges to wetlands, lakes, and springs along both the north and south side of the river. Along the Mississippi River at Minneapolis about 5 miles upstream of the confluence of the Minnesota and Mississippi Rivers, the Mississippi River lies in a post-glacial valley cut through thin glacial drift into the St. Peter aquifer. Beneath the river, ground water flows from the St. Peter aquifer through the overlying post-glacial alluvium to the Mississippi River. No confining unit separates the St. Peter aquifer and the river.