Agenda - Public Works Committee - 10/19/2021

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rai SEH Building a Better World MEMORANDUM for All of Us" TO: Chris Larson, PE FROM: Mark Sherrill DATE: March 12, 2020 RE: Northwest Metro Aquifers SEH No. 150732 14.00 This memo presents a summary of the regional groundwater aquifers in the Northwest Metro area including the Cities of Corcoran, Dayton, Ramsey, and Rogers. While variation and extent of bedrock aquifers occur, in general five regional aquifers support much of the potable water for the Twin Cities region, from oldest to youngest: (1) Mt Simon-Hinckley(2)Tunnel City-Wonewoc(3) Prairie du Chien-Jordan (4)St. Peter, and (5) Quaternary aquifers. These aquifers are hydrologically disconnected by a variety of interbedded confining layers. In the Northwest Metro area, Quaternary deposits are highly variable in the types of materials present. Large areas of sand and gravel are required for high production municipal wells. For purposes of this report, Quaternary deposits will not be relied upon for drinking water wells. Based upon the geologic bedrock map represented in Figure 1, the St. Peter aquifer only exists in a small portion of Corcoran and is largely not available as a bedrock aquifer. Jordan Aquifer The Jordan Aquifer is generally considered to be hydrologically connected to the Prairie Du Chien Unit. However, as evident from the geologic bedrock map (Figure 1)the Prairie Du Chien Unit was either not deposited or has been eroded through much of this area. The thickness and presence of this aquifer through this area is scarce and laterally disconnected. Where present, the thickness of the Jordan aquifer is generally around 70 feet, with some areas within the Cities of Corcoran and Dayton as thick as 170 feet. Within the City of Ramsey, the Jordan Sandstone thickness is minimal at around 20-30 feet and appears heavily eroded. Quaternary deposits directly overlay this unit and the Jordan Sandstone is likely recharged by these deposits. Based on the geologic bedrock map and lack of existing Jordan wells, it assumed that the Jordan aquifer is not available for municipal wells throughout most of the study area. Tunnel City—Wonewoc Aquifer The Tunnel City Group and underlying Wonewoc Sandstone supply water for much of the Northwest Metro region. Presence and thickness of the Tunnel City is depicted on Figure 2 and for the Wonewoc on Figure 3. These units appear laterally continuous through much of this area and greater Twin Cities region aside from where it has been eroded away.Areas where the Aquifer is not present primarily occur within bedrock valleys where previous streams and surface water features carved away the bedrock unit. The productivity of the Tunnel City—Wonewoc Aquifer is generally regarded as variable. Yields tend to be moderate to low with some of the highest yields reported where bedrock units are highly fractured. Engineers I Architects I Planners I Scientists Short Elliott Hendrickson Inc.,3535 Vadnais Center Drive,St.Paul,MN 55110-3507 SEH is 100%employee-owned I sehinc.com 1 651.490.2000 1 800.325.2055 1 888.908.8166 fax