Impact of tunnel dewatering on surface water bodies in Mil. County

The interaction of Milwaukee County’s Northshore Tunnel with surface water bodies was analyzed.  Seepage meter measurements established that the tunnel induced recharge into the dolomite and glacial aquifer of at least 1,600 m³/day from Lake Michigan and 534 m³/day.  The tunnel’s influence extends up to 4 km away from it under the lake and about 2 km upstream in the river.

In surface water bodies near the tunnel, flow of water through the bed is a combination of vertical downward flow toward the bedrock system and the tunnel and lateral inflow from the unconsolidated sediments.  The low conductivity Milwaukee Formation, topmost rock unit in most of the study area, restricts exchange between the sediments and rock, thus limiting the extent and magnitude of exchanges between tunnel and surface waters.  It limits vertical outflow toward the tunnel and simultaneously causes groundwater in the sediments to flow predominantly horizontally to discharge in the lake or river.  Locally this discharge can exceed any efflux induced toward the tunnel.  High conductivity units, such as the Thiensville Formation, expand the extend of the tunnel’s trough of depression.  If such units are continuous between a surface water body and the tunnel’s trough, they then increase the quantity of flow toward the tunnel.  Thus accurate determination of the tunnel’s impact requires a full understanding of the hydrogeologic system in which it’s located and a well designed monitoring program.

The value of piezometers and seepage meters for determining the effect of a major groundwater stress on a surface waters was assessed.  Neither could accurately define the full extent and magnitude of the tunnel’s influence by themselves, but they can be successfully combined in a scheme whose design is based on the characteristics of the hydrogeologic system.  Piezometers provide necessary information on the distribution of heads and Ks, but are too expensive to be spaced densely enough to define the distribution of the tunnel’s influence.  Less conventional seepage meters can be deployed across the lake or river bed as densely as needed.  They provide a cheap, accurate, and relatively rapid direct measure of the net flux across the bed.