MODELING GROUNDWATER IMPACTS AND POTENTIAL DEWATERING OF WETLANDS ALONG THE PROPOSED T.H. 212 ALIGNMENT

A detailed study of wetland groundwater interaction was undertaken to determine indirect impacts to wetlands along the proposed 12-mile T.H. 212 alignment in Carver and Hennepin Counties, Minnesota. This was the first integrated groundwater/surface water study to be completed at this linear scale in Minnesota. Results from the groundwater study were incorporated into surface water models to calculate the total indirect impacts to wetlands due to construction related activities. An existing water table map was produced using Minnesota Department of Transportation borehole data and surface expressions of groundwater using Kriging interpolation and was used to determine where construction activities would intersect the water table. Sixty-four wetlands located in or near these regions were assumed to be at risk and included in the study. Seventy-four piezometers were installed to characterize the local hydrogeologic setting. Specially designed hand installed piezometers were critical to the success of the project by allowing greater data density. Wetlands were grouped by proximity, topography, and aquifer parameter similarity and modeled using Multi Layer Analytical Element Model, assuming steady state conditions. Groundwater elevations were calculated based on groundwater discharge at each wetland. Twenty-seven of the 64 wetlands were integrated into the surface water SWMM model and evaluated for indirect effects of groundwater surface water interaction. For these wetlands three different zones of infiltration were assigned based on elevations of successive stages: groundwater intersection, wetland, and upland. Infiltration at groundwater intersection and below was assumed to be zero. Infiltration rates for the wetland area were based of on previous wetland monitoring data. Upland infiltration was assigned based on the upland soil types surrounding the wetlands. The surface water models were used to predict changes in size and type of wetlands based on groundwater loss.