MODELING BEDROCK TRANSMISSIVITY; IMPLICATIONS FOR CONTAMINANT TRANSPORT IN AN OVERLYING GLACIAL AQUIFER SYSTEM

Glacial drift aquifer systems supply reliable groundwater to much of the northern United States and southern Canada. Regional groundwater models for these systems often treat the underlying bedrock as an impermeable (i.e., no-flow) boundary. Although this approach may be acceptable for groundwater supply applications, it may be inappropriate for contaminant transport models. For example, below the city of Ann Arbor, Michigan, the migration of 1, 4-dioxane in a glacial aquifer system has led to the establishment of a 10 km² groundwater use prohibition zone along the expected transport pathway. Moreover, located just outside of this zone, the city’s Northwest Supply Well is no longer pumped due to 1, 4-dioxane contamination. Existing groundwater flow models in Ann Arbor incorporate Mississippian Coldwater Shale bedrock as an impermeable basal layer. These models incorporate large Kx/Ky anisotropy ratios to correct for contaminant transport that does not follow observed flow path directions. An alternative explanation for contaminant flow pathway orientations is the potential influence of bedrock as a transmissive basal layer.

This study evaluates the effects of changes to basal layer bedrock conditions in an existing 11 km x 15 km x 116 m numerical groundwater model. Bedrock characteristics were investigated by examining the Coldwater Shale in available core and outcrop. Bedrock topography was remapped using newer well data. Local structural trends were evaluated using regional geologic bedrock maps. Results of core analysis reveal matrix permeability and low angle horizontal fractures. Observations of mapped Coldwater Shale outcrops document near vertical joint sets along with low angle fractures indicating plausible transmissivity at scales that could affect contaminant transport model performance. The combined results of these investigations inform ongoing modifications to the bedrock erosional topography, matrix and fracture permeability, and structural features influencing anisotropy in the current model. Sensitivity analysis will be used to quantify the influence of those parameters on 1, 4-dioxane transport in the model.