INFLUENCE OF BEDROCK TRANSMISSIVITY ON 1,4-DIOXANE TRANSPORT IN A COUPLED BEDROCK-GLACIAL AQUIFER SYSTEM

For the past 40 or more years, 1,4-dioxane (a miscible organic solvent resistant to adsorption to soil particles and microbial degradation in the subsurface) has migrated several kilometers through approximately 80 m of Quaternary glacial drift beneath western Ann Arbor, Michigan, USA. Monitoring wells have been installed at more than 160 locations to investigate and remediate 1,4-dioxane as it advances toward the Huron River through an ~10 km²groundwater use prohibition zone established in 2005. Until recently, the underlying Paleozoic shale bedrock was assumed to be impermeable and was treated as a no-flow boundary in numerical groundwater flow and contaminant transport models. However, examination of core and outcrop exposures of the Mississippian Coldwater Shale suggest the possibility of vertical and sub-horizontal fractures that represent potential transport pathways through the bedrock.

This study evaluates alternative representations of the bedrock surface and bedrock transmissivity on advective transport predictions in an 11 km x 15 km x 116 m groundwater model. Bedrock topography, initially based on a map by Kunkle (1960), was reinterpreted using data from more than 200 additional bedrock penetrations drilled in the last 55 years. In addition, varying assumptions of hydraulic conductivity were employed to model a range of bedrock conductivity from 1 to 3 orders of magnitude less than the overlying glacial aquifer system. MODPATH forward particle tracking was employed to explore the influence of bedrock configuration and conductivity variability on steady state pathways and travel times. For particles released along the eastern edge of the suspected source zone, results indicate that the revised interpretation of bedrock topography produces longer particle travel times to the Huron River than the Kunkle bedrock topography model. Results also show that as the transmissivity of the bedrock is increased, median travel times generally increase as more particles travel through the bedrock. Incorporation of alternative conceptual models will improve our ability to assess uncertainty in contaminant transport within this coupled bedrock-glacial aquifer system.