Modeling Bedrock Transmissivity; Implications For Contaminant Transport In An Overlying Glacial Aquifer System

This study evaluated alternative representations of 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 (1961), was reinterpreted using data from 227 additional bedrock penetrations drilled in the last 55 years. 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 and MT3DMS advective-dispersive transport modeling were employed to explore the influence of bedrock configuration and conductivity variability on predicted steady state contaminant transport pathways, travel times, and concentrations. For source conditions positioned along the eastern edge of the suspected 1,4-dioxane source zone, results indicate that model predictions differ on the order of a few years to decades in travel times and 10s to 100s of µg/L in concentrations, supporting the hypothesis that bedrock configuration and transmissivity influence groundwater flow and 1,4-dioxane transport in the overlying glacial aquifer system