Simulating Chloride Migration Potential and Assessing Particle-Transport Uncertainty in the Vicinity of Poplar, Montana

Chloride migration from historic oil and gas operations in eastern Montana has the potential to impact municipal water supplies. Local agencies need estimates of timing and pathways in order to anticipate the need for alternative future water supplies. A regional groundwater flow model was developed to estimate flow conditions. Particle tracking using the simulated flow field provided estimates of particle pathways and timing. Heterogeneity of the aquifer system was developed using a combination of zones, delineating sharp contrasts in aquifer properties, and pilot points reflecting aquifer-test data. This approach provided an efficient way to incorporate sharp transitions between formations while allowing the observations to drive within-zone variability using parameter estimation techniques. Water quality information was interpreted in an effort to develop a range of estimated effective porosities. Uncertainty of hydrogeologic conditions, including stream-aquifer interaction, inter-formation connectivity, and effective porosity, were summarized to estimate advective transport prediction uncertainty and provide a range of travel-time predictions.