Groundwater Modeling of a Permeable Reactive Barrier to Enhance System Performance

Experience has shown that one of the most common reasons permeable reactive barriers (PRBs) fail to meet performance criteria is inadequate understanding of the groundwater flow system. For example, contaminated water may flow around the ends of the barrier or funnelling slurry walls, under the barrier walls, or through the slurry walls. An analysis of the groundwater flow regime resulting from the installation of a PRB can lead to an effective design that significantly reduces the escape of untreated water through associated slurry walls and/ or around the ends of the PRB system. A three-dimensional groundwater flow model using Modflow was developed to evaluate the effectiveness of groundwater capture and treatment resulting from the installation of a funnel-and-gate PRB. The results of the model analysis provided for a design that incorporated several barrier and gate segments at varying orientations to groundwater flow for optimization groundwater capture. The model indicated the ideal location for gates to minimize head increases along the upgradient portion of the slurry wall, which resulted (as compared to a more simplistic design) in reduced seepage of untreated groundwater through the wall and reduced flow around the end points of the barrier by an estimated 100 per cent. The length of the gates were also optimized to provide sufficient resident time of groundwater in the PRB to accomplish treatment. The model results were also used to locate optimum sites for downgradient monitor wells to assess performance of the system.