Evaluation and design of permeable reactive barriers amidst heterogeneity

Permeable reactive barriers (PR13s) are in-situ walls constructed from porous media that react with groundwater contaminants. Currently, PRBs are designed assuming the aquifer and PRB are homogeneous and isotropic. This study investigated the impact of aquifer and PRB heterogeneity on influent and effluent from horizontal flow PRBs (HFPRBs), funnel and gate PRBs (FGPRBs), and caisson PRBs (CPRBs), and makes design recommendations for each type of PRB.; Spatial variability in the reaction rate constant (k_r) and hydraulic conductivity (K_P) of the PRB, and variations in the mean (μ_InK), standard deviation (σ_InK), and correlation scale of the logarithm of hydraulic conductivity (InK) of the aquifer were considered in a parametric study. Spatial variability of k_r and K_P, and correlation scale were found to have a minor effect on influent and effluent concentrations. The (μ_InK), and σ_InK were found to strongly impact influent and effluent concentrations. Decreasing (μ_InK), does not affect influent concentrations, but yields lower and more broadly distributed effluent concentrations. Increasing σ_InK causes lower but more broadly distributed influent concentrations, and higher and more broadly distributed effluent concentrations.; Recommendations for PRB design are made in the second half of this report. The FGPRB was determined to be the most economical PRB design and the HFPRB the least economical design. Design of PRBs using a plug-flow model was found to require a factor of safety between two and twelve, depending on σ _InK. In place of a factor of safety, scaling factors that are functions of μ_InK, σ_InK, correlation scale, and allowable risk are suggested for the three types of PRBs. The most efficient monitoring systems for detecting higher effluent concentrations from each type of PRB are also recommended.; Recommendations for future PRB design are to place greater importance on aquifer characterization (i.e., σ_InK, μ_InK, and correlation scale) and PRB selection. PRB design should begin with appropriate PRB selection and adequate aquifer characterization to choose a scaling factor that corresponds to a low risk of failure. Then, economical monitoring systems that verify the proper operation of the PRB and have a reasonable chance of detecting higher effluent concentrations can be used.