Effects of heterogeneity on influent and effluent concentrations from horizontal permeable reactive barriers

Numerical models were used to evaluate how aquifer and barrier heterogeneity affect influent and effluent concentrations for permeable reactive barriers (PRBs). Spatial variability in the reaction rate constant kr and hydraulic conductivity KP of the PRB and hydraulic conductivity of the aquifer (in terms of variations in the mean μlnK, standard deviation σlnK, and correlation scale λ of the logarithm of hydraulic conductivity (ln K)) were considered. Spatial variability of kr and KP was found to change influent and effluent concentrations by less than an order of magnitude. Spatial continuity in hydraulic conductivity parallel to flow, described by the correlation length λx, has a modest effect, with greater continuity yielding higher effluent concentrations. Decreasing the hydraulic conductivity of the aquifer (i.e., decreasing μlnK) does not affect influent concentrations but decreases the median effluent concentrations and broadens the distribution of effluent concentration due to increased residence time in the PRB. Increasing the variability of the aquifer hydraulic conductivity (i.e., increasing σlnK) decreases the median influent concentration and broadens the distribution of influent concentrations due to additional dispersion caused by greater heterogeneity. Greater variability in aquifer hydraulic conductivity also results in a higher median effluent concentrations and a broader distribution of effluent concentration. Comparison of effluent concentrations predicted using a one‐dimensional deterministic model and the three‐dimensional numerical model shows that longer residence times and lower effluent concentrations are usually predicted by the one‐dimensional model. The results indicate that designers should carefully consider factors of safety used for design, perhaps opting for a more conservative approach until more guidance on designing amidst aquifer heterogeneity is available.