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

Abstract

[1] 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 k r and hydraulic conductivity K P of the PRB and hydraulic conductivity of the aquifer (in terms of variations in the mean m lnK , standard deviation s lnK , and correlation scale l of the logarithm of hydraulic conductivity (ln K)) were considered. Spatial variability of k r and K P 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 l x , has a modest effect, with greater continuity yielding higher effluent concentrations. Decreasing the hydraulic conductivity of the aquifer (i.e., decreasing m lnK ) 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 s lnK ) 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 threedimensional 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.

Volume-based inverse model (VINMOD): innovative approach to quantify groundwater pollution in heterogeneous aquifers

“…The total mass flow to be calculated from the inversion of pumped concentration in homogeneous and isotropic conditions can correspondingly differ from that of the heterogeneous formation (e.g. Elder 2002). …”

Application of Fe-Cu/Biochar System for Chlorobenzene Remediation of Groundwater in Inhomogeneous Aquifers

“…The average residence time in the heterogeneous porous media was shorter compared to the homogeneous porous media. The contaminant's residence time is associated with the infiltration velocities of the homogeneous or heterogeneous porous media rather than the innate hydraulic conductivity of the PRB in homogeneous system A or the heterogeneous porous media in system B [38]. Additionally, in the heterogeneous porous media, a slight curvature of the path lines was generated near the edges of the reactive barrier, indicating a modestly higher gradient in this region.…”

“…Additionally, compared to the PRB fillings of the second columns, the natural aquifer materials were unlikely to have a significant impact on effluent concentrations because the hydraulic conductivity of the PRB is greater than that of the aquifer. Here, the PRB provides a preferential connection between the permeable units of the influent and effluent faces [38]. Nevertheless, the concentration changes in the effluents of the second columns between homogeneous system A and heterogeneous systems B were different.…”

Assessment of optimum width and longevity of a permeable reactive barrier installed in an acid sulfate soil terrain

“…Considering the time-dependent performance of a PRB with respect to 95 mineral fouling on reactive surfaces, as well as seasonal changes in the 96 hydraulic gradient and direction of flow, Elder et al (2002) used a SF of two. 97 Hemsi and Shackelford (2006) discuss the SF associated with variable flow 98 and aquifer heterogeneity in more detail.…”

“…There have been several past studies carried out to 83 optimise the barrier thickness or the width, in order to obtain the maximum 84 usage of a PRB configuration. Elder et al (2002) calculated the required 85 thickness using a one-dimensional plug-flow model with first order reactions 86 as given by: 87…”