[1] The influence of heterogeneity in aquifer hydraulic conductivity (K) on contaminant plume patterns and the required thickness and length of permeable reactive barriers (PRBs) used for in situ remediation is evaluated using stochastic modeling. The results provide a quantitative means for evaluating the effects of (1) the level of aquifer heterogeneity as reflected by the standard deviation of the logarithm of K, s lnK , (2) the aquifer correlation structure anisotropy represented by the ratio of correlation lengths, l x /l y , and (3) D PRB representing the distance from the contaminant source zone to the PRB. In terms of PRB thickness, a probabilistic factor of safety related to uncertainty in influent groundwater seepage velocities (FS 1,90 ) at the location of the PRB is quantified. In terms of PRB length, a probabilistic factor of safety related to uncertainty in the length of a PRB required to capture the contaminant plume, defined as the capture length ratio (CLR), is quantified. The mean and standard deviation of FS 1,90 significantly increase as s lnK increases from 0.2 to 1.6, and slightly increase as l x /l y increases from 1.0 to 3.0 and D PRB increases from 15 to 45 m. The values for the factor of safety for PRB thickness versus s lnK compare favorably with previously published values based on a different methodology. The mean and standard deviation of CLR increase with increasing s lnK and with increasing D PRB , and decrease slightly with increasing l x /l y . Finally, the ranges in CLR are correlated with strongly divergent and strongly convergent plume patterns.Citation: Hemsi, P. S., and C. D. Shackelford (2006), An evaluation of the influence of aquifer heterogeneity on permeable reactive barrier design, Water Resour. Res., 42, W03402,
The influence of decomposing organic solids on sulfate (S04(2-)) reduction rates for metals precipitation in sulfate-reducing systems, such as in bioreactors and permeable reactive barriers for treatment of acid mine drainage, is modeled. The results are evaluated by comparing the model simulations with published experimental data for two single-substrate and two multiple-substrate batch equilibrium experiments. The comparisons are based on the temporal trends in SO4(2-), ferrous iron (Fe2+), and hydrogen sulfide (H2S) concentrations, as well as on rates of sulfate reduction. The temporal behaviors of organic solid materials, dissolved organic substrates, and different bacterial populations also are simulated. The simulated results using Contois kinetics for polysaccharide decomposition, Monod kinetics for lactate-based sulfate reduction, instantaneous or kinetically controlled precipitation of ferrous iron mono-sulfide (FeS), and partial volatilization of H2S to the gas phase compare favorably with the experimental data. When Contois kinetics of polysaccharide decomposition is replaced by first-order kinetics to simulate one of the single-substrate batch experiments, a comparatively poorer approximation of the rates of sulfate reduction is obtained. The effect of sewage sludge in boosting the short-term rate of sulfate reduction in one of the multiple-substrate experiments also is approximated reasonably well. The results illustrate the importance of the type of kinetics used to describe the decomposition of organic solids on metals precipitation in sulfate-reducing systems as well as the potential application of the model as a predictive tool for assisting in the design of similar biochemical systems.
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