Effects of aquifer heterogeneity and reaction mechanism uncertainty on a reactive barrier.

This paper addresses impacts of aquifer heterogeneity and reaction mechanism uncertainty on permeable reactive barrier (PRB) performance and describes modeling tools and preliminary guidelines for risk-based design of reactive barriers at heterogeneous sites. A braided stream aquifer was generated stochastically, using a fixed correlation structure and four levels of variability in the hydraulic conductivity field. A vertical, homogeneous barrier was placed in the aquifer. Based on a deterministic design, the size of the PRB for uniform conditions was considered conservative (factor of safety=3.3). Monte Carlo simulation was used to model cis 1,2-DCE reduction by iron metal with uncertainty in the reaction mechanism rate constants. These results were combined with flow and particle tracking results to predict the spatial distribution and flow-averaged concentrations of cis 1,2-DCE and vinyl chloride at the exit face of the PRB. Evaluated on a risk basis, the deterministic design method was found to be unconservative for more heterogeneous aquifers. Uncertainty in the reaction mechanism accentuated the negative effects of aquifer heterogeneity. Several compensating factors that may reduce the vulnerability of reactive barriers to aquifer heterogeneity are discussed.