Reactive transport modeling of an in situ reactive barrier for the treatment of hexavalent chromium and trichloroethylene in groundwater

Multicomponent reactive transport modeling was conducted for the permeable reactive barrier at the Coast Guard Support Center near Elizabeth City, North Carolina. The zero-valent iron barrier was installed to treat groundwater contaminated by hexavalent chromium and chlorinated solvents. The simulations were performed using the reactive transport model MIN3P, applied to an existing site-specific conceptual model. Reaction processes controlling the geochemical evolution within and down gradient of the barrier were considered. Within the barrier, the treatment of the contaminants, the reduction of other electron acceptors present in the ambient groundwater, microbially mediated sulfate reduction, the precipitation of secondary minerals, and degassing of hydrogen gas were included. Down gradient of the barrier, water-rock interactions between the highly alkaline and reducing pore water emanating from the barrier and the aquifer material were considered. The model results illustrate removal of Cr(VI) and the chlorinated solvents by the reactive barrier and highlight that reactions other than the remediation reactions most significantly affect the water chemistry in the barrier. In particular, sulfate reduction and iron corrosion by water control the evolution of the pore water while passing through the treatment system. The simulation results indicate that secondary mineral formation has the potential to decrease the porosity in the barrier over the long term and illustrate that the precipitation of minerals is concentrated in the upgradient portion of the barrier. Two-dimensional simulations demonstrate how preferential flow can affect the reduction of electron acceptors, the consumption of the treatment material, and the formation of secondary minerals. In addition, the model results indicate that deprotonation and the adsorption of cations down gradient of the barrier can potentially explain the observed pH buffering.