Simulation of Two Strategies to Enhance Permeable Reactive Barriers in Heterogeneous Aquifer

Ground water flow (MODFLOW) and geochemical reactive transport models (RT3D) were used to assess the effectiveness of two strategies in limiting mineral fouling and its impact on hydraulic behavior of continuous-wall permeable reactive barriers (PRBs) employing granular zero valent iron (ZVI). A geochemical algorithm including kinetic expressions of oxidation-reduction and mineral precipitation-dissolution was developed for RT3D. The two strategies that were evaluated are (i) adding pea gravel equalization zones upgradient and down gradient of the reactive zone and (ii) placement of sacrificial pretreatment zones upgradient of the reactive zone. The PRB locates at a three-dimensional heterogeneous sandy aquifer. The sacrificial pretreatment zone contains mixtures of pea gravel and ZVI. Results of simulations show that installation of pea gravel zones provides a more conductive path for ground water flow through the ZVI, which enhances preferential flow and causes greater porosity reductions and shorter residence time in the PRB. After installation of pea gravel zones, the esidence time decreases which is caused by short travel distances in the ZVI due to short circuit of preferential flow. Sacrificial pretreatment zones can be used to elevate the ground water pH and consume many of the mineral forming ions to form secondary minerals in before the reactive zone is reached. The remaining mineral forming ions that pass into the reactive zone cause less mineral fouling. However, mineral fouling by Fe(OH)2 still occurs, and this mineral is formed regardless of the influent mineral forming ions. Addition of the sacrificial pretreatment zone slightly decreases the initial median residence time. However, the pretreatment zone retains higher residence time after 30 yrs due to less mineral fouling in the pure ZVI zone.