Impact of Mineral Fouling on the Long-Term Performance of Permeable Reactive Barriers

A modeling study was conducted to investigate the impact of mineral fouling on the long-term performance of permeable reactive barriers (PRBs) using zero-valent iron. MODFLOW was used to simulate flow in an aquifer containing a PRB. RT3D was used to simulate geochemical reactions and to predict mineral fouling in the PRB. A geochemical algorithm including kinetic expressions of oxidation-reduction and mineral precipitation-dissolution was developed for RT3D. Predictions made with the model after calibration were in general agreement with field measurements reported for PRBs at Moffett Field, CA and Elizabeth City, NC. Mineral precipitation and related porosity reductions were simulated for different scenarios. Porosity reduction in PRB is found to be spatially variable as a result of flow heterogeneity. The largest porosity reductions occur between the entrance face and the mid-plane. Carbonate minerals precipitate in this region, and the amount of precipitated carbonates is closely related to the Darcy velocities. The relationship between mineral precipitation and Darcy velocity reflects the balance between the rate of mass transport and the geochemical reaction rates. Porosity reductions decrease and then level out as carbonates precipitates in front half of the PRB. Porosity reductions in the rear half of the PRB are primarily due to precipitation of ferrous hydroxide, and are not related to the Darcy velocity. The analysis shows that reduction of hydraulic conductivity and porosity by mineral fouling causes seepage velocities through the PRB to increase over time, which reduces the residence time. Shorter residence times make a PRB less effective because less time is available for contaminants to be treated. When precipitation in the reactive medium is so extensive that very large reductions in hydraulic conductivity occur in the PRB, changes in the flow paths occur, along with flow bypassing. Both the reduction in residence times caused by mineral fouling and bypassing may have a significant impact on the long-term effectiveness of PRBs.