Impact of Solids Formation and Gas Production on the Permeability of ZVI PRBs

The permeability of zero-valent iron permeable reactive barriers (ZVI PRBs) may be reduced by the production of gas and solid precipitates. The reduction in permeability was examined using column experiments, which showed that permeability loss was correlated with influent oxidant concentration. The column containing 100 mg/L nitrate experienced the greatest loss, approximately two orders of magnitude over the course of 200 pore volumes. However, the permeability loss owing to precipitated solids was largely independent of oxidant concentration, accounting for only 24% of the observed loss in the 100 mg/L nitrate column, suggesting that the majority of loss was attributable to gas, not precipitates. Geochemical modeling corroborated these findings, indicating that precipitation of solids in the 100 mg/L nitrate system does not account for more than a 10% permeability reduction. These findings suggest that in field PRBs in which a high reduction in permeability is observed, gas production may be implicated. Design choices that impede gas migration out of ZVI PRBs, such as capping, must be considered in light of the possibility of a high potential for permeability reduction.