Influence of hydrogeochemical processes on zero-valent iron reactive barrier performance : A field investigation

Abstract Geochemical and mineralogical changes were evaluated at a field Fe 0 -PRB at the Oak Ridge Y-12 site concerning operation performance during the treatment of U in high NO 3 − groundwater. In the 5-year study period, the Fe 0 remained reactive as shown in pore-water monitoring data, where increases in pH and the removal of certain ionic species persisted. However, coring revealed varying degrees of cementation. After 3.8-year treatment, porosity reduction of up to 41.7% was obtained from mineralogical analysis on core samples collected at the upgradient gravel–Fe 0 interface. Elsewhere, Fe 0 filings were loose with some cementation. Fe 0 corrosion and pore volume reduction at this site are more severe due to the presence of NO 3 − at a high level. Tracer tests indicate that hydraulic performance deteriorated: the flow distribution was heterogeneous and under the influence of interfacial cementation a large portion of water was diverted around the Fe 0 and transported outside the PRB. Based on the equilibrium reductions of NO 3 − and SO 4 2− by Fe 0 and mineral precipitation, geochemical modeling predicted a maximum of 49% porosity loss for 5 years of operation. Additionally, modeling showed a spatial distribution of mineral precipitate volumes, with the maximum advancing from the interface toward downgradient with time. This study suggests that water quality monitoring, coupled with hydraulic monitoring and geochemical modeling, can provide a low-cost method for assessing PRB performance.