|Capstone Report on the Application, Monitoring, and Performance of Permeable Reactive Barriers for Ground Water Remediation
This report describes research on the geochemical and microbiological processes within zero-valent iron permeable reactive barriers (PRBs) that may contribute to changes in iron reactivity and decreases in reaction zone permeability over time. Two full-scale PRBs were evaluated in this study: the U.S. Coast Guard Support Center PRB located near Elizabeth City, North Carolina, and the Denver Federal Center PRB in Lakewood, Colorado.
Detailed water sampling and analysis, core sampling, and solid-phase characterization studies were carried out to:
Trends in geochemical parameters (e.g., pH and oxidation reduction potential) may signal changes in system performance, but no clear correlations between these parameters and decreased system performance have been observed to date at the sites studied. Long-term trends in geochemical parameters are consistent with contaminant removal trends observed at both sites.
Spatial and temporal variations in the concentration distribution of terminal electron-accepting species (e.g., sulfate), specific conductance, and Eh suggest that both anaerobic iron corrosion and microbial activity play important roles in controlling the oxidation reduction potential in iron barriers. Low Eh values (≤100 millivolts) relative to the standard hydrogen electrode and decreases in the specific conductance of ground water between upgradient contaminant plumes and sampling points within reactive iron media are consistently observed in normally operating PRB systems. The rate of mineral and biomass buildup was evaluated at both sites. The principal factors that determine the amount of mineral precipitation in zero-valent iron PRBs are flow rate, ground water chemistry, and microbial activity.
After five years of operation, the Elizabeth City and Denver Federal Center PRBs have developed consistent patterns of spatially variable mineral precipitation and microbial activity. The development of precipitation and biomass fronts result from abrupt geochemical changes that occur at upgradient interface regions, coupled with ground water solute transport. Upgradient regions at both sites have the greatest accumulation of mineral mass and biomass. However, neither of the sites shows complete filling of available pore space after five years, suggesting that flow characteristics should not be affected by the accumulation of authigenic components.
For zero-valent iron systems, the reactive medium is a long-term sink for carbon, sulfur, calcium, silicon, magnesium, and nitrogen.
Porosity loss in the iron media due to precipitation of inorganic carbon and sulfur minerals can be estimated by integrating the concentrations of inorganic carbon and sulfur as a function of distance in the iron and estimating the volume loss by using the molar volumes of zero-valent iron, calcium carbonate, iron carbonate, and iron sulfide. Porosity loss estimates have ranged from about 1 percent to 4 percent per year in this study. Based on these estimates, the average porosity of the PRB at Elizabeth City, for example, would not be expected to approach that of the surrounding aquifer for 15 to 30 years.
As corrosion minerals form on the surface of the iron media, reactive surfaces are coated, presumably decreasing the effective reactive surface area. However, corrosion products formed include some minerals that are highly reactive and capable of transforming inorganic and organic contaminants into immobile or nontoxic forms. This phenomenon must also be factored into lifetime projections.
While long-term performance observations of the Elizabeth City and Denver Federal Center sites are now more than five years old, there has still not been sufficient time to adequately predict the lifetime of these or most other PRBs. It is clear that lifetimes exceeding 10 years are reasonable to expect under some conditions and that PRBs may function adequately for much longer. Continued studies are needed to better predict longevity based on ground water composition, flow rate, and contaminant flux.
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