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INFLUENCE OF GROUNDWATER GEOCHEMISTRY ON THE LONG-TERM PERFORMANCE OF IN-SITU PERMEABLE REACTIVE BARRIERS CONTAINING ZERO-VALENT IRON
Citation:
Wilkin*, R T., F A. Khan*, AND R W. Puls*. INFLUENCE OF GROUNDWATER GEOCHEMISTRY ON THE LONG-TERM PERFORMANCE OF IN-SITU PERMEABLE REACTIVE BARRIERS CONTAINING ZERO-VALENT IRON. Presented at GSA Annual Meeting, Reno, NV, 11/9-18/2000.
Description:
Reactive barriers that couple subsurface fluid flow with a passive chemical treatment zone are emerging, cost effective approaches for in-situ remediation of contaminated groundwater. Factors such as the build-up of surface precipitates, bio-fouling, and changes in subsurface transport control long-term performance of such Permeable Reactive Barriers. Surface precipitates can impact long-term performance by decreasing iron reactivity, and by decreasing pore space/permeability that may result in detrimental rerouting of subsurface flow paths. Reactive barriers containing zero-valent iron alter ambient groundwater chemistry by increasing pH, PH2, and decreasing the oxidation-reduction potential. Depending on the composition and oxidation state of ambient groundwater, these conditions can favor the precipitation of mixed iron oxyhydroxides, Ca-Fe carbonates, magnetite, or mixed valence Fe(II)/Fe(III) ternary compounds (such as green rusts). In sulfate-rich groundwaters, reducing conditions promote microbial sulfate reduction and the precipitation of iron monosulfides and subsequent transformation to pyrite. We present results of long-term monitoring studies (groundwater chemistry and soil/iron characterization) from PRBs at the U. S. Coast Guard Base (Elizabeth City, NC) and the Denver Federal Center (Lakewood, CO), each with contrasting groundwater compositions. At both sites, reactive barriers have been in place for about 4 years. We present a comparison of groundwater equilibrium modeling with the results of mineralogical characterization using microscopy, XPS, and extraction studies.