Gas and Sulfide Generation from Ground Water Interaction with an Iron Permeable Reactive Barrier, Fry Canyon, Utah

Permeable reactive barriers (PRBs) represent a viable tool for remediation of metal-contaminated groundwater associated with abandoned mine lands (AML) on private, State, and Federal properties. Because many AML sites throughout the world are in remote areas, low-maintenance remediation technologies, such as PRBs, are especially relevant to current and future remediation activities. Since 1997, the Fry Canyon site in southeastern Utah has functioned as a field-scale demonstration area to assess the feasibility of long-term treatment of U-contaminated groundwater with a zero-valent iron (ZVI) PRB. Periodic monitoring of the incoming and treated groundwater in the ZVI PRB since 1997 indicates virtually 100% removal of dissolved U from groundwater. Although U removal is high, concurrent geochemical reactions have also occurred in the PRB. Gas generation from groundwater interaction with the ZVI PRB was measured and simulated with the geochemical reaction path model PHREEQC. Due to anaerobic corrosion of Fe⁽⁰⁾ within the ZVI PRB, measured total dissolved gas pressure exceeded hydrostatic pressure, resulting in ebullition and depletion of dissolved noble gases. Geochemical modeling indicates that Fe⁽⁰⁾ corrosion coupled with CH_4(g) production yields the measured partial pressures of CH_4(g) within the ZVI PRB required to exceed the hydrostatic pressure. Decreases in Ne_(g) and Ar_(g) in groundwater from the ZVI PRB indicate that the over-pressuring from CH_4(g) production has been sufficient for bubble formation and subsequent ebullition. Since the installation of the ZVI PRB, increasing proportional amounts of influent sulfate have been removed from groundwater. The increased proportion of sulfate removal is probably the result of bacterially mediated sulfate reduction. Isotopic enrichment of stable isotopes of sulfur and oxygen in the sulfate anion from groundwater samples collected within the ZVI PRB support the presence of bacterially mediated sulfate reduction.