ASSESSMENT OF HYDROLOGIC AND BIOGEOCHEMICAL CHANGES DURING THE AGING OF A ZERO-VALENT IRON PERMEABLE REACTIVE BARRIER

Zero-valent iron permeable reactive barriers (ZVI PRBs) may represent a cost-effective technology for removal of uranium from contaminated ground water; however, little is known about the effects of aging on the hydrology and biogeochemistry of ZVI PRBs. Long-term hydrologic and biogeochemical data have been collected from a ZVI PRB installed in a U-contaminated aquifer near Fry Canyon, Utah, and operated continuously from September 1997 to present (2005). Tracer tests conducted during 1999, 2003, and 2005 indicate a substantial decrease in ground-water velocities from an average of 1.54 meters/day (m/d) in 1999 to 0.95 m/d in 2003 and 0.4 m/d in 2005. The decrease in flow velocities is accompanied by a slight mounding of ground water in the ZVI PRB indicated by increased water-level ratios measured in upgradient/downgradient well pairs. Increased amounts of carbonate and sulfide mineral precipitation may be causing the observed reduction in flow velocities. Dissolved calcium (Ca) removal ratios (1-Ca(input)/Ca(treated)) have steadily increased from 0.4 in 1997 to near 1.0 in 2004, suggesting increasing amounts of carbonate precipitation. Consistent with the increased amounts of Ca removal is the observed increase in pH and calcite saturation index of the treated water. The pH increased from a mean of 8.5 units between 1997 and 2000 to more than 10.0 between 2003 and 2005. An increased amount of sulfate reduction and likely sulfide precipitation has also occurred during PRB aging, as indicated by decreasing concentrations of ferrous iron and sulfate, and increasing concentrations of dissolved sulfide. For example, ferrous iron concentration ranged from 1,000 to 20,000 micrograms per liter (ug/L) during 1997 to 2000, decreasing to less than 30 ug/L during 2003 to 2005. Increased rates of bacterially mediated sulfate reduction are also supported by an increase in the enrichment of the delta sulfur-34 of dissolved sulfate in the treated water from approximately 12 permil in 1999-2000 to more than 18 permil in 2004. Results from sequencing of DNA extracted from water-suspended sediments indicate that approximately 30 percent of the microbial community in the ZVI PRB were found to be related to sulphate-reducing Desulfotomaculum species, consistent with the delta sulfur-34 results.