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THE APPLICATION OF PRB TECHNOLOGY AT TWO SITES: LESSONS LEARNED AFTER 7 YEARS OF PERFORMANCE MONITORING
Citation:
Puls*, R W. AND R T. Wilkin*. THE APPLICATION OF PRB TECHNOLOGY AT TWO SITES: LESSONS LEARNED AFTER 7 YEARS OF PERFORMANCE MONITORING. Air Force 2003 Technology Transfer Workshop, San Antonio, TX, 02/25-27/2003.
Description:
In June of 1996, a 46 m long, 7.3 m deep, and 0.6 m wide permeable reactive barrier (continuous wall configuration) of zero-valent iron was installed at the USCG-SC site. The reactive wall was designed to remediate hexavalent chromium-contaminated groundwater, in addition to treating portions of an overlapping, larger plume of trichloroethylene (TCE). A monitoring network of over 130 subsurface sampling points was installed in November of 1996 to provide detailed information on spatial and temporal changes in pore water geochemistry and hydrology.
In the fall of 1996, the Federal Highway Administration (FHWA) and General Services Administration (GSA) installed a permeable reactive barrier at the eastern edge of the DFC in Lakewood, Colorado to treat a contaminant plume containing volatile organic compounds, primarily TCE, cis-DCE, and TCA. In contrast to the continuous wall design used at the USCG-SC site, the DFC PRB has a funnel-and-gate design configuration. The funnel component of the PRB employs metal sheet pile that was driven into unweathered bedrock or into resistant, weathered layers of the local bedrock. The DFC PRB has 4 reactive gates, each 12.2 m long, 7.5 to 9.5 m deep, and from 0.6 m (Gate 3 and 4 ) to 1.8 m (Gate 1) wide. The design thickness varied because of anticipated differences in contaminant fluxes to the PRB at different locations along the plume front.
A comparison of groundwater chemistry between upgradient and downgradient wells indicates that the iron zones at Elizabeth City and the DFC are long-term sinks for C, S, Ca, Si, Mg, N, and Mn. Solid phase characterization studies indicate average rates of inorganic carbon and sulfur accumulation of ~0.1 and ~0.05 kg/m2y at Elizabeth City where upgradient waters contain up to 400 mg/L total dissolved solids (TDS). At the Denver Federal Center site, upgradient groundwater contains up to 1100 mg/L TDS and rates of inorganic carbon and sulfur accumulation are as high as ~2 and ~0.8 kg/m2y, respectively. At both sites, consistent patterns of spatially heterogeneous mineral precipitation and microbial activity are observed. Mineral precipitates and microbial biomass accumulate the fastest near the upgradient aquifer-Fe0 interface. Porosity loss in the iron zones due to precipitation of inorganic carbon and sulfur minerals was 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. The highest concentrations of mineral precipitates and rates of porosity loss are found adjacent to upgradient interfaces. At Elizabeth City, a maximum of 5.9% loss of the initial available volume (50%) is estimated at 2.5 cm into the iron media after 4 years. At distances >8 cm, volume loss decreases significantly to <0.1% of the initial available volume. In Gate 1 of the DFC, the precipitation front is spread out over a greater distance, which may be the result of higher average flow rates in Gate 1 (0.38 m/d) compared to Elizabeth City (0.15 m/d). In Gate 2 of the DFC, the annual porosity loss in the first 2.5 cm of iron is equivalent to about 14.2% of the available porosity filled-in from 1996 to 2000. This anomalous buildup of mineral precipitates and microbial biomass was identified during this study and may be a function of low flow and ground water geochemistry. Concentrations of inorganic carbon in DFC Gate 2 are as high as 8000 mg/kg and total sulfur values approach 4500 mg/kg, or about a factor of about 4x the maximum amounts observed in DFC Gate 1, DFC Gate 3, and the Elizabeth City PRB.
Success of the Elizabeth City zero-valent iron PRB for treating a hexavalent chromium plume over its first six+ years correlates with consistent patterns in the commonly measured field parameters (pH, specific conductance, and Eh). At this site, six + years appears to be too short a period of time to observe a clear correlation between changes in geochemical parameters and declining performance. At the Denver Federal Center, successful performance in Gate 1 and Gate 3 is also reflected in long-term patterns of pH, specific conductance, and Eh. At Elizabeth City and the Denver Federal Center, regions of high pH do not necessarily correlate with subsurface regions of low Eh. 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.
At Elizabeth City, concentrations of chromium >5 mg/L have not been observed in any of the downgradient compliance wells since November 1996. Similarly concentrations of chlorinated organic compounds (TCE, cis-DCE, vinyl chloride) at Elizabeth City are below regulatory target levels in downgradient compliance wells. At the DFC, treatment of chlorinated compounds with Fe0 has been equally successful in Gates 1 and 3. In Gate 2 of the DFC, detection of 1,1-DCE at downgradient sampling points has been linked to impacts of the funnel-and-gate system on groundwater flow and/or perhaps residual contamination in downgradient sediments. Contaminant breakthrough, particularly of 1,1-DCE is likely related, either directly or indirectly, to anomalous build-up of authigenic precipitates and biomass on the reactive iron surfaces, which would lead to decreases in the hydraulic conductivity of the reactive media, the development of zones of preferential water movement, and a consequent decrease of the residence time of contaminants in the reactive media. In addition to mineral/biomass accumulation in Gate 2, potential indicators of decreased performance are increased Eh values, decreased dissolved hydrogen values, and increases in relative specific conductance values.
Water level surveys provide information on groundwater gradients and capture zones of PRBs. Water level surveys were regularly conducted at the Elizabeth City site on a quarterly basis. Inspection of the water level maps show that the primary flow direction is across the PRB. In general, the hydraulic gradient in June varies from about 0.001 to 0.004 and this range further captures annual variability in the hydraulic gradient at this site. Ground water flow direction varies about 15 degrees and is a generally in a northerly direction. This variation in hydraulic properties was taken into account in PRB design and the chromate shop plume is captured by the wall. Some underflow of an adjacent chlorinated solvent plume has been observed in the central portion of the wall. This may have been caused by disturbance of an unrecognized source near the wall and/or a large influx of recharge water into the aquifer near the wall following installation and prior to repaving of the parking lot covering the site.
Installation of the funnel-and-gate system at the DFC resulted in the mounding of groundwater on the upgradient side of the sheet pile due to insufficient flow through the system. The buildup of groundwater on the upgradient side of the PRB raised concerns about the increased potential for groundwater bypass, either flow under, over, or around the PRB. Follow-up studies suggest that underflow and overflow are not occurring, but that some bypass occurs around the southern side of gate 1. The head differential across gate 2 has averaged approximately 7 ft. It is believed that the head differential in gate 2 is a result of a low permeability zone that was produced by backfilling pre-excavation trenches with muddy material. Pre-excavation was required in order to install the sheet pile. Alternatively, a smear zone of fine materials could have resulted as a consequence of the installation and removal of sheet piling installed for gate construction. In either case, flow velocity through gate 2 is reduced compared to that through gate 1.