Williamson, D; Hoenke, K; Wyatt, J; Davis, A; Anderson, J
Over thirty years of handling and blending pesticides at the Marzone Superfund site, Tifton, Georgia, resulted in soil and groundwater contamination. Contaminants detected at the site include DDT, Toxaphene, Atrazine, Dieldrin, Chlordane, Lindane, alpha-BHC, Endosulfan, methyl parathion, xylene, and ethyl benzene. Faced with implementing a Superfund Record of Decision (ROD) specifying a pump-and-treat remedy for contaminated groundwater, Chevron Chemical Company worked with CH2M HILL to evaluate passive, in-situ groundwater treatment alternatives. A Funnel-and-Gate (F&G)-type passive system was selected as a viable alternative to pump-and-treat. The Marzone project team successfully designed, built, and started-up a full-scale F&G groundwater treatment system at the Marzone Superfund site to test its efficacy as a permanent groundwater remedy. Results from the first 1.5 years of operations indicate the system is successfully operating as intended. The F&G system has operated since August 1998, treating approximately 1 to 2 gallons of hydrocarbon- and pesticide-contaminated groundwater per minute. Total xylene concentrations are reduced from approximately 12,000 to 20,000 ug/L in the influent to less than 1 ug/L in the effluent. Total BHC concentrations are reduced from approximately 1 to 4 ug/L to less than 0.05 ug/L in the effluent. Other contaminants are similarly removed. EPA has developed a ROD Amendment to make this system the permanent groundwater remedy. The reactive media in the treatment gates is granular activated carbon (GAC). The gates are constructed of three subsurface, precast-concrete vessels which provide significant process flexibility. The system can operate in series, parallel, upflow, or downflow mode. The system is designed to operate in a low-flow, low-head aquifer. A process modification of the standard F&G approach, using a trench for contaminated groundwater collection, was selected to maximize hydraulic efficiency. A similar trench was used to distribute the treated groundwater downgradient of the cut-off wall. The "funnel" portion of the system was constructed using the vibrated beam method. A proprietary technology was used to install an impermeable 4-inch-thick cut-off wall similar to a conventional soil-bentonite (SB) slurry wall, but without the excessive site disturbance associated with SE wall construction. The biopolymer slurry method was used for construction of the collection and distribution trenches. After startup, gas accumulation within the system piping resulted in periodic flow stoppages. Vents were installed in the piping to allow the gas to escape. This enabled the system to operate more effectively and significantly decreased the frequency of flow stoppage incidents. The source of the gas was believed to be generated by microbiological activity in the groundwater system. The amount of gas accumulating in the system decreased significantly over the first 6 to 9 months. A significant excess of TOC (on the order of 100 mg/L), above that attributable to known contamination, was also noted initially and decreased with time. The large amount of biogas in the system and initial presence of high TOC were both eventually attributed to the use of guar as part of the biopolymer slurry construction method.