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f j CLEARWATER G R O U P, I N C. Environ mrrrtn! Sr ire. water table will mitigate the original concern of vapor screen occlusion, as the water table fluctuates from a depth of approximately 9 feet below ground surface (bgs) to 17 feet bgs. However, the water table is generally deeper than 12 feet bgs the majority of the year. All parties at the meeting tentatively agreed to this corrective action strategy. Accordingly, a brief revised corrective action plan is presented in this letter. Conceptual air sparge and soil vapor extraction air-flow paths and radii of influence are shown on the attached figures. Predicted flows, vacuums, and influent hydrocarbons anticipated at start-up are summarized in the attached table. The basis for much of the technical data discussed here can be located in Clearwater's Problem Assessment Report, Feasibility Study, and Remedial Action Plan, dated February 21, 1996. Air Sparging and Soil Vapor Extraction at a Low Water Table During periods of a low table (i.e. >12 feet bgs), Clearwater proposes to extract soil vapors from existing wells RW-1, MW-1, MW-2, VW-1, VW-2, and VW-3, and proposed well PVW-4 (which will be constructed the same as VW-3). These wells are located along the axis of the subsurface contaminant plume (Figure 1). The effective radii of influence of these wells should exert a significant vacuum over the majority of the contaminant plume (Figure 1). Clearwater proposes concurrent air sparging into well SW-1 to aid in the volatilization and removal of dissolved contaminants. Sparge well SW-2 will also be plumbed to the sparge manifold; however, sparging through this well will only be initiated if it appears the vapor recovery system can adequately capture additional air flow introduced through this well, as measured by vacuum pressure measured in MW-7. Conceptual air sparge and soil vapor extraction pathways for the vertical wells are shown on Figure 4. It is anticipated that vacuums ranging from approximately 30 inches of water column (" w.c.) to 50" w.c. will be applied to each vertical vapor well, based on the results of feasibility testing at the site performed by Clearwater and reported their Problem Assessment Report, Feasibility Study, and Remedial Action Plan, dated February 21, 1996. Vacuums will be adjusted based on water table elevation in order to prevent screen occlusion by upwelling, which will be most significant in those wells which penetrate the water table. The sum of flow from all six vertical wells will be approximately 90 to 160 cubic feet per minute (cfm) with an average influent concentration of total petroleum hydrocarbons as gasoline (TPHg) of approximately 14,000 parts per million by volume (ppmv) (approximately equal to 42,000 mg/m3) at start-up. Based these concentrations and flow rates, initial hydrocarbon recovery rates should range from approximately 338 and 600 pounds per day (lbs/day). Air will be sparged into SW-1 at a flow of approximately 10 cfm at 3 pounds per square inch (psi). More detailed information regarding air sparge and soil vapor extraction Ruiz.4491 W.Durham Ferry.Tracy.CA 2 November 12,1996