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plume approximately 400 feet long by 250 feet wide. Once the source is removed (which is the objective of AS/SVE), all that remains is dissolved-phase groundwater contamination. Fate and Transport The current fate of smear zone NAPL is stripping by AS/SVE and biodegradation through enhanced aeration of groundwater and soil vapor. In brief, the current fate mechanisms affecting dissolved-phase gasoline are hydrodynamic dispersion; dilution, adsorption, volatilization, and biodegradation through aerobic and anoxic processes. Groundwater movement is estimated to be at a rate of 5 to 10 feet per year horizontally and 1.0 foot per year, or less, vertically. An estimated 3 percent organic fraction in the A Zone will contribute. to retardation of contaminant migration. The estimated 160 pounds total mass of dissolved phase gasoline in the plume is based on the following assumptions. { + Soil porosity is 25 percent. + The contaminated groundwater zone is a triangularly shaped block 400 feet long by 250 feet wide by 60 feet deep. + The average concentration in the plume is 3.3 mg/L TPH(g) or '/, of the 10 mg/L TPH(g)concentration measured in MW-DIS-2. If the first order rate coefficient for biodegradation of BTEX constituents is assumed to be eo' between -0.01 and -0.001 per day, the initial. rate of BTEX removal by biodegradation is between 1.4 and 0.14 pounds per day. The removal rate is expected to decline asymptotically with concentration. Exposure Path Analysts The nearest groundwater receptors are deep water supply wells located more than 3,000 feet from the Disco site. Closed wells within a 3,000-foot radius include three Del Monte production wells and two California Water Company wells that are all screened below 250 feet or so, significantly deeper than the naturally confining A and --LB- onn the gasoline plume from the Disco site. Inexpensive domestic drinking water supplied by California Water Company and excessive depth to the water bearing formation drastically limit the potential.for new private water-supply wells. It is very unlikely that gasoline constituents from the Disco site could ever reach a drinking water source or a surface water course, unless a new well is constructed with a screen interval in the shallow A or B Zone. Soil contamination above the groundwater is limited to the area directly below the footprint of the former service station, as indicated by soil borings documented in the RDFS. NAPL gasoline staining of auger cuttings was first encountered at a depth of 30 feet directly below the former tanks and piping. Therefore, contact with contaminated soil is not likely unless a,_ deep excavation is,made in the immediate vicinity o f the former tanks No such activity is f planned or anticipated.... SF010030983.DOC 4-2