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Figures 6A, 613, and 6C, respectively These maps were constructed from the 3rd quarter 2004 groundwater monitoring and sampling event conducted on August 17, 2004 Historical groundwater analytical data is included in Appendix B Maximum historical constituent concentrations in groundwater were 166,000 ug/L TPHg (well MW-5 on November 13, 2002), ' 20,000 ug/L benzene (well TB-7 on May 14, 1996), and 40,400 ug/L MTBE (well MW-5 on November 13, 2002 by EPA Method 8260) The areal extent and core concentrations of the dissolved hydrocarbon plume have diminished significantly, due primarily to the SVE system ' operations Maximum dissolved hydrocarbon constituent concentrations as of August 17, 2004, were 3,240 ug/L TPHg (well MW-5), 3 3 ug/L benzene (well TB-10) and 3,650 ug/L MTBE (wellMW-5) The dissolved hydrocarbon plume extends approximately 115 feet downgradient ' (north) from the center of the source area(USTs near well MW-5), where a MTBE concentration of 36 ug/L was recorded during the August 17, 2004 sampling of well MW-3R The dissolved plume also extends approximately 41 feet east, 43 feet south, and 42 feet west of the center of the 1 source area(USTs) As reported by AGE (2004), TPHg was detected in groundwater in borings CPT-2, CPT-3, and CPT-4 at depth intervals of up to 107 to 111 feet bgs at a maximum concentration of 414 ug/L (CPT-2) AGE concluded that while the vertical extent of TPHg-impacted groundwater was apparently not defined, benzene and MTBE were not detected in groundwater below 65 to 70 ' feet bgs interval which is dust above the fine grained interval of 75 to 90 feet bgs 7.3 Screened Intervals and Vertical Distribution of Hydrocarbon in Soil Continuous well screen lengths extend kb 50 feet in wells TB-7 (25 to 75 feet bgs) and TB-10 (20 ' to 70 feet bgs) and 70 feet in well MW-5 (20 to 90 feet bgs) Wells TB-9, TB-11, and TB-12 also have a total of 50 feet of screen length, but are dual nested with two separate casing and screen intervals per well (Table 1) As shown on Table 1, well TB-10 is completed in a sand interval at 70 feet bgs and well TB-7 is completed in a sand interval that extends from 70 to 75 feet bgs Well MW-5 has a coarse-grained interval from 65 to 75 feet bgs, otherwise, the entire screen length penetrates fine-grained soil Hydrocarbons were not detected in soil in well TB-10 Soil analytical results indicate that a significant concentration of TPHg was detected at 20 feet bgs (5,300 mg/Kg) in well TB-7 with lower concentrations at 10 feet bgs (6 6 mg/Kg) and 60 feet bgs (14 mg/Kg) Hydrocarbon-impacted soil is present in well MW-5 from 30 feet bgs (2,000 mg/Kg) to 70 feet bgs (760 mg/Kg) 8.0 PLUME TRAVEL TIME The plume travel time for this site was estimated using BIOSCREEN Natural Attenuation Decision Support System BIOSCREEN is an easy to use screening model that simulates remediation through natural attenuation (RNA) of dissolved hydrocarbons at petroleum release sites The software, programmed in Microsoft Excel spreadsheet environment and based on the Domenico analytical solute transport model, has the ability to simulate advection, dispersion, adsorption, and aerobic decay as well as anaerobic reactions that have been shown to be the dominant biodegradation process at many petroleum release sites BIOSCREEN includes three different model types as shown on the following page 1 8