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:y Remedial Action Plan Frontier Transportation June 17, 2004 Page 8 [ l concentrations have fluctuated from a low of 100 µg/L to 9,750 µg/l, identified in the last ! I groundwater sample collected from MW-3 prior to abandonment. Dissolved TPH-g was identified in monitoring well MW-3R as 3 ,790 gg/L in October 2002. In March 2004, no l detectable concentration of dissolved TPH-g was identified in MW-3R. Dissolved TPH- J g concentrations in MW- 1 have attenuated from 1 ,800 µg/l, in August 1996 to <50 µg/L since January 2000 and has remained <50 µg/L. The fluctuation in dissolved TPH-g in , [ .� the area described by these wells supports that dissolved plume migration is occurring S beneath the site. IJ Historically concentrations of dissolved BTEX above maximum contaminant levels (MCLs) set by the California Department of Health Services were identified in F- monitoring wells MW- 1 , MW-2, MW-3 , MW-3R, MW-4, and MW- 13 . 'Maximum dissolved BTEX concentrations were 302.6 µg/L, 145 . 1 µg/L, 8,374 µg/L, 846 . 7 µg/L, 6 µg/L, and 62 µg/L, respectively. Dissolved BTEX concentrationsmonitoring wells I MW- 1 , MW-2, and MW-4 have remained non detect since at least Tuly of 1999 . Only Jmonitoring well MW-3, which was replaced by MW-3R, contained dissolved BTEX from every groundwater sampling episode. The identification of dissolved BTEX in the area described by these wells further supports migration of a dissolved plume beneath the site. Historically concentrations of dissolved MTBE above its MCL of 13 µg/l, set by the California Department Health Services were identified in monitoring wells MW4 , MW-3 , MW-3R, MW- .�9-13, MW44 and MW47 . Maximum dissolved MTBE l concentrations were 5,800 µg/L, 29,400 µg/L, 5,500 µg/L, 42 µg/L, 46 µg/L, 286 µg/l, and 99 µg/L, respectively. A maximum concentration of dissolved MTBE in MW-3 was detected using EPA method 8020, which is not a quantitative method for detecting MTBE. Dissolved MTBE concentrations in monitoring wells MW-4 and MW- 13 have -- remained non detect since at least July of 2002. However, monitoring well MW-3, which was replaced by MW-3R, contained dissolved MTBE from every groundwater sampling I l episode 'again supporting migration of dissolved MTBE and gasoline constituents l resulting from significant concentrations in the soil under the shallow first water under the site. i i Laboratory analysis indicates that historically the dissolved MTBE plume has been for the most part contained within the site boundaries . Please see Figure 7 for 1 estimated plume extent using the maximum dissolved MTBE concentrations from all J sampling episodes )d katuvdF'" The impacted groundwater region starts at approx�ifnately 9' bgs and ends at S �` approximately 17' bg's (assumption based on soil samples collected every five feet). The estimated area of impacted groundwater was defined by existing groundwater sample results . The estimated mass of dissolved petroleum hydrocarbons were calculated as follows : '�J '_l