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ifig Work Plan Frontier Transportation =1 1099466 ! ;) October 27, 1999 In January of 1998, BEII advanced four (4) groundwater monitoring wells (MW4 ! ' through MW-7) to assess the extent of subsurface contamination beneath the site (See Figure 2, Site Layout Map). Soil samples were collected every five feet. Laboratory analysis of soil -� samples identified no detectable concentrations of TPH-g, BTEX, or MTBE from 5 ' bg to 25 ' bg. Groundwater samples were subsequently collected. Results are tabulated in Table 1 . 3.0) SITE GEOLOGY/HYDROGEOLOGY The site is located on the boundary of the Sacramento Valley and the San Joaquin Valley l within the west-central portion of the Great Valley Geomorphic province of California. The I Great Valley has been filled with a sequence of older to younger alluvium of Pliocene to Holocene age which overlay sedimentary rocks of Cretaceous to Tertiary age. These sedimentary units, in turn, overlay a crystalline basement of Paleozoic and Mesozoic metamorphic and igneous rocks. The shallow subsurface geology in the site vicinity is a heterogeneous mix of gravel, sand, silt, and clay (Reference; Site Characterization Report, Twining Laboratories, October 16, 1996). BEII encountered clay and a heterogeneous mix of clay from 0' to 25' bg in January of 1998. -J First subsurface water was encountered at depths from 11 ' to 12' bg during tank removal and monitoring well installation at this site in 1995 and 1996 . Since then, shallow subsurface water has been encountered between 6' and 10' bg. Data collected in July of 1999 identified the l flow of groundwater in a generally western direction beneath the site (See Figure 3 , Groundwater Gradient Map), 4.0) GROUNDWATER SAMPLE RESULTS Groundwater collected from all monitoring wells identified a maximum dissolved TPH-m I concentration of only 260 µg/L using EPA method 8015 since initially tested. Dissolved TPH-d - I concentrations have attenuated to 260 µg/L from a maximum of 1 , 800 µg/L as TPH-d exclusively identified in April 1997 using EPA method 8015 . In addition, laboratory analytical results from j groundwater samples collected in MW-4, MW-5, MW-6, and MW-7 identified no detectable dissolved concentrations of TPH-g above 110 µg/L since initially tested using EPA method 8015 _. modified. Groundwater collected from monitoring wells MW- 1 , MW-2, and MW-3 identified i_ attenuation in TPH-g concentrations since initially tested. TPH-g concentrations in MW- 1 decrease from 1 , 800 µg/L in August 1996 to 75 µg2 in July 1999. Monitoring wells MW-2 and MW-3 also identified decreases in TPH-g concentrations from 850 µg/L in August 1996 to non- detect in July 1999 and from 8,000 µg/L in August 1996 to 100 µg(L in July 1999, respectively. Please see Figure 4, for recent TPH-g isoconcentration map. No detectable concentrations of BTEX were identified in groundwater samples collected 2 i-