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--•-a�sxlu.»rl}- f' ; Site°Background Information Former'Stockton Plating Facility ,Pa e 2`of 7 { { The following is a summary of milestone events at the site } I rY - • � In September 1991, six soil bonngs were drilled at the site {; ,_, On 9 and 10 February 1994;ground water monitoring wells MW71,MW-2 and MW-3 were ' installed at the site • In November 1994, ground water monitoring well MW-4 was installed at the site ' On 13 June 1995, three soil probe borings were advanced at the site On 14 and 15 June ' 1995„soil vapor samples were obtained from the site r • On 25 and 26 September 1995,four soil bonngs were dnlled at the site,bonngs VE-1, VE-2, } Y ' VE-3 and VE-4 were completed,as dual-screened soil vapor extraction*wells • On 24 April 1997, ground water monitoring well MW-5 was installed at the site • On 22 May 1998, four soil probe bonngs (ASB4 to AS137) were advanced at the site • On 04 November 1998, ground water monitoring well MW-6 was installed on-site 9 •, On 10, 11 and 12 January 2000 ground water monitoring wells MW-7, and MW-8 were installed on-site, after a failed sanitary seal (grout) had penetrated the transition seal ' ! (bentonite) and infiltrated the sand filter pack • To date, a total of 18 ground water monitoring events have occurred at the site I .r , a Assessment activities at the site to date indicate that I I } The vadose zone at the site is composed of fine`grairied sediments Below tlie'water table, ' the hydrologic structure is composed primarily of fine grained sediments with discontinuous sand units I II ' , • The majority of the,unsaturated, hydrocarbon-impacted soil resides below former UST TK1-90, to a depth of 30 feet below surface grade (bsg) Saturated hydrocarbon-impacted ' soil extends to a depth of 80 feet bsg, however, below 40 feet bsg, hydrocarbon concentrations generally decrease with depth The mass of hydrocarbons in the unsaturated ' soil is greater than the saturated soil (Figures 3a and 3b) f • The 'highest concentration of hydrocarbon-impacted ground water has remained below 'I t�UST,area ,former UST TK1-90 The maximum benzene concentration was 41,000 micrograms per liter ,(pg/l), detected in a water sample from vapor extraction well VW-1, located at the former 4 { r ,r , TPH-g and BTEX concentrations have steadily declined in the ground water samples , collected from the deep screened well MW-6 (screened 70 to 80 feet bsg), located at the ` a . former UST TK1-90 area, the benzene concentration detected on 28 January 2002, was , f s � � Advanced Geo Env}ronmental,Inc ' I , f r ,