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. Appendix A - Site Background Information Former Stockton Plating Facility Page 2of7 The following is a summary of milestone events at the site • In September 1991, six soil borings were drilled at the site • On 9 and 10 February 1994, ground water monitoring wells MW-1,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 • On 25 and 26 September 1995,four soil borings were drilled at the site,bonngs VE-1, VE-2, 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 ASB7)were advanced at the site • On 04 November 1998, ground water monitoring well MW-6 was installed on-site • 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 Assessment activities at the site to date indicate that • The vadose zone at the site is composed of fine grained sediments Below the water table, the hydrologic structure is composed primarily of fine grained sediments with discontinuous sand units • 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) • The highest concentration of hydrocarbon-impacted ground water has remained below formerUST TKl-90 The maximum benzene concentration was 41,000 micrograms per liter (µg/1), detected in a water sample from vapor extraction well VW-1, located at the former UST area • 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), Iocated at the Advanced GeoEnvironmentaL Inc.