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FORMER AMERICAN FOREST PRODUCTS 2.0 Background Closure Summary Report July 2009 URS Project 17322736 Page 2-2 the western side of the building were taken out of service. In 1987, Kleinfelder Inc. conducted a soil and groundwater investigation(Kleinfelder, 1987). The purpose of the investigation was to further characterize the extent and type of petroleum hydrocarbons identified in the maintenance shop soils and to assess any consequent groundwater contamination. Three groundwater-monitoring wells,MW-I,MW- 2, and MW-3, were installed by Kleinfelder in June 1987. In 1988 the DHS assigned the SJCEHD as the oversight agency for the maintenance shop area. In April of that year AFP employed Dames &Moore(D&M)to review existing data and perform supplementary site investigation in the maintenance shop area. In May 1988 the 10,000 gallon diesel and 500 gallon waste oil tanks were removed by D&M under supervision of the SJCEHD in 1988. A Feasibility Study/Remedial Action Plan(FS/RAP) was completed by Dames & Moore in 1988 (D&M, 1988). The FS/RAP recommended the demolition of the shop to access impacted soil and excavation of soil with TPH-d concentrations greater than 10,000 milligrams per kilogram (mg/kg). Conditional approval of the FS/RAP was provided by the SJCEHD in a November 1988 letter. Comments . in the letter required in addition that three on-site production wells and three groundwater monitoring wells be sampled, that vicinity production wells be identified, and that well placements be re-assessed. A revised work plan was provided by D&M in January 1989 (Dames &Moore, 1989). In February 1990 the RWQCB agreed with the proposed plan of excavation and on-site treatment, combined with long-term groundwater monitoring. The RWQCB required preparation and submittal of a detailed work plan for soil removal and the long-term monitoring plan, including new monitoring wells. Dames & Moore submitted the work plan in April 1990 (Dames & Moore, 1990). The maintenance shop demolition and soil excavation was carried out in mid-1990. Excavations were completed to 22 feet below grade at the former 10,000-gallon diesel tank and to 26 feet below grade at the location of the former 500-g4llon_waste-oil, tank. The soil was treated in an on-site bioremediation cell,the excavation was backfilled with clean silty �. sand from a local quarry, and then capped with asphalt. Three dry monitoring wells were abandoned, and wells DMW-1 through DMW-4 were installed. A series of groundwater investigations, resulting in a total of 14 groundwater monitoring wells, has established the location of the former waste oil UST (Tank 1882005)as the source of groundwater contaminants. A series of closure request reports(Dames & Moore, 1994; URS, 2001 and 2002a) demonstrated that hydrocarbon impacts at the site have been adequately characterized by a series of " investigations, beginning in 1985. However the solvent,1,2-DCA,remains as the site's constituent of concern. An investigation to assess the lateral and vertical extent of 1,2-DCA was initiated in 2002 and completed in 2005 (URS, 2002, URS, 2005). Data obtained during the investigation lead to installations of groundwater monitoring wells in 2005 (URS, 2005, LTRS, 2006). In 2006, URS provided a RWQCB �.. Tier I environmental risk assessment using 2005 maximum 1,2-DCA concentrations. The results of that risk assessment indicated no indoor air risk due to the presence of 1,2-DCA in groundwater. In the spring of 2008 five additional groundwater monitoring wells were installed with the objective of delineating a "zero-line" for 1,2-DCA downgradient of the former tank location on the east end of the foriner maintenance shop (URS, 2008). By the first quarter 2009 four rounds of groundwater monitoring had been completed subsequent to the installation of the additional wells. That report.(URS, 2009) recommended that the site be considered for closure based on the four quarters of data showing a predictive trend of 1,2-DCA concentrations for DMW-6 and 9, and favorable delineation laterally and vertically. In commenting on the recommendation, the SJCEHD agreed and requested that a closure summary report be prepared. K:`.V mcess?-- 6`.An—i-1 Forest•(insure-Summary-kepn7-key-Gl.doc