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Additional 1,2-DCA(Investigation attt- "ell Survey, Former American Forest Products Faciity Table of Contents URS Project No. 17322736 6/1312005 Page 1-1 1 1.0 INTRODUCTION ` This work plan documents procedures for an additional investigation of 1,2-dichloroethane (1,2-DCA) impacts in groundwater at the.former American Forest Products Company LP - p y (AFPC LP) facility (Site) in Stockton, California (Figures 1 and 2). The principal purpose of the investigation is to provide data to define the downgradient lateral extent of 1,2-DCA in groundwater to enable site closure to proceed. 1.1 PROJECT BACKGROUND Two underground'storage tanks at the site were excavated and removed in 1988. Associated piping was excavated and removed from the site in 1991. Soil impacts were partially remediated in 1990. A series of closure request reports (Dames & Moore, 1994; URS, 2001; and 2002a) have demonstrated that hydrocarbon impacts at the site have been adequately characterized by a series"of investigations, beginning in 1985. Following a Stratoprobe TM groundwater investigation in July 2002, and subsequent to installation of monitoring wells DWM-8 and DMW-9 downgradient of the source area, four quarters of groundwater fmonitoring data were collected and assessed (URS, 2005).These data showed that: • The site groundwater gradient remains consistently to the northwest • Totalpetroleum hydrocarbons(TPH)have not been reported since June 2004. • 1,2-DCA is reported in DMW-9, the shallower of the two downgradient wells, at concentrations of 10 to 31 micrograms per liter (µg/L}. DMW-9 is screened between 60 and 70 feet below ground surface (bgs). • The lack of detections of 1,2-DCA in DMW-8 compared to the shallower well DMW-9 suggests a lack of vertical migration of that constituent. " The trend of concentrations of 1,2-DCA is either level or downward for all four wells in which'the constituent is reported. Based on the demonstrated low concentrations and stability, URS requested closure..However, in their letter of May 5, 2005, the San Joaquin County Environmental Health Department (SJC-EHD) indicated that remaining data gaps included definition of the downgradient lateral extent of the 1,2-DCA plume, using direct-push sampling and an onsite laboratory, and preparation of an adequate well survey to identify potential receptors in the vicinity. e K_lwprocess\22736\American Forestlwork Plan 20051work Plan_2005_revOl.doc r i