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Closure Request Addendum Report 1,2-DCA Investigation July 22,2002 - Former American Forest Products Site,Stockton,California Page 1 F'rr 1.0 INTRODUCTION _� This report documents an investigation of 1,2-dichloroethane(1,2-DCA)impacts in groundwater at the :., Former American Forest Products Company(AFPC) site(site), in Stockton, California(Figures 1 and 2). The primary objective of this investigation is to assess the extent and degree of 1,2-DCA impacts in groundwater in an area downgradient of where 1,2-DCA has been reported during previous sampling events at the site. The purpose of the investigation is to resolve the 1,2-DCA data gap, which has prevented site closure from being approved. The 1,2-DCA data gap was identified during the review of the Closure Request Report(URS, 2001)by the Regional Water Quality Control Board (RWQCB). The document Work Plan for 1,2-DCA Investigation, Former American Forest Products Stockton Facility, Stockton, California(Work Plan)(URS, 2002)was prepared following a telephone discussion of the additional requirements for closure with Mr. Marty Hartzell of the RWQCB. This investigation was conducted in accordance with the Work Plan, which was reviewed and approved by the San Joaquin County Public Health Services Environmental Health Division (PHS/EHD). The Work Plan presented the following scope of work: • Install a total of eight direct-push strataprobes; • Collect two groundwater samples at each location: — At a depth of 40 feet below ground surface(bgs) where the water table was expected; and ==# — At a depth of 55 feet bgs; • Analyze the water samples for volatile organic compounds (VOCs), including 1,2-DCA, using EPA Method 8260B; • Prepare an Addendum Report to the Closure Request Report; and :.: • Prepare an updated map of the drinking water supply wells within 2,000 feet of the site. 2.0 SAMPLING METHODS AND PROCEDURES This section describes the methods and procedures associated with the collection of groundwater samples. Sampling was conducted by URS on May 15 and 16, 2002. Sampling locations were based on existing water quality data and groundwater flow directions. The objective of the investigation was to assess 1,2-DCA concentrations in the area downgradient of existing monitoring wells. The direction of the groundwater gradient has varied approximately 180 degrees during the previous fourteen years, based on data presented in Figure 3. The groundwater gradient is currently - oriented toward the north-northeast, which has been the prevalent flow direction since 1998. The existing monitoring well network provides adequate coverage in the directions west and south of the wells where 1,2-DCA has been detected(DMW-2,DMW-5 and DMW--6). Therefore, sampling locations for this investigation were placed in the areas north and east of these three wells, as indicated in Figure 3. A total G:\AFP\REP0RT\DCA_FinaLrpt.doc __J