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Work Plan—1,2-DCA Investigation $ Former American Forest Products Site,Stockton,Califomia. April 2,2002 URS Corporation Americas Page 1 1.0 INTRODUCTION This Work Plan documents the procedures of a proposed investigation of 1,2-dichloroethane (1,2-DCA) impacts in groundwater at the Former American Forest Products Company (AFDC) site (site), in Stockton, California (Figures 1 and 2). Diesel impacts to soil and groundwater have been the primary focus of site investigations and remediation, which began in 1988. However,residual diesel impacts have been found to i be limited, and the current focus is on the extent of 1,2-DCA in groundwater. 1,2-DCE has been reported at ' low concentrations in a limited area of the site. The objective of the proposed investigation is to assess the vertical and horizontal extent of 1,2-DCA in groundwater at locations downgradient from existing monitoring wells. The purpose of the investigation is to resolve the 1,2-DCA data gap, which has prevented site closure from being approved. Site closure.was requested in 2001 (URS, 2001). Closure was based on data which indicate that the site meets the criteria for designation as a"low risk groundwater site."These criteria are as follows: • The leak has been stopped and sources removed or remediated; • The site has been adequately characterized; • The dissolved plume is not migrating,and chemical concentrations are not increasing with time; • No water wells, deeper aquifers, surface water or other sensitive receptors are likely to be impacted; and • The site represents no significant risk to human health or safety. Two underground storage tanks at the site were excavated and removed from the site on May'11, 1988. Associated piping was excavated and removed from the site during rune 1991. Soil impacts were partially remediated in 1990. The site has been adequately characterized by a series of investigations, beginning in f 1985. The dissolved hydrocarbon plume shows indications of natural degradation, and the plume not migrating. There are no water wells in the vicinity that could be impacted by the plume. The site does not pose a significant threat to water resources, or human health and safety. Discussions with the Regional Water Quality Control Boar(RWQCB)on February 14,2002, indicated that the site meets the criteria, with the exception of the 1;2-DCA data gap,for site closure. This investigation has been proposed in: exchange for postponement of quarterly monitoring for diesel, as discussed with the RWQCB. (Also requested by the RWQCB was a revision to the map identifying drinking water wells within 2,000 feet of the site). In order to resolve the 1,2-DCA data gap, the following scope of work has been proposed: • Install a total of eight direct push stratapobes; • Collect two groundwater samples at each location: KA840191AFMCA_W ORKPLAN.DOC c F