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1 .0 INTRODUCTION <br /> This work plan documents procedures for an additional investigation of 1,2-dichloroethane(1,2- <br /> DCA) impacts in groundwater at the former American Forest Products Company LP(AFPC-LP) <br /> facility (Site)in Stockton,California(Figures 1 and 2). The purpose of the investigation is to <br /> f collect sufficient information regarding the extent of 1,2-DCA impacts to enable site closure to <br /> proceed. <br /> 1.1 Project Background <br /> L Two underground storage tanks at the site were excavated and removed in 1988. Associated <br /> piping was excavated and removed from the site in 1991. Soil impacts were partially remediated <br /> in 1990. A series of closure request reports (Dames &Moore, 1994; URS, 2001; and 2002x) <br /> have demonstrated that hydrocarbon impacts at the site have been adequately characterized by a <br /> 6 series of investigations,beginning in 1985. <br /> 1 Data regarding the extent of 1,2-DCA impacts was presented and site closure was most recently <br /> requested in July 2002 (URS, 2002a). Closure was requested based on data that indicates the site <br /> meets the criteria for designation as a "low risk groundwater site." These criteria are as follows: <br /> r+ 0 The Ieak has been stopped and sources removed or remediated; <br /> The site has been adequately characterized; <br /> • The dissolved contaminant plume is not migrating, and chemical concentrations are not <br /> �. increasing with time; <br /> • No water wells, deeper aquifers, surface water, or other sensitive receptors are likely to be <br /> impacted; and, <br /> + The site represents no significant risk to human health or safety. <br /> s Subsequent to discussions with the Regional Water Quality Control Board (RWQCB) on <br /> .� February 13, 2002, indicating that closure criteria were met with the exception of delineation of <br /> the extent of 1,2-DCA impacts in groundwater,URS presented a work plan(URS,2002b)to <br /> provide data to fill that data gap. The work plan, which proposed a direct push Stratoproben" <br /> Lsampling program at eight locations in the vicinity and downgradient of the former waste oil tank <br /> (Figure 2), was approved by the San Joaquin County Environmental Health Division (SJC-EHD) <br /> in their letter of April 15,2002,but included the following conditions: <br /> M <br /> L • Groundwater should be sampled from a permeable zone below 80 feet below ground surface <br /> (bgs)representing historic low groundwater levels in order to define the vertical extent of <br /> I groundwater contamination; <br /> 0 Monitoring wells would potentially be required to demonstrate stability of the 1,2-DCA <br /> plume and decreasing concentrations with time. <br /> The 2002 work plan also provided for a cessation of quarterly groundwater monitoring while the <br /> results of the investigation were evaluated. The report of the 1,2-DCA investigation (URS, <br /> L 2002a) showed, based on six samples collected from five direct push sampling locations, that: <br /> • The lateral distribution of 1,2-DCA was limited to a small area (approximately 60-foot <br /> radius)in the vicinity of the northeast corner of the former maintenance facility building; <br /> 4 <br /> Concentrations of 1,2-DCA decline with depth; and, <br /> 1 <br />