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DRAFT <br /> Introduction <br /> The Department of Toxic Substance Control(DTSC),and the Regional Water Quality Control Board <br /> (RWQCB)have requested additional data collection during the Feasibility Study at the Pacific Gas&Electric <br /> Company(PG&E)former Manufactured Gas Plant site located at 535 Center Street in Stockton, California <br /> (Figure 1). The scope of the additional investigation was agreed upon during meetings on August 8, 1996, <br /> August 28, 1996,and September 12, 1996 and a site walk on September 23, 1996. This workplan presents the <br /> objectives, scope,and methodology for the additional field work <br /> Objectives <br /> The additional field work is designed to meet the following objectives: <br /> Confirm/Modify Dimensions of Source Areas 1 and 4 <br /> • Based on discussions with DTSC and RWQCB regarding the source areas,further data collection <br /> is needed to help define the vertical and horizontal extent of Source Areas 1 and 4 in Area 1. This <br /> information will be used to confirm or modify the dimensions of Source Areas 1 and 4. This data <br /> will be incorporated into the VLEACH model and will result in the creation of additional <br /> polygons to further refine the model. <br /> Determine Leachability of COCs Using Deionized Waste Extraction Test(DI WET) <br /> • Selected soil samples will analyzed using the Deionized Waste Extraction Test(DI WET) to <br /> determine the leachability of PNAs,TPH-gasoline,TPH-diesel and BTEX. These results will be <br /> compared with the VLEACH modeling results. The VLEACH model was used to examine the <br /> potential downward movement of chemicals of concern from the vadose zone to the groundwater. <br /> Investigate Source of Diesel Near MW-23R (Area 11) <br /> • Further soil characterization is requested near soil boring location MW-23R in Area II. During <br /> the Phase IV groundwater investigation,HydropunchTm sample results indicate that diesel is <br /> present in groundwater near the western corner of Area II.Diesel was not observed in the vadose <br /> zone above the Hydropunch'm sample. Additional borings will be drilled to determine if the <br /> source of the TPH-diesel found in Area II groundwater is on PG&E property or is offsite. <br /> Establish Downgradient Extent of COCs in Zone B Groundwater <br /> • Zone B groundwater requires further characterization. Results from groundwater sampling <br /> during the Phase IV groundwater investigation and the most recent quarterly sampling events <br /> indicate that PNAs are present in groundwater samples from Zone B wells in Area II. An <br /> additional Zone B well will be installed to define the extent of PNAs in Zone B groundwater. <br /> Scope Of The Investigation <br /> Confirm/Modify Dimensions of Source Areas 1 and 4(Area 1) <br /> The DTSC and RWQCB requested additional soil samples be collected to support the definition of Source <br /> Areas 1 and 4. To provide this information,four boring locations in Area I are proposed as described below <br /> and shown in Figure 2. This information will also be used to create additional polygons to further refine the <br /> VLEACH model(Evaluation of Potential Groundwater Impacts from Residues in Soil, C112M HILL, 1996). <br /> Boring locations include: <br /> SF01GAA:V t.Doc 1 <br />