SITE INFORMATION AND CORRESPONDENCE 1995-2004

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DRAFT • Two borings to define the perimeter of Source Area I (SB96-1 and S1396-2) • Two borings to better define the northern edge of Source Area 4 (SB96-3 and S1396-4) Each of the four borings will be continuously sampled to saturated soil,approximately 35 feet bgs. Samples will be collected every five feet for analysis. Additional samples from a boring may be collected and analyzed if gas plant residue is observed in the field. The boring locations were selected in joint agreement with the RWQCB,DTSC, and PG&E during the site visit on September 23, 1996. A total of four borings is proposed, however,the number of borings may increase based on field observations. To avoid the need for further characterization of the source areas and minimize the number of borings necessary to characterize the site,the following decision criteria are proposed to evaluate the need for additional borings: 1. Soil samples will be collected from boring locations designated S1396-1, -2, -3, and-4 in Figure 2. 2. The project hydrogeologist will review field observations(odors,discoloration, textural changes). 3. The project hydrogeologist will recommend an additional boring offset five feet from current location if residues are observed in the soil. Drilling will stop if no residues are observed. Observations and recommendations will be reviewed with PG&E and the agencies, if possible. 4. When no additional borings are required,the source area will be considered fully defined. Acceptable locations are limited for the Source Area 4 borings due to high voltage powerlines running both overhead and underground. The locations shown in Figure 2 were field checked and appear to be clear of overhead obstructions. The final locations of the borings are subject to final clearance from an underground utility locator. Determine Leachability of COCs Using Deionized Waste Extraction Test(DI WET) A fifth boring(SB96-5)in Area I is proposed inside Source Area 2 near known residues. The boring will be sampled to a depth of 35 feet bgs. Soil samples directly beneath residue and, if possible,at the saturated zone will be collected and analyzed using the DI WET(California Code of Regulations Title 22). If unable to drill below the residue due to a subsurface barrier,a sample directly above the residue will be collected. If the sample above the residues does not appear to contain any residues,the boring will be relocated to within Source Area 4 near Boring 11. In addition, two samples from the borings outside of Source Areas 1 and 4 will be selected for the DI WET analysis. Total PNA analysis from the borings outside of Source Areas 1 and 4,will be completed within 10 days.The borings with the highest PNA concentration near groundwater will be selected for the DI WET analysis.The agencies will be consulted when determining the specific borings to analyze for DI WET. To stay within holding times the DI WET analysis must be initiated by the 14th day. The purpose of collecting these samples is to directly compare the leaching of a soil sample containing high concentrations of COCs within the source area and directly beneath a source area(near the water table)to the leaching characteristics of affected soils outside of a source area. Investigate Source of TPH-Diesel Source Near MW-23R (Area H) In March 1996 (Phase IV Hydrogeologic Investigation, CH2M HILL, June 1996)a soil boring(MW-22R)was drilled in the northwest portion of Area U. A HydropunchTm groundwater sample from this location indicated the presence of TPH-gasoline and TPH-diesel. TPH-gasoline and TPH-diesel were not observed by the geologist in soils during the drilling of MW-22R.Historical maps and photographs indicate that this portion of the site had been used as an auto repair garage. Solvents may have been used as part of the auto repair SF0/G:W:1011.D0C 2