SITE INFORMATION AND CORRESPONDENCE 1995-2004

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9 0 James L. Tjosvold, P.E. August 19, 1996 Page 3 • The presence of concentrated and extensive residue, such as tar or oil; • Shallow groundwater in the immediate vicinity that contains chemicals that are present in soil; • Locations where hazardous materials were formerly stored or handled; and • Elevated concentrations of COPCs over an extensive area. During our meeting on November 30, 1995, PG&E agreed to conduct a quantitative evaluation using the RWQCB's "Designated Level Methodology' or equivalent to determine whether treatment of these soil source areas is protective of groundwater, as well as expanding on the qualitative definition of the source areas. The conceptual model presented in the Evaluation of Potential Groundwater Impacts from Residues in Soil report submitted in March 1996 describes the behavior characteristics of the COPCs. This report also documented the use of three types of modeling calculations to evaluate: 1) the presence of mobile organic fluid (residual saturation calculation) to determine if there are areas at the site where such organic fluids are present(which increases the possibility of COPCs being mobilized), 2) chemical-specific and site-specific migration velocity calculations (to demonstrate that PNAs are very immobile unless mobile organic fluids are present), and 3) using the VLEACH model (to estimate future groundwater concentrations resulting after source area treatment). As discussed in this report and during our May 22, 1996 meeting, the modeling results show that based on the nature of the COPCs and site- specific characteristics, treatment of the four source areas as proposed in the March 1996 report, is protective of groundwater concentrations. The source areas have changed slightly in volume since the draft Soil OUFS was submitted. Table 6-2 in the March 1996 Evaluation of Potential Groundwater Impacts from Residues in Soil report is a revised version of the OUFS Table 4-2. The revised volumes reflect a correction in the scale used for area calculation as well as a revision in some of the treatment depths. The depths recommended for treatment have changed as follows: • Source Area 1, former fuel tanks and gas holder: The affected depth has been revised from 10 to 30 ft bgs to 15 to 30 ft bgs. The total volume is now 2,960 cubic yards. • Source Area 2, former lampblack pit and gas holder: The total affected depth has been revised from 25 ft bgs to 30 ft bgs. The total volume is now 3,400 cubic yards.