SITE INFORMATION AND CORRESPONDENCE

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October 21,2015 Reference: Response to California Regional Water Quality Control Board, Central Valley Regional Letter dated June 15, 2015 and Request for Additional Methane and Soil Gas VOC Investigation A screening level for TPHg will be retained for the purpose of evaluating its odor threshold against Site data. However, it should be noted that the most recent Region 2 guidance (December 2013), provides a bench mark of 300,000 µg/m3 as opposed to the previously recommended 295,000 µg/m3. This benchmark may be adjusted accordingly if Region 2 updates any ESL documents and/or if any additional memorandums are released adjusting the 0.002 default attenuation factor for soil gas investigations. For purposes of evaluating potential human health risk, since TPH alone is no longer a recommended means of evaluating potential human health risk from petroleum hydrocarbon mixtures, soil vapor samples should be analyzed for the various carbon ranges for which toxicity data are available. Stantec will continue to work with the lab to address this range during future assessment and sampling for areas with elevated TPH of concern. It is noted that TPH-g is analyzed through measurement of compounds generally in the C5 to C8 carbon range including BTEX. However as observed at the Site, where BTEX is absent any reported TPH-g would include a wide number of compounds such as branched alkanes and straight chain alkenes which either have little to no toxicity data or contribute little to potential risk. 2. We also note that soil vapor probes APSV-9 and APSV-10 have been recommended for elimination from the soil vapor monitoring program because they are not considered to be representative of current vapor intrusion protection measures. We do not find that the reasons stated in the report (and reiterated in this letter, above) are sufficient to warrant ignoring TPH-g concentrations above the screening level in these probes, particularly since they are near the apartment buildings. Soil vapor probes APSV-9 and APSV-10 should be retained in the soil vapor monitoring program and should also be addressed in the proposed work plan since they have TPH-g concentrations well above the 295,000 ug/m3 screening level. Stantec recently installed shallow soil vapor probe APSV-17, immediately adjacent to and in between Delta View Apartment (Delta View) Buildings 7 and 8. Stantec feels that this location is more representative for evaluating potential impacts at these buildings. Stantec has continued to sample APSV-9/APSV-10 on a consistent basis and will continue to do so pending further evaluation. Furthermore, Stantec has also installed shallow soil vapor point APSV-1 1 and continues to sample APSV-1, APSV-2, and APSV-3 which are directly adjacent to Delta View Buildings 1 and 2. Stantec is proposing to perform additional soil gas investigations at the western portions of these building to evaluate potential impacts in soil gas to the immediate west of Buildings 1, 2, and 3 (See Figure 2). 3. We agree with the observation in the report that there is a correlation between high levels of methane and relatively high levels of TPH-g. Although the results of the methane measurements taken at the 37 locations around the site indicate no immediate hazard, we are still concerned about methane levels above the 5% LEL that are caused by 2