SITE INFORMATION AND CORRESPONDENCE

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(3 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 degradation of petroleum constituents being a potential threat to the apartment buildings and utility worker under conditions that may not have been anticipated (i.e. changing atmospheric conditions, varying HVAC operations in apartments, etc. . The report recommends methane monitoring during any future sub-surface disturbance; however, this would need to be recorded in a deed covenant on the property to require methane monitoring during any future soil disturbance activities. Since this option is very unlikely to occur given that the L&M Group does not own the property and the high methane levels are still considered to be a potential threat to the apartment buildings and utility workers, the work plan needs to address reducing methane caused by degradation of petroleum constituents to levels below the 5%LEL. Stantec demonstrated in Section 4.3 of the soil vapor assessment report submitted May15, 2015 that the observed methane in soil vapor from all locations monitored was well below the 5% LEL threshold that would pose an immediate concern to residents and/or the commercial and utility/maintenance worker. Based on the 37 point survey of confined accessible spaces, methane was measured at very low levels; far less than 1% LEL, and nowhere near 5% LEL (50,000 ppmv). The highest concentration measured was 1,300 ppmV. In response to CVRWQCB comments and to address concerns regarding methane levels "at depth", Stantec is proposing additional sampling to further assess site-wide methane levels in shallow soil gas, investigation as outlined in the work plan below. 4. Regarding the request to suspend the quarterly soil vapor monitoring, the quarterly monitoring should continue so that the data ore available for use in further assessment of the work to be performed to address the areas with TPH-g and methane, and to make sure that only UPSV-6 and APSV-15 are of concern for benzene and/or naphthalene. Stantec will continue with quarterly soil vapor monitoring, beginning with the third quarter of 2015 (a second quarter soil vapor monitoring was not conducted). Soil vapor sampling was performed on July 1 sl and 2nd; and the data were presented in the second quarter 2015, site status report submitted July 31, 2015. However, Stantec requests that after the proposed additional soil gas investigations as outlined below and beginning 1Q16 that soil gas sampling be conducted on a semi-annual basis. Data collected to date indicates that this frequency of sampling would be adequate for aiding RWQCB risk-management decisions. 3