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51 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 SOIL VAPOR INVESTIGATION WORK PLAN FOR METHANE AND VOCS INTRODUCTION On behalf of CEMC and P66, Stantec has prepared this Soil Vapor Investigation Work Plan to conduct an assessment of methane and petroleum hydrocarbons in shallow soil gas in the immediate vicinity of Delta View Buildings 1, 2, 3 and 5. The Delta View was constructed on a portion of the L&M OU in the early 1980s after the City of Stockton took possession of the former petroleum storage facilities that historically operated at the L&M OU, and demolished those facilities. The City of Stockton continues to own those portions of the L&M OU that are not occupied by Delta View. The L&M OU is bounded by the Delta View Apartments to the west and south, the Unocal Operable Unit to the east and West Weber and Waterfront Office Towers to the north and is depicted in Figures 1 and 2, and referred to as "the Site." Figure 1 also depicts the location of Delta View. The work plan was prepared in response to correspondence from the CRWQCB dated June 15, 2015 (Attachment 1) and previous soil vapor assessment findings submitted by Stantec on May 15, 2015 (Attachment 1). PURPOSE AND SCOPE OF WORK Previous site assessment activities including quarterly groundwater and soil gas sampling have identified hydrocarbon and methane impacts to shallow soil vapor,which historically has occurred at depths less than 11 feet below ground surface (bgs) at the site and site vicinity. Evaluation of hydrocarbons and methane in soil vapor potentially associated with shallow soil and groundwater impacts have been evaluated since 2005 for both areas on-Site in the potential source area as well as off-Site at Delta View. In early 2015, elevated hydrocarbons and methane in shallow soil gas, either above RWQCB ESL or Lower Explosive Limits (LEL) at depth, were identified in several locations adjacent to Buildings 1, 2, 3, and 5. Stantec is proposing to perform shallow soil vapor sampling to profile impacts in the upper 6 feet at locations surrounding Delta View Buildings 1, 2, 3 and 5 and as shown on Figure 2 (SSGP-1 through SSGP-12). Shallow soil vapor profiling will be performed as follows: • At each proposed location, samples will be collected from approximately one-foot below the ground surface (bgs) and at every one-foot interval to a depth of approximately 5 to 6 feet bgs. Methane, carbon dioxide (CO2), and oxygen (02) will be analyzed in each sample by an on-Site mobile laboratory.Site-wide soil gas data for VOCs already exists from 6 to 1 1 feet bgs in shallow soil gas. • During in-situ shallow soil gas investigations, percent 02 will be measured at each interval. Per DTSC, when 02 levels approach at least 4%, there is indication of sufficient oxygen to support aerobic bioattenuation(the bioattenuation zone). CO2 measurements will also 4