ARCHIVED REPORTS_QUARTERLY GROUNDWATER MONITORING REPORT 1ST QUARTER 2016

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QUARTERLY GROUNDWATER MONITORING REPORT — FIRST QUARTER 2016 L&M Operable Unit —West Weber Avenue and North Lincoln Street, Stockton, CA • Appendix B Page 12 Soil vapor data obtained on July 1, 2015 indicated additional soil vapor assessment is required in areas where elevated benzene (UPSV-6 and APSV-15) and naphthalene (APSV-15) were observed adjacent to Delta View Apartment Buildings 2 and 6. The additional assessment would also be used to determine if risk based closure may be acceptable. In the 2Q15 QMR, Stantec proposed preparing a work plan describing the additional assessment activities. On October 21 , 2015 Stantec submitted a Response to CRWQCB Letter dated June 15, 2015 and Request for Additional Methane and Soil Gas VOC Investigation to the in which Stantec responded to comments from a CRWQCB letter dated June 15, 2015. The responses included a proposal for analyzing carbon ranges of gasoline in soil vapor for which toxicity data is available and comparing the results to risk assessment numbers for these carbon ranges and a proposal for a reduction in soil vapor monitoring from quarterly to semi-annual. Stantec also proposed shallow soil gas assessment of methane and petroleum hydrocarbons on the Delta View site (buildings 1, 2, 3, and 5). The CRWQCB • concurred with Stantec's proposals in a letter dated November 16, 2015. Between December 16 and 18, 2016 Stantec oversaw TEG's mobile laboratory staff sample newly installed shallow soil gas probes SSGP-1 through SSGP-13. Results of the investigation were presented to the CRWQCB in a Soil Vapor Investigation Results Report submitted by Stantec dated January 29, 2016. The report concluded that the data collected documented the presence of an aerobic zone in the upper three to five feet that is capable of preventing any upward flux of VOCs and methane and that consequently there is no hazard to the current Delta View Apartments residents. In a letter dated February 29, 2016 the CRWQCB generally agreed with Stantec regarding the Delta View Apartments however expressed concern that methane at five foot depth and below could pose a hazard to future utility workers. The CRWQCB also expressed concern about relatively high concentrations of TPHg and benzene at certain locations posing a threat to underlying groundwater. Consequently the CRWQCB requested Stantec submit: • By March 14, 2016 a plan for bringing the SVE system back online for pulsing and rebound testing of VOCs and methane. • By April 15, 2016 a report with remedial options for addressing remaining • soil vapors. Stantec