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February 19, 2021 Mr. Olivier Theard Page 5 SLR International Corporation slrconsulting.com will be conducted using a PID to identify potential locations of ambient/background sources, or potential points of soil vapor entry into the building. • Place passive diffusive sorbent samplers for monitoring time-weighted average concentrations of VOCs at 6 (5 + 1 duplicate) indoor locations inside the building and at 2 outdoor locations, as shown on Figure 6. (As stated above, the locations may be adjusted based on the Tier I results.) The samplers placed outside the building will be used to collect control samples of ambient/background air. Air sampling will be performed in accordance with the DTSC Final Guidance for the Evaluation and Mitigation of Subsurface Vapor Intrusion to Indoor Air (revised October 2011), except where the DTSC Supplemental Guidance: Screening and Evaluating Vapor Intrusion (February 2020) supersedes. This includes sampling during standard building operating conditions, collection of samples in each heating, ventilation, and air conditioning (HVAC) zone, placement of the samplers in the typical breathing level of a worker inside the building (approximately 3 to 5 feet above the ground), and away from air ventilation intake or discharge points. • Time-integrated samples will be collected using passive diffusive sorbent samplers over a 48-to-72- hour period to characterize the average daily inhalation exposure for building occupants based on the commercial use of the Site. As described in the Office of Solid Waste and Emergency Response Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air (June 2015), USEPA generally recommends beginning outdoor air sampling at least one hour before indoor air monitoring begins, but preferably two hours, and continuing to sample until at least 30 minutes before indoor air monitoring is complete. The indoor and outdoor air samples will be shipped to a State certified laboratory under strict chain-of-custody protocol and analyzed for VOCs by EPA Method TO-17 on a standard turn-around basis. • Local meteorological data from a public weather station (Weather Station ID: KCASTOCK93) will be obtained for the sampling period to record temperature, dew point, humidity, barometric pressure, visibility, wind direction, and wind speed to assess diurnal or seasonal fluctuations in atmospheric conditions that may affect the sampling results. The weather station is located in Stockton, CA at 37.99° N latitude, 121.33° W longitude, at approximately 16 feet above mean sea level (ft amsl). Following completion of the Task 4 activities, SLR will update the Supplemental ESA report to include a summary of indoor and outdoor air sampling activities and results. SCHEDULE SLR will commence work immediately upon authorization and approval of Site access. The findings of the Tier I and II evaluations will be communicated verbally or via email to SheppardMullin throughout the assessment process and will be compiled in a Supplemental ESA report (as described above) upon completion. SLR will complete the draft report within 60 days following notice to proceed. SLR can deliver the final report within a week of receiving comments on the draft report. SLR will coordinate with SheppardMullin to meet mutually agreeable deliverable dates. The above estimated schedule is contingent on Site access being granted within two weeks of authorization of this scope of work.