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Joe Niland - 2 - 7 April 2020 collected from each suite, as well as duplicate samples, and three outdoor air samples. The indoor air samples will be collected over an 8-hour period. Based on the results of the initial sampling, building pressure control may be implemented to create realistic, worst-case scenario conditions be depressurizing the suite to induce vapor intrusion. During each phase of sampling, pressure data will be collected from the interior and exterior of the suites and the sub-slab monitoring points using data loggers. As is recommended in the draft vapor intrusion guidance, temporal data will be collected to assess seasonal effects on vapor off-gassing and migration, and at least two seasonal sampling events are proposed. Following the receipt and validation of all the data, a memorandum will be prepared that will summarize the findings and propose next steps as warranted. Central Valley Water Board staff notes that the results of the first round of seasonal indoor air data should be compared with indoor air screening levels. If indoor air exceeds the screening levels, a preliminary risk analysis should be performed for any occupied suites and mitigation measures should be implemented if the risk analysis indicates they are needed following consultation with Central Valley Water Board staff. Central Valley Water Board staff also requests that all vapor intrusion data be entered into the GeoTracker in Electronic Data Format and building-specific information as Electronic Submittals of Information. The "Guidance on Uploading Vapor Intrusion Information Into GeoTracker" can be downloaded at: https://www.waterboards.ca.gov/ust/electronic submittal/. Central Valley Water Board staff concurs with the work proposed in the work plan. Given the proposed collection of seasonal data, please submit the results, assessment, and proposed next steps in the proposed memorandum by 31 December 2020. Please provide us with at least 72 hours' notice prior to initiation of the field work so that we have an opportunity to observe the work. Feel free to contact me with any questions or concerns regarding this letter at (916) 464-4622 or Bill.BrattainCo�waterboards.ca.gov. William Brattain, P.E. Water Resource Control Engineer Private Sites Cleanup Unit P(tOf ESSIOh w H iTTFN�fF FiC C 057461 Expires 12131 021 Steven W. Meeks, P.E. Senior Water Resource Control Engineer 04/07 CIVILc Chief, Private Sites Cleanup Unit kcr cc: Arthur Forma, PG, CEG, CHG, Geosyntec Consultants, Inc., Rancho Cordova