AgdaPkt 2017-09-11 Closed and Joint SA PFA

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7.A. - Page 40 LSA ASSOCIATES, INC. ROCKETSHIP REDWOOD CITY ELEMENTARY SCHOOL PROJECT SEPTEMBER 2016 INITIAL STUDY/MITIGATED NEGATIVE DECLARATION a. Passive Sub-Slab Depressurization System. The passive sub-slab depressurization system is designed such that VOCs would be captured by the negative pressure conditions created by three horizontal rows of perforated pipes constructed below the building sub-slab. These sub-slab perforated pipes are connected to vents on the roof through vertical air-tight pipes. These vents discharge any captured VOCs to ambient outdoor air. The ambient air discharge point is located near the southern portion of the building,approximately 30 feet away from the nearest HVAC unit. The layout and design of the system is further described in Appendix A of this document. As part of the ongoing monitoring and oversight of the effectiveness of the system,an Operations and Maintenance Plan(O&M Plan)would also be developed. The O&M Plan would identify the specific mitigation goals and objectives,performance measures,and any potential contingencies.Performance measures include indoor air sampling and sub-slab monitoring to ensure that the passive system is operating correctly and preventing contaminants from entering the building. Indoor air sampling,which is described in detail in the Work Plan(Appendix A),will include four events to be completed by February 2017 (approximately seven months prior to the proposed opening of the school in the Fall of 2017),with semi-annual monitoring thereafter. Six vapor monitoring points would assist in verifying the effectiveness of the system. b. Active Sub-Slab Depressurization System. In the event that indoor air sampling and sub-slab monitoring indicates that the passive system is not effective (i.e.,indoor air concentrations of VOCs exceed acceptable levels),the system would be converted to an active system. The difference between the passive and active systems is that an active system would involve installation of exhaust fans to force the vapors out through the existing sub-slab depressurization system. This active air flow would expand the capture area of the sub-slab perforated pipes. The active system would involve the installation and continuous operation of 120 volt exhaust fans at each of four roof vent locations. Each fan would be capable of transferring a maximum of 188 cubic feet of air per minute. Should DTSC determine that indoor air monitoring results are not meeting screening levels,DTSC would require that the system be converted to the active system and monitored to ensure effectiveness.. 8. Discretionary Actions As previously discussed,although the project site is partially within unincorporated San Mateo County,Redwood City is the Lead Agency for environmental review. The County of San Mateo is a Responsible Agency and will also review the environmental document. A Conditional Use Permit from the County is also required to allow the school use. In addition,DTSC will oversee the implementation of the Work Plan,preparation of the O&M Plan, and monitoring events associated with the sub-slab depressurization system. P:1RWC1401B Rocketship Sch-I\PRODUCrSUS-MND1Public\Ra ketship Public Review 09-29-16.docx(09/29/16)PUBLIC REVIEW DRAFT 16 ATTY/RESO.0084/CC RESO ADOPTING CEQA-ROCKETSHIP CHARTER SCHOOL APPEAL REV:09-05-17 VR Page 24 of 124