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Sub-slab sample locations will be collocated with indoor air samples. The sub-slab sampling ports will be installed after the indoor air sample collection is completed (8- 24 hours later) to avoid biasing the indoor air concentrations with potentially higher sub-slab gas infiltration rates during port installation. Installation and sampling will be conducted in general accordance with DTSC, RWQCBs, and EPA guidance (DTSC 2011; DTSC, LARWQCB and SFBRWQCB 2015; EPA 2015) (Appendix E). Vapor Pins will be utilized to collect the sub-slab gas samples. The sub-slab monitoring points will be installed using a roto hammer drill, and will consist of a countersunk, stainless steel Vapor Pin that will be installed according to the manufacturers standard operating procedures (Appendix C). The shut-in test and helium leak check method will be conducted following the Active Soil Gas Advisory.The Vapor-Pins° will be set in countersunk holes with secure covers to protect the installation and allow for repeat sampling as necessary. At least two hours will be allowed prior to sampling to allow the subsurface to equilibrate. Vapor Pins sampling procedures are as follows: 1. Background sources of indoor air contamination from the building will be identified and removed several days prior to sampling. 2. Immediately prior to sampling, activities that could cause contamination, such as cleaning activities, permanent marker writing, etc. will be stopped to clear the area. Each sample area should be cleared of dirt, dust and debris prior to sampling. 3. Weather information including temperature, barometric pressure, relative humidity, wind speed, and wind direction, will be recorded at the beginning of the sampling event. Also, any substantial changes to these conditions that may have occurred over the past 24 to 48 hours should be noted. 4. Remove the protective cap and connect sample tubing to the Vapor Pin barb fitting using a new section of Teflon tubing (1/8 to % inch in diameter), with the appropriate internal diameter to ensure a complete seal with the Vapor Pin hose barb. 5. The purge volume is the internal volume of the tubing. 6. The Summa canister will be provided by the laboratory, along with a flow controller equipped and vacuum gauge. 7. Sample train shut-in test and sampling leak check will be completed at each location. See section below for leak check details. 8. One purge volume will be removed from the Vapor Pin. 9. Purge flow rate will be between 100 and 200 milliliters per minute (mL/min). 10. Vacuum during purging and sampling will be maintained at less than 100 inches of water (approximately 7 inches of mercury). 11. Vacuum will be measured using an in-line gauge during purging. FINAL RIPON PCE INVESTIGATION WORK PLAN.DOCX 4-8 PROJECT NO.631021512