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4 STANDARD OPERATING PROCEDURES FOR SOIL GAS PROBE INSTALLATION Twining is a C-57 licensed water well drilling contractor (Contractor's License No. 506159). The soil gas probes will be installed under the direction of a California-registered geologist or -certified professional engineer. Soil borings for gas probes will be drilled and sampled according to Twining's "Standard Operating Procedures for Drilling and Soil Sampling". Equipment used for drilling, sampling, and constructing wells, and well construction materials will be thoroughly cleaned according to Twining's "Standard Operating Procedures for Equipment Decontamination". Soil gas probes will be constructed of Schedule 80 polyvinyl chloride (PVC) casing. Screen will be machine-slotted. Rubber stoppers will be placed at the top of the screened section and at the top of the casing. Teflon tubing will extend from approximately one foot below the bottom stopper to approximately one foot above the top stopper for sample collection. Casing and well screen specifications will depend upon project objectives, site conditions, and project budgets. The well screen and casing sections will be joined by factory-made flush-threaded connections. The well filter pack material will consist of appropriately-sized pre-washed sand or pea gravel. The filter material will be placed in the annulus between the boring wall, from the bottom of the boring to approximately 2 feet above the top of the screen. Approximately one foot of bentonite will be placed on top of the sand pack as a seal. The bentonite may consist of 1/4- or 1/2-inch pellets, or may be in the form of a slurry. If pellets are used, they will be hydrated prior to grouting the well, by adding at least one gallon of water for every bucket of pellets. The thickness of the bentonite seal may depend upon local regulatory requirements. The remainder of the well annulus will be sealed with a 6-sack sand-cement, or cement grout slurry containing 3 to 5 percent bentonite. For wells more than 15 feet deep, the seal will be emplaced through the hollow augers, which will extend to within 15 feet of the bentonite seal. If required by the local regulatory agency, the seal will be emplaced by pumping through a tremie pipe which extends to within 15 feet of the bottom of the boring. The tremie pipe or augers are withdrawn from the boring in 5-foot increments as the boring fills with grout. A 3-foot steel standpipe with locking cover or a traffic-rated box will be installed around each wellhead. Gas probes will be locked to restrict access to unauthorized personnel. Gas probe construction details will be recorded on the field well construction form. Construction information will be transferred to Soil gas probe construction schematics for presentation in final reports.