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bentonite between the sand pack and the hydrated bentonite is to preclude Temporary Soil Vapor Wells: Post-run tubing and drive point methods' the hydrated bentonite grout from infiltrating the sand pack. Follow a similar used to create temporary soil vapor wells are not recommended for soil gas procedure for deep well construction with multiple probe depths,in that one sampling(McAlary,et al,2009). The tubing used to create temporary soil foot of dry granular bentonite should be emplaced on top of the sand pack vapor wells for the post-run method is prone to sealing issues associated with encasing each probe,followed by hydrated bentonite grout. The hydrated connecting the tubing to the drive point. Additionally,the drive point probes bentonite grout should continue until the next sand pack,as shown on Figure may be deflected by cobbles,which can create gaps between the outer wall 2. A cement/bentonite mixture in accordance with California well construction of the casing and the geologic materials that are difficult to observe and standards may also be used above the dry bentonite layer to seal the equally difficult to seal. A hydrated bentonite plug at ground surface does not borehole annulus. stop communication along the outer wail of the casing between different depth intervals. Samples collected under these circumstances will draw soil C. A tremie pipe should be used for soil vapor wells deeper than 15 feet,to avoid gas primarily from the most permeable layer above the tip of the probe which bridging or segregation during placement of the sand pack and bentonite seal. may introduce a significant bias. Moreover,this condition cannot be identified by any tracer applied at or near ground surface. Temporary soil vapor wells D. The use of a down-hole probe support may be required for vapor wells in may also yield questionable results in moderate to low permeability soils such excess of 40 feet bgs. The probe support may be constructed from a one- as clay and/or silt clay lenses,where the Flow of gas through the geologic inch diameter bentonite/cement grouted PVC pipe or other solid rod,or materials is low. In such case,soil gas will be.collected from the path of least equivalent,allowing probes to be positioned at measured intervals prior to resistance at any depth along the drive shaft. installation. The support should be solid or properly sealed to avoid possible cross contamination or ambient air intrusion. Alternative probe support Equilibration Time designs with accompanying descriptions may be proposed in the project workplan. Justification should be included in the project workplan if the Subsurface conditions are disturbed during probe placement. To allow for project proponent chooses not to use probe support for deep vapor wells. subsurface conditions to equilibrate and vapor concentrations to stabilize,the following procedures are recommended: Tubing Protection: Tubing should be protected from damage or clogging from subsurface soil materials by placing the tubing inside a Flush-mount 1. For soil vapor wells installed with the direct push method,do not conduct the casing. For deep vapor wells ensure the probe tip and tubing are properly purge volume test,leak test,and soil gas sampling for at least 30 minutes placed and tubing is not damaged or kinked. Properly mark tubing at the following vapor probe installation. surface to identify the probe location and depth. 2. For soil vapor wells installed with hollow stem or hand auger drilling methods, Soil Gas Well Completion: Soil gas wells should be properly secured, do not conduct the purge volume test,leak test,and soil gas sampling for at capped and completed to prevent infiltration of water or ambient air into the least 48 hours after vapor probe installation. subsurface and to prevent accidental damage or vandalism.For surface completions,the following components may be installed: 3. For soil vapor wells installed with the rotosonic or air rotary method,do not conduct the purge volume test,leak test,and soil gas sampling until it can be a. Gas-tight valve or fitting for capping the sampling tube; empirically demonstrated that the subsurface equilibrium time is sufficient for representative sample collection. b. Utility vault or meter box with ventilation holes and lock; Vapor well installation method and equilibration time should be recorded in c. Surface seal;and the field log book or field form. d. Guard posts. Decontamination Decontaminate all reusable equipment to prevent cross contamination. I Drive point methods may be appropriate for certain site conditions or circumstances depending on DQOs.The use of post-run tubing should be discussed with the Agency prior to inclusion In the workplan. 3/3/2010 -21- 3/3/2010 -22-