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Figure C-1:Example Apparatus for Field Screening and Sampling using Helium Tracer APPENDIX D SOIL GAS SAMPLING IN LOW-PERMEABILITY SOIL(EXAMPLE) Shroud o eas Probe Design and Construction:The following is example the probe How Controller and construction and design,which may be used for collection of soil al gas from a low- s Micron Filter _ ___ permeability soil. Alternative designs may be acceptable;however any probe in low permeability material should meet the following conditions: Summa canister Pump 1. Probes installed in a cored borehole(e.g.,Geoprobe°Dual Tube 3.25-inch . Helium meter diameter core barrel system(DT325),or similar); monitoring port 2. Probes constructed of clean inert materials(i.e.,Teflon, PEEK,nylon);. 3. Probes having a sand filter pack around the probe tip with a pore volume of - Lung Box about 1 L, 4. Probes having an annular seal of well-hydrated bentonite and 5. Probes having a ball valve with compression fitting(e.g.,SwagelokTM),or Tedlar Bog Teflon'-taped threaded fittings on the top. If lilhology with low permeability is anticipated,a continuous core should be collected Soil Gas/ and visual inspection of the core should be performed to assess the moisture profile and Sub-Slab Probe the probe screen should be installed at a depth above the capillary fringe. Laboratory Helium Addition Point analysis of the grain size distribution(sieves and hydrometer)and moisture content in selected samples from the soil core is also recommended. k- Solt Vapor Well Emplacement Method: In this example,a screened tip(e.g., :,..° Geoprobe®6-inch permanent implant screen with conical stainless steel tip)is used h attached via compression fittings to the 14-inch tubing. As a precaution,the implant References: .1�' screens are soaked in isopropyl alcohol and oven-dried to remove any cutting oils. An �,", equipment blank sample(i.e.,blank air sample)is collected from a fully assembled McAlary,T.A.,P.Nicholson,, H,Groenebelt,D.Bertrand.2009.A Case Study of Soil- probe prior to subsurface installation to identify any target analytes that may be Gas Sampling in Silt and Clay-Rich(Low-Permeability)Soils,Ground Water Monitoring attributable to contaminant from the equipment. &Remediation,.29,no.1,pp.144. - The well design described below is a modification of the design in main[ext. In this • USEPA 1996 United States Environmental Protection Agency EPA Publication SW- example,.filter sand is installed in the bottom of the borehole before the probe is 846,Test Methods for Evaluating Solid Waste.Ptrystcal/Chemical Methods 0040. inserted then sand is added to surround and extend above the screen. The total 5amp,J�yg of Principle Organic Hazardous constituents from Combustion o it s Using volume of sand should be sufficient to provide a pore volume of at least 1 L,assuming dlar Baos•National Technical Information Services U.S.U S. Department of Commerce. 40%porosity. For a 3.25-inch diameter borehole,this corresponds to a filter sand Springfield,VA December 1996,Section 4 1 2 Figure-1 interval of about 18 inches. Above the filter sand,3 to 6 inches of dry granular bentonite fhttD://www.el)a.Qoviepaoswerlhazwaste/tesLlpdfsIO04pdf 1 is placed to trap any liquid that might separate from the overlaying bentonite slurry or the water used to seal the borehole annulus. The bentonite seal should preferably be mixed as slurry. The actual volume of slurry added to the borehole should be verified to be equal to the calculated volume of the annular space between the probe tubing and borehole wall. Alternatively,the seal may be placed using alternating lifts of water and granular bentonite,provided that a tamping rod is used to facilitate mixing of the two components in-situ. A bolt-down flush-mounted casing or a well box should be installed as a ground surface completion. 12/1012009 -59- 12/10/2009 .60-