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1.0 INTRODUCTION LIST OF FIGURES The Advisory—Active Soil Gas Investigation(ASGI or Advisory)provides Figure and Description Pace technically consistent approaches for collecting and analyzing soil gas samples. The Advisory is not a regulation,nor does it impose any requirements or Soil Gas Vapor Well Emplacement Methods........................................................39 obligations on the regulated community. Rather,it provides a technical Figure 1 —Permanent/Semi-Permanent Gas Vapor Well Construction Diagram framework and reference for addressing soil gas sample collection and analysis. Figure 2—Multi-depth Gas Vapor Well Construction Diagram Other technically equivalent procedures may exist,and this Advisory is not Figure C-1: Example Apparatus For Field Screening and Sampling using Helium intended to exclude alternative approaches or methodologies. -= , = Tracer...................................................................................................................50 Active soil gas sampling and analysis refers to the methods utilized to collect Figure D-1:An example of soil-gas permeability testing equipment.....................52 vapor phase data at sites potentially affected by volatile organic compounds • Figure G-1:Soil Drainage Curves(Gardner,eta], 1970)......................................51 (VOCs),including chlorinated solvents,petroleum hydrocarbons,methane, Figure G-1:Soil Drainage Curves(Sisson,et al, 1980)......._...............................51 hydrogen sulfide and semi volatile organic compounds(SVOCs). The data obtained from a soil gas investigation can be used to identify the source and spatial distribution of contamination at a site or to estimate indoor air concentrations for risk assessment purposes. Collecting and analyzing soil gas samples allows the practitioner to: (1) Determine if discharges of contaminants have occurred which may impact indoor-air,outdoor-air and groundwater; (2) Determine the spatial patterns and extent of vapor phase soil contamination; (3) Establish the distribution of contaminated vapor for effective design of soil vapor extraction(SVE)systems;and (4) Determine the effectiveness of remedial action to reduce the threat of contaminated vapors to receptors,including groundwater. Within the unsaturated vadose zone,contaminants may partition into the following phases: (1) Solid phase by adsorbing onto the organic fraction of soil; (2),. Aqueous phase by dissolving in groundwater; • (3) Free liquid phase;and/or (4) Gaseous phase,by accumulating in the interstitial space of soil particulates as soil gas. Thus,soil matrix,groundwater,and soil gas sampling and analysis should be performed for site characterization in order to ensure that all potential phases of VOCs are evaluated. Using appropriate methodology,soil gas sampling is practicable and preferred for many geologic materials including fine-grained soil formations. For evaluating the risk associated with vapor intrusion to indoor air, including sub-slab sampling,consult the vapor intrusion guidance(Cal/EPA 2005a or its revisions). 3/3/2010 -ix- 3/3/2010 -10-