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2.2.3 Sample Depth: Sample depths should be chosen to minimize the <br /> effects of changes in barometric pressure, temperature, or <br /> breakthrough of ambient air from the surface; and to ensure that <br /> representative samples are collected. Consideration should be given <br /> to the types of chemicals of concern and the lithology encountered. <br /> A At each sample location, soil gas probes should be installed at a <br /> minimum of one sample depth, generally at five(5)feet below <br /> ground surface (bgs), in accordance with the most current ASGI. <br /> B. Samples should be collected near lithologic interfaces or based on <br /> field instrument readings (e.g., Flame Ionization Detector[FID], <br /> Photo Ionization Detector[PID]) from soil cuttings and/or cores to <br /> determine the location of maximum analyte concentrations at the <br /> top or bottom of the interface depending upon the analyte. <br /> C. Multi-depth sampling is appropriate for any of the following <br /> locations: <br /> 1. Sites identified with subsurface structures (e.g., USTs, sumps, <br /> clarifiers,waste or chemical management units), subsurface <br /> sources (e.g., oil fields, artificial fill, buried animal waste), <br /> changes in lithology, and/or contaminated groundwater. Soil <br /> gas probes should be emplaced below the base of any <br /> subsurface structures, sources or backfilled materials in the <br /> vadose zone. Collection of deeper samples should be done in <br /> consultation with Agency staff; <br /> 2. Areas with significantly elevated VOC concentrations detected <br /> during shallow or previous vapor sampling; <br /> 3. Areas where elevated field instrument readings are <br /> encountered from soil matrix cuttings, cores or samples; or <br /> 4. In the annular space of groundwater monitoring wells during <br /> construction, where an assessment of the vertical extent of soil <br /> gas contamination is necessary. <br /> D. If no lithologic change or contamination is observed, default <br /> sampling depths may be selected for multkdepth sampling. For <br /> example, soil gas samples may be collected at 5, 15, 25,40 feet <br /> bgs, etc., until either the groundwater is encountered or VOCs are <br /> not detected,whichever comes first. <br /> 1. Additional samples may be necessary based on site <br /> conditions. <br /> 2. For Preliminary Endangerment Assessments: When 40 feet <br /> bgs is reached, collection of deeper samples may be waived. <br /> 01/28/2003 - 5- <br />