Text Box: Text Box: Page # Text Box: Volume 10, issue 2 Text Box: to be identified prior to interior sampling. This approach results in less inconvenience to building occupants and avoids the complexities of potential background sources of COCs”. “Other investigators and some states prefer going directly to interior testing, combining indoor air and subslab soil gas sampling, or just conducting indoor air sampling. The usual reason given for adopting this approach is concern over the reliability of exterior measurements or assumptions/models used to predict indoor air concentrations versus taking direct indoor air measurements. Perhaps the most reliable approach is to base the investigation and determination of pathway completeness on site-specific conditions and multiple lines of evidence with offsetting strengths and weaknesses”. On Page 33… “Groundwater contaminant plumes migrating under or near buildings may be a source of vapor intrusion. Because of the distance interval between groundwater monitoring wells, it is often necessary during the early stages of a site investigation to estimate the contaminant concentrations at locations that are closer to the building(s) of potential concern. While the concentrations between monitoring wells may be interpolated based on the distance between the wells, it is generally safer to use the higher concentration found in the nearby wells initially to estimate the contaminant levels that may partition into the soil pore space, Text Box: possibly leading to vapor intrusion. Since water concentrations at the top of the water table are the potential source of vapor partitioning, understanding the well construction and how groundwater samples were collected can be beneficial in making more accurate determinations. The vertical profile of concentrations and water table fluctuations may be necessary data for assessing the potential for off-gassing”. “Soil matrix data are less than ideal for evaluating vapor intrusion risk because of the uncertainty associated with using partitioning equations to calculate soil gas concentrations and the potential loss of VOCs during sample collection. Numerous studies have identified sampling biases, and USEPA (1993a) has determined that the scale of the biases can be as large as several orders of magnitude. Scientific studies have failed to show a good correlation between soil and soil gas sampling and analysis on a consistent basis. Consequently, soil data should be used as a screening or secondary line of evidence in vapor intrusion assessments”. On Page 34… “Measurement of soil gas is a common approach around the country for evaluating the vapor intrusion pathway. Soil gas data are reflective of subsurface properties and allow for real-time results. Risk-based screening levels for soil gas are usually higher than indoor air screening levels. Drawbacks to this method include the lack Text Box: of knowledge of the proper attenuation factor to apply when interpreting soil gas data; debate over the number, location, and method of sample collection; and uncertainty over temporal and/or spatial variations in soil gas concentrations. Soil gas data are recommended over other data, specifically soil matrix and groundwater data, because soil gas data represent a direct measurement of the contaminant that can potentially migrate into indoor air”. “Two methods are available to measure soil gas contamination in the unsaturated zone, active and passive soil gas sampling”. “Soil gas samples can be further differentiated by the location of the samples. Near-slab soil gas samples are collected outside a structure but within a short distance (usually 10 feet) of the building’s foundation. Soil gas samples collected 10 feet or more from the perimeter of the building are generally referred to as “exterior” samples. Finally, subslab soil gas samples are collected from below the building foundation or slab”. “Subslab soil gas sampling is the preferred approach of many regulatory agencies for investigating vapor intrusion, primarily because of the proximity of the sample location to the receptor and the elimination of background interferences (when proper sampling methods are employed). Subslab soil gas sampling involves collecting

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