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ADVISORY - ACTIVE SOIL GAS INVESTIGATIONS 2.0 INITIAL PROJECT PLANNING AND WORKPLAN DEVELOPMENT 2.1 STUDY PURPOSE AND DATA QUALITY OBJECTIVES A soil gas investigation may be undertaken for a number of different reasons and a single investigation may have multiple objectives. The data quality objectives (DQ05) for each investigation will vary according to the overall goals of each specific investigation. Examples of different purposes for performing a soil gas investigation are provided below: Determining if discharges of contaminants have occurred which may impact indoor air, outdoor air and groundwater, such as leaks at aboveground storage tanks (AST), underground storage tanks (USTs) or other underground pollution sources; Determining the spatial patterns and extent of vapor phase soil contamination, Designing and monitoring the performance of a soil vapor extraction system; Mapping soil vapor plumes to select buildings for indoor air monitoring; Creating a data set for performing a vapor intrusion risk assessment using either generic attenuation factors or a mathematical model to estimate indoor air concentrations from soil gas data; Remedy performance monitoring; and Providing data for no-further-action determinations at impacted sites. The DQO process is a systematic planning tool based on the scientific method for establishing criteria for data quality and for developing data collection procedures. By using the DQO process to plan environmental data collection efforts, the effectiveness, efficiency and defensibility of decisions can be improved. DQ0s should be established before an investigation is started. Example input parameters to the DQ0s include past, current and future land uses, regulatory action levels for contaminated media, laboratory method reporting limits, and the appropriate sample collection method. The expected output is the most resource-effective design for the study. Information concerning DQ0s is provided in USEPA (1994a, 1994b, 2000a). A critical step in developing site-specific DQ0s is the generation of a conceptual site model (CSM), discussed below in Section 2.3.2. Within the subsurface, contaminants may exist in the following phases: Solid phase by adsorbing onto the organic fraction of soil; Aqueous phase by dissolving in groundwater and pore water; Non-aqueous phase liquid (NAPL); and/or Gaseous phase, by accumulating in the interstitial space of soil particulates as soil gas. Thus, soil matrix and groundwater sampling and analysis should be considered for site characterization in addition to soil gas sampling to ensure that all potential phases of VOCs are evaluated and their associated exposure pathways. Soil gas sampling is July 2015 2