API PUBL 4741:2005 pdf download.Collecting and Interpreting Soil Gas Samples from the Vadose Zone
In the case of petroleum hydrocarbons, aerobic biodegradation has the potential to attenuate concentrations of chemicals of concern in soil gas and fluxes significantly as the vapors move toward buildings from soils or groundwater containing concentrations of petroleum hydrocarbon chemicals of concern. Therefore, soil gas data collected for the subsurface-vapor-to-indoor-air exposure pathway need to be focused on the purpose and use of the data and be based on these specific data-quality objectives. The collection and use of soil gas data to evaluate the subsurface-vapor-to-indoor-air exposure pathway is a relatively new approach for this exposure pathway, and limited information is available on the appropriate methodologies. To address this need, options for the collection, analysis, and interpretation of soil gas data are presented in this document. This document emphasizes conceptual models for vapor transport in the formulation of sampling plans and data analysis and presents information needed to support the selection of sample locations and depths. The overall process of collection and analysis of soil gas samples is supported through important reminders and checklists at the end of each section. A systematic process is provided in the data analysis section to check the collected data for inconsistencies and for determination of situations requiring further study.
2.2 Measured Soil Gas Profiles at Petroleum Hydrocarbon Impacted Sites
To better illustrate the connection between the conceptual model shown in Figure 2-2 and measured vertical soil gas profiles, sample soil gas profiles are shown in Figure 2-3 and Figure 2-4. Each of these profiles is consistent with the conceptual model, yet each is qualitatively different from the others. In these plots, normalized soil gas concentrations (actual values divided by the maximum concentration at that site) are plotted as a function of depth below ground surface (z/L = actual depth to soil gas sample/depth to the source at that site). Roggemans et al. (2002) performed an empirical assessment of soil gas profiles from petroleum hydrocarbon impacted sites and classified the data in terms of generalized hydrocarbon-oxygen soil gas profiles. Figure 2-3 represents specific examples of these profiles; all data originate from sites impacted by gasoline or other petroleum products. Note that most of the profiles presented by Roggemans et al. (2002) were measured near buildings or beneath paved surfaces; few were measured beneath buildings. The profiles in Figure 2-3 and Figure 2-4 show oxygen utilization (as evidenced by decreasing concentrations with depth below ground surface) and some level of hydrocarbon concentration reduction, although it is variable. In profile A of Figure 2-3, the oxygen penetrates about half of the distance down to the vapor source, but then is consumed by aerobic biodegradation over a short distance. This aerobic biodegradation is also reflected in the hydrocarbon concentration profile that shows the hydrocarbon concentration decreasing several orders of magnitude over a short distance near the anaerobic/anoxic transition zone. Profile A was the most frequently observed by Roggemans et al. (2002).