API PUBL 4741:2005 pdf download

API PUBL 4741:2005 pdf download.Collecting and Interpreting Soil Gas Samples from the Vadose Zone.
o Thickness of the aerobic region
c Rate of biodegradation reactions.
Because oxygen transport and aerobic biodegradation play a significant role when soil gas profiles are established, the soil gas profiles measured near buildings may be quite different from those that would be measured beneath them, especially if the building foundation reduces the oxygen flux to the subsurface. In those cases, the effects of aerobic biodegradation on the soil gas profile will be minimal beneath the foundation.
The discussion of soil gas profiles assumes that the releases that have generated the vapor source have been in place for some period of time and that the concentrations of chemicals of concern in soil gas have reached a near-steady condition. Some data indicate that soil gas profiles are affected by seasonal changes: therefore, the near-steady conditions still exhibit some temporal variability.
2.2 Measured Soil Gas Proflles 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 sie) are plotted as a function of depth below ground surftice (zIL = 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 surftices; 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 surtlice) 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, hut 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 anacrohic/anoxic transition zone. Profile A was the most frequently observed by Roggemans et al. (2002).
In profile B, the oxygen is present throughout the vadose zone, except at the vapor source zone interface. The corresponding hydrocarbon profile reflects reduction in hydrocarbon concentration by aerobic biodegradation with distance above the vapor source. The effect of aerobic biodegradation, however, is less dramatic than in profile A. Profile 11 might be observed at shallow sites where transport distances are short and biodegradation is slow relative to the oxygen diffusion time scale through the vadose tone, or where vapor source concentrations are relatively low with respect to atmospheric oxygen concentrations (as might be the case above dissolved hydrocarbon groundwater plumes).
Profile C was collected beneath a basement overlying a high concentration vapor source. It is distinguished from the other profiles by the lack of oxygen at the monitoring points and less attenuation of the hydrocarbon vapor concentration. Relative to profile A. which also corresponds to a high concentration vapor source (but one beneath an uncovered surface) data suggest that in this case the building affects the oxygen transport and significance of the resulting aerobic biodegradation.
Profile D has an oxygen profile similar to that in profile B. but the hydrocarbon attenuation with distance away from the source is much more significant (i.e.. a four-order-of-magnitude decrease in concentration over a very short depth). This profile occurs when the source is located in a zone having lower diffusion rates than the overlying soils: for example. this data set corresponds to a case where the vapors origInate from within, or below, the capillary fringe.