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API PUBL 4741:2005 pdf download

API PUBL 4741:2005 pdf download.Collecting and Interpreting Soil Gas Samples from the Vadose Zone.
Figure 2.4. Soil gas profile at a site with methane production in the source zone Johnson et al. 2003). This figure shows the soil gas proliles for oxygen (circles) and methane diarnonds.
Roggemans et iii. (2002) attempted to correlate soil gas prolilts with site characteristics and surluce cover conditions. but were unable to find any obvious correlations (i.e.. soil gas profiles below paved surfaces did not necessarily resemble those below buildings, and soil gas profiles beneath paved surfaces varied from site to site. Thus, the ability to anticipate the reduction in hydrocarbon vapor flux caused by biodegradation. based on site properties, is limited at this time. Soil-gas-profile data, therefore. are critical to understanding the subsurface processes and the net effect on hydrocarbon vapor migration to enclosed spaces.
To help visualize the impact that oxygen and aerobic biodegradation can have on vapor profiles at a site. Figure 2.5 and Figure 2-6 from Abreu (20051. show results from three-dimensional numerical simulations of vapor transport and aerobic biodegradation under homogeneous conditions near a building with a hasenient. Figure 2-5 resents how significantly the chemical of concern and oxygen soil gas profiles and the attenuation factor can be affected by changes in the source concentration, for a progression of source concentralions from 20 mgL to 20(1 mjiL For reference, the Roggemans et al. (2002 report suggests that source vapor concentrations> 200 mg/L would be representative of gasoline source zones above the water table, while source soil vapor concentrations < 20 mg!L. would be expected to occur near dissolved plumes down- gradient of source tones. In the 200 mg/I case, the effect of biodegradation is minimal relative to simulations without biodegradation, while the 2 mgL source vapor concentration case corresponds to attenuation that is six orders of magnitude different from the 200 mgL case (a=5.6 x 101 versus 7.1 x l0’). The major difference between the figures is the oxygen penetration depth beneath the building. In the 20 mgfL case, oxygen is found at elevated levels beneath the building footprint, so that chemical vapors are subjected to aerobic biodegradation along most of the transport pathway. It should be noted that these results are presented here simply to visualize trends and that they are specific to this depth; the influence of concentration on attenuation factors for aerobically biodegradable chemicals is expected to be more significant as the source depth is increased (Abreu 2005). FIgure 2-6 illustrates the effect of depth on soil gas profiles and vapor attenuation coefficients for aerobically biodegradable chemicals. Simulations are shown for slab-un-grade foundations and relatively high (200 mgi.) soil vapor source concentrations at depths ranging from I mto m below ground surfacc (Ahreu 2005). The cffcct of biodegradation relative to non-biodegradation simulations is minimal for the shallower depths. but is very signiticant for the source located K m below ground surface. Overall, the simulations illustrate that the significance of the effect of aerobic biodegradation is expected to be linked to the presence of oxygen beneath a foundation, and that attenuation due to aerobic biodegradation will increase with increasing source depth and decreasing source concentration.


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