API PUBL 4775:2009 pdf download
API PUBL 4775:2009 pdf download.Simulating the Effect of Aerobic Biodegradation on Soil Vapor Intrusion into Buildings.
4.2 Effect of First-Order Biodegradation Rate
The effect of first-order biodegradation rates on soil gas concentration profiles for the low source concentration range is illustrated in Figures 7 and 8 for basement and slab-on-grade scenarios, respectively. For these simulations, a vapor source concentration of 4 mg/L located at 4 m bgs was assumed. The hydrocarbon soil gas concentration profiles in Figures 7 and 8 show about 5 orders-of-magnitude reduction in hydrocarbon vapor concentration at about 2 m bgs as A increases from 0.079 to 2 h1. Additionally, for the source concentration and depth simulated, the oxygen concentration profiles beneath the slab show increasing oxygen utilization as A increases. This illustrates the sensitivity of the degradation rate for the vapor intrusion pathway. The ct-values decrease about 5 orders-of-magnitude for the basement scenario and about 10 orders-of-magnitude for the slab-on-grade scenario as A increases from 0.079 to 2 h1.
4.3 Effect of Source Depth
The effect of source depth on the soil gas concentration distribution for a vapor source concentration of 1 mg/L and a first-order biodegradation rate of 0.79 h1 is illustrated in Figures 9 and 10. Figure 9 shows the results for basement scenarios with sources at 3, 5 and 7 m bgs (corresponding to source-foundation separation of 1, 3 and 5 m). Figure 10 shows the results for slab-on-grade scenarios with sources at 1, 3, 5 and 7 m bgs (corresponding to source-foundation separation of —1, 3, 5 and 7 m). These simulations indicate multiple orders of magnitude decrease in cx-values with an increase in source depth of approximately 2 meters. For these scenarios, oxygen rich conditions (oxygen concentrations> 1 % vol/vol) are present throughout the subsurface, even for very shallow source depths.
sirrH I suer mufti-component sources. The contour ots also show s,Nar profNes for benzene from a single component source and BTEX and other aromatics for the multi-component source scenarios. This similarity in the aromatic hydrocarbon distribution is a result of similar chemical properties and biodegradation rates for these constituents. The concentration profiles indicate less biodegradation for aliphatic hydrocarbons than for the aromatic hydrocarbons even though the biodegradation rate constants for the aliphatics are higher than those for the aromatic compounds. This is a result of the different gas-water partitioning for these classes of compounds and the initial assumption that biodegradation is occurring only in the dissolved phase. Because the Henry’s law coefficient for aliphatics are much higher than those for the aromatics (see Table 3), only a relatively small portion of the total mass of aliphatic hydrocarbons is present in the aqueous phase assumed to be available for biodegradation. Under these assumed conditions the overall consumption of oxygen by the multi-component source is similar to the single compound source for the biodegradation rates used in this study.
Aliphatic hydrocarbons generally degrade faster aerobically than aromatic hydrocarbons, based on available data reviewed to date. DeVaull (2007) reported average degradation rates for aliphatics ranging from 4 to 1,100 hr1, with a geometric mean of 71 hr1. For aromatics, reported average degradation rates ranged from
0.4 to 2 h(1, with a geometric mean of 0.79 h(1. The vapor concentration profiles simulated in this work have not yet been compared to field measurements for all scenarios, so they may differ from site-specific conditions. Many of the simulated hydrocarbon concentrations are lower than typical laboratory reporting limits, so a direct comparison between the model and field data may not be possible at many sites. For low concentration hydrocarbon sources, the oxygen concentration profiles are not expected to change significantly from those presented in this report with changes in the aliphatic degradation rate constants because oxygen concentrations are typically much higher than the hydrocarbon concentrations, and are therefore less sensitive to changes.
Tables 6 and 7 present the attenuation factors predicted for the scenarios shown in Figures 16 and 17, respectively. The attenuation factors calculated for the aromatic hydrocarbons in the multi-component scenarios are very similar to the attenuation factor for benzene in the single component case. These results indicate that the calculated attenuation factors for the single component simulations may be applicable to the vapor intrusion evaluation for aromatic compounds in multi-component petroleum hydrocarbon sources. Note that the total concentration of all biodegradable species in the multi-component vapor source must be used to select an applicable attenuation factor and not just the concentration of the compound of interest.
