During the course of the 1993 field season, a total of approximately 35.3 lb (16 kg) of hydrocarbons (out of the 88.2 Ib [40 kg] spilled) was removed from the soil in the form of vapor, as determined by vapor off gas measurements. At the same time, the measured concentrations in the extracted air dropped by about an order of magnitude (e.g., see the benzene and toluene data in Figure 21). (In Figure 21 and subsequent figures with time in hours on the horizontal axis, time refers to “pump-on” time rather than real time.) This drop is due to a number of factors which are difficult to decouple from one another, including: 1) gasoline became increasingly less available with time (due to its dissolution and diffusion into the matrix); 2) changes in the composition of the NAPL as the result of weathering reduced the overall vapor pressure of the mix; 3) flow rates (and applied vacuums) were changed; 4) water levels changed as the result of precipitation; and 5) airflow changed due to the probable establishment of preferential pathways in the soil.
At most SVE remediation sites, soil vapor concentrations drop relatively quickly. In many cases this probably results because the most easily removed mass (i.e., the most volatile or the most accessible) is quickly extracted from the system, leaving progressively less volatile and less-accessible mass to be removed. This process almost certainly occurred at the site.
Preferential removal of the more volatile contaminants from the mixture would result in an overall reduction in the volatility of the gasoline. As a consequence, under steady-state flow conditions, the recovery rate of the gasoline would be expected to go down. This decrease should also be reflected in changes in the composition of the extracted hydrocarbons. As will be discussed in the next section, there is some evidence of this in the ratios of compounds removed by the SVE system.
Based on the tracer test data listed in Table VIII, it is clear that air flow within the cell was irregular. It is likely that some of these pathways existed at the onset of the SVE process. It is also likely that some of these pathways became enhanced due to desiccation during the SVE process. As a result, portions of the soil were probably not effectively vented. Ail of the above factors probably played a role in controlling mass removal.
The cumulative hydrocarbon mass recovery in the SVE off-gas is seen in Figure 22.
Superimposed on the figure are the flow rates which were active during the various stages of pumping. The figure shows that mass recovery is fairly steady throughout a variety of changes in extraction conditions. Close inspection of the data indicates that there is some increase in mass recovery at the higher flow rates, but in some cases the higher flow rate also resulted in increased problems with water production.
44
Copyright American Petroleum Institute Provided by IHS under license with API
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S T D - A P I I P E T R O P U B L DR 225-ENGL 1 7 7 8 O732290 Ob12276 6 7 7
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Figure 2 1. SVE off-gas concentrations (g/m3) of benzene and toluene during the 1993 field season. SVE operating conditions are indicated along the top of the figure.
45
Copyright American Petroleum Institute Provided by IHS under license with API
Not for Resale No reproduction or networking permitted without license from IHS
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STD.API/PETRO PUBL DR 225-ENGL 1998 0 7 3 2 2 9 0 0632297 523
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Figure 22. Cumulative mass of hydrocarbons recovered (kg) during operation in the 1993 field season and the SVE conditions.
46
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S T D . A P I / P E T R O PUBL DR 225-ENGL I998 m 0732290 O b 1 2 2 9 8 4bT
Mass recovery of individual gasoline components is perhaps more revealing, although stili difficult to interpret. Figure 23 shows the recovery of isooctane, TCE, toluene and benzene as a function of time. The least soluble compounds, such as isooctane, did not move into the matrix (by dissolution and difision) to the same extent as did the more soluble compounds (e.g., TCE and MTBE). As a consequence, isooctane remained largely in the fractures where it was accessible to the SVE air, while MTBE was less accessible to the extraction system.
TCE, whose solubility is intermediate to toluene and benzene, showed greater recovery than either of those two compounds. This is likely to have occurred because both toluene and benzene were more readily biodegraded than TCE, however, the uncertainties in mass recovery are on the same order as the observed differences. (TCE was originally added to the mixture because it was thought to be non-degradable. However, it is possible that TCE could be degraded Co-metabolically in the soil, in part due to the degradation of toluene.)
The fraction of each of the compounds in the gasoline mixture recovered during extraction from the trenches in 1993 is seen in Figure 24. As expected, the less soluble and more volatile compounds were recovered to the greatest extent. The ratios of MTBE and isooctane to TCE are shown in Figure 25 and indicate that MTBE:TCE and isooctane:TCE have opposite responses. Also, demonstrated on the figure are the changes that occurred when system operating conditions were changed. For example, changes in air flow resulted in large changes in response for MTBE:TCE and isooctane:TCE. These increases and decreases are likely because changes in the system operation changed the flow distribution within the soil, thus changing air contact with the plume allowing the more accessible compounds (i.e., greater percent in fractures or higher vapor pressure) to be more readily removed.
The recovery of MTBE was greater than might have been expected considering it should have moved almost entirely into the soil matrix. However, MTBE has the highest aqueous-phase effective diffusion coefficient (into or out of the matrix) due to its high solubility and low sorption. It is also less degradable than most of the other compounds. This may cause it to move more readily from the mảix into the fracture where the SVE air can remove it. In contrast, benzene is more biodegradable and more sorptive, therefore, it will move from the matrix to the fracture at a slower rate.
Figure 26 shows the ratio of MTBE to isooctane and the inverse of that ratio. Isooctane was chosen because it is the least water soluble and it probably biodegrades slowly. The data indicate that at the time of system start-up isooctane recovery is much greater than MTBE. This is thought to occur because most of the isooctane is in the fractures, whereas most of the MTBE is in the matrix. As time progresses, the ratio of MTBE to isooctane increases to about 0.5.
This probably occurs because most of the mass transfer at that point is coming from the matrix.
This situation continues until extraction is switched to the other (east) trench, at which point the isooctane again increases dramatically.
47
Copyright American Petroleum Institute Provided by IHS under license with API
Not for Resale No reproduction or networking permitted without license from IHS
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~~
STD.API/PETRO PUBL D R 225-ENGL L79A 0732290 0632299 3Tb
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500 1000
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1500
Figure 23. Mass recovery of isooctane, TCE, toluene and benzene and the SVE operating conditions during the 1993 field season.
48
Copyright American Petroleum Institute Provided by IHS under license with API
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STD.API/PETRO PUBL D R 225-ENGL 1 9 9 8 0732290 Ob12300 748
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Figure 24. Fraction of mass recovered for each of the NAPL components in 1993.
49
Copyright American Petroleum Institute Provided by IHS under license with API
Not for Resale No reproduction or networking permitted without license from IHS
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S T D - A P I / P E T R O P U B L D R 225-ENGL 1778 0732270 0612301 884
: I
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I I I I I I I I I I I 1 I I I I I 1 r I I I
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1500
Figure 25. Mass ratios of MTBE and isooctane to TCE in the offgas and the SVE operating conditions during the 1993 field season.
50
Copyright American Petroleum Institute Provided by IHS under license with API
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~ ~
S T D . A P I / P E T R O P U B L DR 225-ENGL 1998 I 0732290 Oh32302 710
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Figure 26. Mass ratios of MTBE to isooctane and isooctane to MTBE in the offgas and the SVE operating conditions during the 1993 field season.
51
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S T D . A P I / P E T R O PUBL D R 225-ENGL 3 9 9 8 m 0 7 3 2 2 9 0 O b 3 2 3 0 3 b57 m