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Soil Vapor Extraction Test Report for SWMU B-3
Section 4 - Treatability Study Results and Data Evaluation
The purpose of this section is to summarize the sampling results and provide an evaluation of the data collected during the installation of the full-scale SVE system at SWMU B-3. The pilot SVE system was expanded from the original six VEWs to 18 VEWs. Soil samples were collected during system expansion to characterize the pre-study conditions and to further delineate the extent of VOC contamination present in the SWMU B-3 trench area. Additionally, resampling was performed in the spring of 2000 due to concerns regarding the accuracy of 1996 treatability study soil boring analytical results.
4.1.1 Confirmation Soil Sample Data
Confirmation soil samples were collected in December 1996 near five VEWs installed as part of the initial pilot test phase. The results of the 1997 confirmation sample analyses are compared with the 1996 results in Table 4.1.
The analytical results show a significant reduction in contaminant concentrations in the general vicinity of the VEWs with the exception of samples collected from VEW‑01 (9 to 11 feet bgs). The highest concentrations of VOCs were detected in VEW-01 (13 to 14 feet bgs) during the initial pilot test sampling. VEW-01 continued to exhibit the highest concentrations in the 1997 data, only from a shallower depth. During the confirmation sampling event, sample recovery was minimal when attempting to collect the VEW-01 sample from 9 to 11 feet bgs using the normal split spoon sampler, so a larger diameter spoon was used to collect the required sample volume. This sampling spoon likely penetrated below the 11 foot interval to obtain the necessary volume, so the actual sampling depth is probably deeper than the initial sample. This may explain the results obtained, since the deeper soils near this VEW had significant contaminant levels detected in 1996.
It is important to note that the only sample collected from a depth of less than 13 feet bgs (VEW-01, 9 to 11 feet bgs) also exhibited the least reduction in contaminant concentration. This finding suggests that the SVE system is very effective at removing VOCs from the deeper soils at the site, which is meaningful because the majority of contamination has been identified in the deeper portions of the trench. The results also indicate that the SVE system is not as efficient at removing contamination from shallower intervals. However, the apparent VOC reductions observed from the shallower sample interval at VEW-01 are probably sufficient to maintain removal from this less contaminated interval during normal operation of the SVE system.
PCE was detected at low concentrations in samples collected during the 1997 confirmation sampling event, however, it was not detected during the 1996 initial pilot testing analyses. PCE concentrations in samples VEW-02 (14-15) and MPD (14-15) were at 0.26 mg/kg and 0.13 mg/kg, respectively. To accommodate the lower PQLs used in 1997, lower calibration standards were used to calibrate the instruments. Consequently, PCE was detected at lower levels than was detected in 1996.
4.1.2 Characterization Soil Sample Data from Newly Installed VEWs
The locations of the sampled boreholes are shown on Figure 2.3. The samples were collected from depths ranging between 13 and 19 feet bgs. This range of depths were identified during the initial pilot study as having the greatest levels of VOC contamination. Soil core samples were also scanned with the Photovac 2020 during drilling activities to identify the most contaminated intervals of soil cores for sample collection.
One rework soil sample was collected from each of the 12 borings drilled for VEW installation in the expanded SVE system. The analytical results are presented in Table 4.2. Table 4.2 also includes the criteria for the detected compounds using the RRS 2 (groundwater protection for residential scenarios) and background concentration ranges established for different soil and rock types at CSSA. The background levels of metals for soils proximal to SWMU B-3 are provided, and fill soils are assumed to be Tarrant Association gently undulating, the native soil found at SWMU B-3. To assess possible cleanup goals for the SVE remediation system, TNRCC RRS 2 closure values (residential groundwater protection) for VOCs were selected as a conservative benchmark. These RRS 2 values were also used to delineate the target treatment area in the main trench for evaluating the volume of contaminated soils and calculating the contaminant mass present in that treatment area. The delineation of the estimated treatment areas, based on RRS 2 values, is shown on Figure 4.1.
The uncharacterized portion of the landfill south of the VEW grid is also shown on Figure 4.1. The landfill boundaries are estimated based on aerial photography and geophysical survey results.
Volatile Organic Compound Contamination Delineation
VOCs were detected in all of the rework soil samples collected during the redrilling near SVE wells VEW07 through VEW18. TCE was detected in four of the samples at concentrations ranging from 0.002 mg/kg in VEW-07 (16-17) to 44.0 mg/kg in VEW-18 (17-18). Three of the samples, VEW-16, VEW-17, and VEW-18, had concentrations of TCE exceeding the TNRCC RRS 2 groundwater protection criteria for residential use. No other VOC contaminant was detected above RRS 2.
Comparison of the rework sample results to the original ITS data indicates there is a general agreement between the results with respect to the analytes present and their distribution. The concentrations detected at each location are generally lower in the 2000 results as compared to the original ITS data collected in 1996, possibly due to ongoing SVE operations at the site. The increases observed in TCE concentrations at VEW16, VEW17, and VEW18 are likely attributable to localized differences and variations within the fill material.
Based on the results of the soil sampling performed during the initial pilot test, the 2000 resampling activities, and the use of the RRS 2 values listed in Table 4.2, the area of the SWMU B-3 trench that requires treatment for VOCs is approximately 8,400 square feet (approximately 70 feet east-west and 120 feet north-south). Estimated thicknesses of this potentially contaminated soil range from 10 to 15 feet bgs inside the trench area. The total cubic footage of the treatment area is conservatively estimated as 126,000 cubic feet (or approximately 4,700 cubic yards). The boundaries of the required treatment area were determined by outlining the VEWs that exceeded the cleanup criteria, and included other proximal sample locations with VOC concentrations greater than ten percent of the cleanup criteria (RRS 2) as a conservative measure (see Figure 4.1). Soil contaminant contours for TCE, cis-1,2-DCE and PCE are shown in Figures 4.2, 4.3 and 4.4, respectively.
The bulk density values from the 1997 testing event are more indicative of actual conditions present in the trench, and in the underlying limestone because they were determined by a test method (ASTM D2937) more commonly applied to vadose soils. The 1996 bulk density values were determined through a laser technique that is geared primarily for rock formations. The average bulk density measured from 1997 soil samples was 1.38 g/cm3 (or 86 pounds per cubic foot). Based on this average, the total mass of fill material in the trench that requires treatment is approximately 10,836,000 pounds of fill material. This is a significant reduction from 25,875,000 pounds that was estimated based on 1995 and 1996 soil gas data, geophysical data, and 1996 pilot test results. Although VOC reductions have already occurred from operation of the pilot test system, the initial pilot test VOC concentrations were averaged with the results of the 1997 treatability study to conservatively estimate the contaminant mass that remains in the trench. The average TCE concentration equals 17.6 mg/kg, and the average cis-1,2-DCE concentrations equal 3.6 mg/kg. The estimated mass of these contaminants in the targeted treatment area is approximately 190 pounds of TCE and approximately 39 pounds of cis-1,2-DCE.
Using the average VOC concentrations derived from the 1996 pilot test (Parsons ES GWIASC, 1996 June), the pretreatment mass of contaminants was approximately 178 pounds of TCE and 36 pounds of cis-1,2-DCE. The contaminant mass removal summaries indicate that mass of TCE and cis-1,2-DCE being removed is approximately equal to or exceeds the mass remaining in the landfill, which suggests that additional contaminants are also being removed from peripheral area (i.e., less contaminated soils in the main trench and limestone fractures).
Metal Contamination Delineation
Similar to the results from the initial 1996 pilot test, metal contamination appears to be located randomly throughout the trench areas. Of the 12 samples analyzed for metals, nine samples collected from the Glen Rose Formation and one sample of the soil fill material (Tarrant) were found to exceed background. Samples VEW-08 (8 to 9), VEW-12 (16.5 to 17), and VEW-14 (16 to 16.5), had metals detected at concentrations greater than background. Barium cadmium, chromium, copper, lead, nickel, and zinc were all detected in sample VEW-08 (8 to 9) at concentrations that exceed the soil background. Lead was present in concentrations significantly above background in sample VEW-12 (16.5 to 17) collected from the Glen Rose Formation. Cadmium was present above the background in the Glen Rose sample from sample VEW-14 (16 to 16.5). Several other metal analytes were detected near or slightly above the natural background levels, but none were as high as detected in these three samples. It is important to note that VEW-08 is located less than 15 feet from MPD, which was the only sample location inside the main trench area that had high levels of metals from the pilot test sampling. This indicates that the northern portion of the SWMU B-3 trench may contain the most significant metals contamination. VEW-06, which is located outside of the main trench, was the only other sample location with significant metals contamination. With the exception of isolated pockets at VEW-12 and VEW-14, the fill material in the main trench south and west of MPD and VEW-08 appears to be relatively free of metal contamination, and very near background levels.
Geotechnical Data
Ten soil samples were collected during drilling of the additional VEWs for geotechnical analyses. Results of the geotechnical analyses are presented in Table 4.3. Six of the samples were collected in weathered limestone; the remaining four samples were collected from fill material. Sample VEW-14 (10-11) contained debris in the
Total organic carbon levels in sample VEW-14 (10-11) were much greater than any of the other samples tested. The debris in the recovered sample volume from this sample also made it impractical to test for particle size distribution or intrinsic permeability. The reported values from VEW-14 are not used in the averaging calculations used to estimate soil volume/mass.
Based on the data from these samples, none of these physical parameters appear to limit the potential effectiveness of SVE in the SWMU B-3 trench. In fact, the values from the 1997 testing indicate less density than was reported for the 1996 pilot test. The 1997 porosities appear more realistic given the types of soil encountered in the trench area. Fill material samples indicated relatively low bulk density (high porosities) and the reported densities for weathered limestone samples are similar to those expected for finely textured soils (clays). Similar to the 1996 data, moisture contents remain highly variable in the trench. However the 1997 data indicates that clay contents are slightly greater than reported in 1996. This clay content is primarily responsible for the low permeability values reported in the 1997 data. Permeability testing was not performed on the 1996 soil samples.
4.1.3 Soil Gas and Air Emission Data
A total of 34 soil gas/air emission samples were collected for analytical testing throughout the duration of this treatability study. These samples were analyzed by ITS Laboratory and due to concerns previously discussed, are of questionable accuracy. Resampling was not performed for these samples because the exact conditions of the original sampling effort could not be duplicated. However, the original sample results can be used as screening results and do allow an assessment of emissions from the system. These results are grouped by testing event in Table 4.4. This table also includes TVH field screening results from corresponding sampling events to provide data on the general relationship between TVH levels and contaminants of concern. The soil gas data are examined more thoroughly in discussions of the testing event results (Sections 4.2 through 4.5.)
The results indicate that a simple correlation exists between the field TVH data and laboratory reported VOC concentrations. Generally, the greater the field TVH, the greater the concentrations of VOCs, and in particular, TCE. This implies that field screening instruments can continue to be a valuable tool for assessing the relative contaminant concentrations in both soil gas and emissions in a flow stream.
4.1.4 Quality Assurance Summary
All laboratory data collected as part of the SVE treatability study was validated using the AFCEE Quality Assurance Project Plan (QAPP) version 1.1 to ensure generation of defensible data. An Informal Technical Information Report (ITIR) is provided as Appendix A to this report. Deviations from the QAPP or the analytical method and a discussion of the overall usability of the data are presented in the ITIR.
In summary, all samples were prepared and analyzed within the specified holding times. Only EPA-approved analytical procedures were used. Noted laboratory problems and deviations from the QAPP are summarized below.
No MS/MSD samples were collected for the metal analysis.
Some methane blank contamination was encountered for sample group 214 during RSK-175 analyses.
Particle size analysis and falling head permeability tests for sample VEW-04 (10 to 11 feet bgs) were not conducted due to insufficient sample volume.
Relative percent differences for several analytes were outside the AFCEE specified control limits in several samples. The appropriate samples have been flagged �J� as estimated.
4.2 - January 1997 System Check
The objectives of the January 1997 system check were to determine the long-term influence on the SVE configuration throughout the test area and to determine the reduction in VOC removal rates that has occurred since the pilot test began operation in March 1996.
Based on the data shown in Table 4.5, the majority of the VOCs after at least 14 days of extraction are being removed from VEW-04 and VEW-05. This can be seen more strikingly in the field TVH, but the volatile contaminants of concern also match this finding.
A comparison of the VOC removal rate data from the pilot test operation and the January 1997 system check indicate that cis-1,2-DCE and TCE continue to be removed near steady state conditions. That is, the rates of removal from the January 1997 system check are approximately equal to the rates of removal from the pilot test after 48 to 96 hours. However, the January 1997 system check indicates removal of PCE, a compound which the pilot test operation did not identify as a contaminant of concern.
A pressure influence test was also performed during the start-up system check by checking the newly installed VEWs for pressure responses prior to shutting the system off for the hydrocarbon recovery test. Figure 4.5 shows the results of this testing. It was later determined that the only two VEWs responsible for these pressure responses were VEW-04 and VEW-05. This was determined by shutting all VEWs in the initial pilot test configuration but one and measuring the responses. As can be seen on Figure 4.5, the areas that are the most influenced by this configuration include the MPs and VEWs located on the eastern perimeter of the main trench. VEW-13 and VEW-15, in the central portion of the main trench, also exhibited some low pressure responses, and VEW-16 was the only point on the western side of the trench that was influenced by the initial configuration. The greatest pressure influence was observed in VEW-11. These findings imply that the VEWs installed outside of the main trench, and screened through limestone on the perimeter are capable of exerting a pressure gradient on over half of the delineated treatment area. Fractures through the limestone are the probable reason that all of these areas can be influenced by VEWs located outside of the main trench. This fracture network is probably very complex and obviously prevalent in the vicinity of VEW-04 and VEW-05.
4.3.1 Soil Gas Equilibration Time
The time required for soils at SWMU B-3 to return to near static conditions was determined to be 24 days. This is the time that the system was idled between testing events so that changes in soil gas from normal conditions could be quantified. The system was shut down on
Most of the changes observed in the soil gas were oxygen decreases and carbon dioxide increases, with rather sporadic changes observed in TVH levels. A few wells exhibited almost no changes in soil gas composition during the entire monitoring period. The 5- and 10-foot intervals of MPB, MPC, and MPD each only experienced a slight decrease in oxygen with no apparent changes in TVH or carbon dioxide. TVH increases were only observed in the four VEWs located on the eastern perimeter of the expanded system (VEW-09, VEW-10, VEW-11, and VEW-13), whereas TVH decreases were observed in the four VEWs on the western perimeter of the main trench (VEW-15, VEW-16, VEW-17, and VEW-18). VEW-15, which initially had very low oxygen, experienced an increase in carbon dioxide and rather sporadic changes in TVH.
These findings suggest that operation of the initial four VEW pilot system had some impact on the soil gas composition throughout the entire portion of the main trench covered by the expanded SVE system. Every location sampled demonstrated that at least minor changes occurred after extraction was discontinued. Expected changes from areas that had been influenced by the pilot test system include decreases in oxygen, increases in carbon dioxide, and increases, or subtle changes in TVH, depending on the surrounding formation. During extraction, subsurface soil gas that is influenced by the extraction process is drawn toward the extraction well�s gradient, and eventually is displaced by air from either the surface (ambient) or surrounding lateral porosity.
4.3.2 Soil Gas Influence Observations
The results of the hydrocarbon recovery test were also evaluated to determine which VEWs or areas of the trench experienced significant changes in soil gas concentrations of oxygen, carbon dioxide, and TVH following shut down of the system. Figure 4.6 shows those VEWs that experienced significant changes in their soil gas composition during the hydrocarbon recovery period. Although these points may not be directly influenced by the pilot test configuration, displacement of soil gas from adjacent locations has resulted in indirect influences on these location, resulting in the changes observed. Only two VEW locations, VEW-14 and VEW-15, did not exhibit significant changes to the soil gas composition during the recovery period. MPB-5, MPB-10, MPC-5, and MPC-10 also exhibited relatively minor changes, although the 15-foot interval at each of those points experienced major changes. The pressure response at those locations was also observed only in the 15-foot interval. This indicates that the majority of the influence on the VEWs is probably associated with the deeper portions of the screened intervals at each VEW.
4.3.3 Oxygen Utilization Evaluation
Consumption of oxygen and generation of carbon dioxide are direct indicators of biological activity. Both of these parameters were measured over time during the hydrocarbon recovery test. After the SVE system was shut down, soil gas samples were also collected from VEW-01 over time for analytical testing to examine data for additional chemical parameters which support the determination of biodegradation of TCE and PCE. VEW-01 was selected because it contained the highest VOC concentrations from soil gas samples collected during the 1996 pilot test. The most common breakdown products from the microbial metabolism of TCE and PCE include cis-1,2-DCE, vinyl chloride, ethane, ethylene, and carbon dioxide.
The biological activity indicator testing results are presented in Table 4.6. The first few analytical samples were collected within a short time frame (three samples in the first 30 hours of the test) to assess the generation of degradation products as the soil environment changed from aerobic to anaerobic conditions. The remaining samples were collected after the system had been shut down for 8 days and 27 days. Method RSK-175, which analyzes soil gas samples for methane, ethane, and ethylene, was not requested for the 8- and 27-day samples, so a final sample was collected from VEW-01 following the 24-day recovery time after MCT-1 to assess the accumulation of degradation products after a longer idle period.
The results provide strong evidence that biodegradation of TCE and/or PCE is actually occurring within the main trench at SWMU B-3. This is indicated by the rapid decreases in oxygen, increases in carbon dioxide, and the increases in the concentrations of cis-1,2-DCE, ethane, and ethylene. Oxygen utilization rates ranging from 2.16 percent per day in VEW-01 to 7.08 percent per day in VEW-11 were observed throughout the main trench, with only a relatively few locations exhibiting only minor decreases in oxygen levels. Figure 4.7 shows the oxygen utilization over time at VEW-01 following the shut down of the SVE system. The degradation rate calculated using pilot test porosity, moisture contents, and assuming a mineralization mass ratio of hydrocarbon to oxygen utilization of 1 to 3.5 (hexane), is approximately 354 milligrams of hydrocarbon per kilogram of soil per year at VEW-01. This value appears to be fairly typical for utilization rates observed at the site. Oxygen utilization determination graphs and biodegradation calculation sheets for VEW-01, VEW-11. MPD-15, and VEW-04 are included in Appendix C.
Although this data provides direct evidence of TCE and PCE biodegradation, it is important to consider that most of the oxygen that was utilized during the recovery period was likely consumed during the metabolism of less recalcitrant hydrocarbons that are present in the landfilled material. In fact, it is difficult to determine the portion of TCE and PCE degradation that occurred during the recovery period. The reason for this is because increases in cis-1,2-DCE could have resulted from the breakdown of PCE and/or TCE, or from volatilization from soils. Likewise, the generation of ethane, ethylene, and carbon dioxide could be the breakdown products of other hydrocarbons that are biodegrading at the site.
4.3.4 Volatilization Observations
Another key item evaluated from the results of the hydrocarbon recovery data is the volatilization rate of VOCs from soils into the soil gas phase. The volatilization of VOCs into the soil phase is the key factor that ultimately controls the rate at which VOCs can be removed from the trench�s soils by SVE. As shown in Table 4.6, the rate of TCE showed only slight increases in soil gas levels through the first 8 days of the recovery period, whereas PCE was not detected.
Meanwhile, cis-1,2-DCE demonstrated a rather marked increase over time, however, it is difficult to determine whether this increase was from volatilization or degradation. It is interesting to note that the most dramatic increase in cis-1,2-DCE levels occurred during the 17-day period following the 8-day sampling event. During this same period, the concentrations of TCE decreased dramatically. This may imply that a large portion of the decrease observed in TCE resulted from biodegradation of TCE to cis-1,2-DCE. There were no dramatic increases in removal rates when SVE was reinitiated after any of the three idle periods.
Regardless of this observation, it appears that significant volatilization of VOCs does not occur while the system is shut down. This indicates that a pulsing system, as compared to continuous operation, would not likely improve the efficiency of VOC removal from the main trench at SWMU B‑3.
4.4 - Subsurface Influence Findings
The primary objective of the multiple configuration tests was to determine the extraction influence from several different extraction scenarios. Data from the multiple configuration testing can be evaluated to identify portions of the subsurface soils that are affected by extraction from the different configurations. Two major types of data were evaluated to assess the relative effect that a configuration exerts on subsurface soils in the main trench, pressure response, and soil gas composition changes. Pressure responses provide direct evidence that subsurface soils in the vicinity of the monitored VMP are or are not influenced by the extraction configuration, and the relative degree of influence. Soil gas changes are less precise and can also result from indirect influences, such as gradients created by displacement of soil gas from adjacent locations. Rainfall is another key factor that may have affected subsurface influence results, because an inordinate amount of precipitation occurred during the MCTs, and in particular during MCT-2.
It was rare for oxygen, carbon dioxide, and TVH to all experience significant changes in influenced VEWs or VMPs. In fact, consistent and significant changes in TVH were relatively uncommon throughout all three MCTs. For purposes of this evaluation, it was concluded that significant changes in soil gas compositions were observed if either oxygen levels increased at least 2 percent, carbon dioxide levels decreased at least 2 percent, and TVH levels changed by at least 100 ppm during the course of each test period. It was also necessary for the changes to follow a trend, such that sporadic changes were given less credence. The results of all three MCTs are summarized in Figure 4.8. Figures for each MCT are included in the description of results for respective tests. The evaluations of the three MCTs are discussed in the following sections. Data collection sheets for the MCTs that were used to assess soil gas changes are presented in Appendix C.
4.4.1 Multiple Configuration Test 1
The results of MCT-1 are graphically presented on Figure 4.9. Figure 4.9 shows the locations of the six-VEW configuration relative to the VEWs and VMPs that were used as monitoring points. Pressure responses observed in the monitoring points were low for this configuration. However, there were only two locations in the main trench area that did not exhibit any soil gas changes indicative of influence: MPC and VEW-02. Although most soil gas responses were small, almost every point exhibited some changes in soil gas. The most prominent changes were observed in MPA, VEW-03, VEW-13, and MPD. The results from this test indicate that MCT-1 is capable of influencing most of the subsurface soils at the site, but that the extent of that influence is limited.
4.4.2 Multiple Configuration Test 2
The results of MCT-2 are graphically presented on Figure 4.10. Figure 4.10 shows the locations of the six-VEW configuration relative to the VEWs and VMPs that were used as monitoring points. Some of the monitored locations exhibited high pressure response readings indicating a strong influence from the MCT-2 extraction configuration over portions of the trench. MPA-15 and VEW-18 had readings of 7.0 and 4.5 inches of water, respectively. Other areas that had pressure response values greater than 0.1 inches of water include MPD-15, VEW-03, VEW-05, MPA-5, MPB-5, and MPC‑5.
The 5- and 10- foot interval in MPB had pressure response of 0.2, whereas the 15-foot interval only had a response of 0.05 inches of water. MPB-5 and MPB-10 are located in fill material and that the edge of the limestone perimeter of the main trench, whereas MPB-15 is screened in limestone. Therefore, the pressure response difference is caused by the inability of this configuration to exert influence over locations in the limestone formation. This conclusion is supported by the low pressure responses also observed in VEW-04 and VEW-05. This lack of influence from VEW-04 and VEW-05 could be related to the high water levels that were observed in VEWs installed at the site during the performance of MCT-2. Water levels were measured at approximately 14 to 15 feet bgs at the beginning of MCT-2, and had dropped to near 17 feet bgs by the end of the test period. Elevated water levels reduce the effective screened intervals, and may create a seal from the more permeable intervals in each VEW. Water was encountered during soil gas sampling at MPA-15, MPC-13, and MPD-15 during most of the sampling events.
Significant soil gas changes were observed at all but three of the monitoring locations: VEW-07, VEW-08 and VEW-17. Given the locations of the extraction wells, it is not surprising that no influence was observed in VEW-07 and VEW-08. However, it is notable that VEW-17 did not exhibit any signs of influence. The areas that appeared to be affected most by MCT-2 include VEW-12, VEW-18, VEW-03, MPA-5, MPA-10, and MPD-5. MPD-5 changes are also interesting given the proximity of VEW-08 to MPD. Neither MPD-10 or MPD-15 experienced any significant soil gas changes.
MCT-2 appears to affect monitoring points located in the main pilot test trench, and VEW-18 and VEW-15, located on the southern portion of the main trench, but appears to be limited in its influence of the areas north of the pilot test VEWs and in the limestone. This may be the result of high water that saturated, and therefore blocked, some of the primary preferential air flow pathways in the fractured limestone. The high water was the result of an unseasonably wet period prior to and during the performance of MCT-2
4.4.3 Multiple Configuration Test 3
The results of MCT-3 are graphically presented on Figure 4.11. Figure 4.11 shows the locations of the six-VEW configuration relative to the VEWs and VMPs that were used as monitoring points. Note that the VEW locations employed in MCT-3 are located across the entire site. These locations also represent the VEWs that exhibited the greatest potential TVH removal based on preliminary results from the January 1997 system check, MCT-1, and MCT-2. Two of the monitored VEWs (VEW-18 and VEW-11) exhibited high pressure responses, and two other VEWs (VEW-02 and VEW-05) exhibited lower responses. No other pressure responses were observed at any of the other monitoring points.
Significant soil gas changes were observed at all but two of the monitoring locations: VEW-11 and VEW-17. The areas that appeared to be affected most by MCT-3 include VEW-09, VEW-16, VEW-18, MPA-5, MPA-10, MPB-5, MPB-10, MPC-5, MPC-10, MPD-5, and MPD-10. This suggests that fairly uniform influence is exerted throughout the trench from this configuration with the exception of the two �dead� areas. Also, only slight changes were observed in VEW-13, VEW-14, VEW-01, and VEW-02, which are in the central portion of the trench. As with MCT-2, the deepest intervals at MPA, MPB, MPC, and MPD were saturated due to high water tables caused by unseasonably heavy rainfall events during the MCT-3 test period.
4.5 - Air Emission/Mass Removal Summary
The Texas Clean Air Act requires permitting of any emitter of pollutants to the atmosphere. The Act is implemented through 30 TAC Chapter 116, �Control of Air Pollutants By Permits for New Construction or Modification�. SVE systems remove chemical constituents from soils, emitting volatile organic compounds (VOC), and possibly other air pollutants into the atmosphere. Generally, most SVE systems involve very low air pollution emissions rates allowing them to be exempted, as outlined in 30 TAC 116.211, under Standard Exemption 68. Standard Exemption 68 incorporates portions of Standard Exemptions No. 80, 88, and 118. In addition, standard exemptions are limited to a total maximum of six pounds per hour emission rate.
For SWMU B-3, the effectiveness of SVE is determined by three main factors: vapor flow rate, contaminant vapor concentration, and vapor flow path. By multiplying the vapor flow rate by the contaminant vapor concentration, the estimated effective removal rate can be determined.
The estimated removal rate for each compound is determined by multiplying a conversion factor, the measured VOC concentration (contaminant specific), the flow rate, Figure 4.11 and the molecular weight (see equation). The actual removal rates are quantified by using the following equation:
Ract = MWQCact
where:
Ract = actual rate of removal (lb/hr),
MW = contaminant molecular weight (lb/lb-mole),
Q = vapor flow rate (ft3/min),
1.581x 10-7 = conversion factor (lb-mole-min./ft3-ppmv-hr)
Cact = measured vapor concentration (ppmv).
Using the 1996 pilot test data, emissions were estimated to obtain a standard exemption for the SVE system. The speciated chlorinated hydrocarbon emission rates are presented in Table 4.7.
To compare a conservative scenario with the emission limits established in the applicable Standard Exemption, the maximum 30 minute soil gas concentrations was used in the calculations. During an SVE operation, the soil gas concentrations typically start at a maximum concentration and decrease asymptotically to steady state conditions. Therefore, the soil gas concentrations presented in the emission calculations are the maximum concentrations of chemical compounds in the initial soil gas 30 minutes after the pilot system was turned on. In addition, emission rates were calculated using the maximum flow rate of 80 standard cubic feet per minute (SCFM). The current Standard Exemption for the SWMU B-3 SVE system includes the use of six wells and a maximum estimated VOC emission rate of 1.1 lb/hr. A modification to this standard exemption is being prepared to allow extraction from a maximum of 18 VEWs from SWMU B-3.
Multiple Configuration Tests
During the MCTs, samples were collected for VOC analysis from a sampling port located on the inlet pipe before the moisture separator to estimate the rate and volume of VOC removal and to assess the quantity of contaminants that may be discharged to the atmosphere during normal operation of SVE system as configured by MCTs. To quantify the mass removal rate during the tests, a total of 19 exhaust air samples were collected at an in-line sampling point for laboratory VOC analysis. Emission samples for each MCT were collected between 15 to 60 minutes after initiating air extraction, after 6 hours of operation, and after 24, 48, 96, and 336 hours of operation. During MCT-3, a sample was collected 672 hours from initiating extraction. VOC measurements from the exhaust port were coupled with air flow rate measurements to estimate the cumulative mass of VOC removal taking place over time. Periodic monitoring with a hydrocarbon meter was also conducted at the vapor outlet port to evaluate emissions.
The first MCT (MCT-1) extracted from VEW-07, VEW-08, VEW-12, VEW-15, VEW-17, and VEW-18. Table 4.8 shows the results of analyses of air emissions sampling along with estimated removal rates and total quantities removed resulting from the SVE MCT-1 operations. The total maximum VOC emission rate from the operation of the SWMU B-3 SVE system as configured by MCT-1 is 0.064 lb/hr. The estimated total mass of contaminants removed during MCT-1 is 17.57 pounds. Table 4.9 presents the estimated total mass of TVH removed from each well of the SVE MCT-1 configuration. VEW-12 was identified as the main contributor of contaminant removal. Data and calculation worksheets are presented in Appendix C.
The second MCT (MCT-2) extracted from VEW-09, VEW-10, VEW-11, VEW-13, VEW-14, and VEW-16. Table 4.10 shows the results of analyses of air emissions sampling along with estimated removal rates and total quantities removed resulting from the SVE MCT-2 operations. The total maximum VOC emission rate from the operation of the SWMU B-3 SVE system as configured by MCT-2 is 0.0343 lb/hr. The estimated total mass of contaminants removed during MCT-2 is 5.56 pounds. Table 4.11 presents the estimated total mass of TVH removed from each well of the SVE MCT-2 configuration. VEW-10 was identified as the main contributor of contaminant removal. Data and calculation worksheets are presented in Appendix C.
The third MCT (MCT-3) extracted from VEW-03, VEW-04, VEW-08, VEW-10, VEW-12, and VEW-15. Table 4.12 shows the results of analyses of air emissions sampling along with estimated removal rates and total quantities removed resulting from the SVE MCT-3 operations. The total maximum VOC emission rate from the operation of the SWMU B-3 SVE system as configured by MCT-3 is 0.014 lb/hr. The estimated total mass of contaminants removed during MCT-3 is 6.37 pounds. Table 4.13 presents the estimated total mass of TVH removed from each well of the SVE MCT-3 configuration. VEWs-10 and 12 were identified as the main contributor of contaminant removal. Data and calculation worksheets are presented in Appendix C.
A general review of the emission rate estimate from the MCTs reveal that all existing VEWs could be incorporated into the configuration for full-scale operation and stay within compliance of the standard exemption criteria. The observed contaminant removal rates are approximately equal to the steady state (>24 hours of continuous operation) contaminant removal rate observed during the 1996 pilot scale test. Thus, the continuous operation of the SVE system has a greater benefit than pulsing the SVE system. An estimated 18 pounds of TCE and 9.7 pounds of cis-1,2-DCE was removed during the performance of all three MCTs. These removal estimates are consistent, or perhaps slightly greater, than would be expected considering the estimates of contaminants present in the soil prior to performing the first MCT (TCE at 124 pounds, and cis-1,2-DCE at 18 pounds). Removal rates are greater than contaminant estimates because the system is also removing VOCs from soils outside of the target treatment area.
4.6 - Investigation Derived Waste Management Summary
IDW generated from the SVE installation effort included drill cuttings, PPE, and miscellaneous debris. The PPE and miscellaneous debris were placed into plastic bags and disposed of along with general plant trash. Drill cuttings generated were placed into labeled 55-gallon containers.
A review of validated 1997 analytical data identified soils from borings VEW-01, VEW-02, VEW-06, VEW-08, VEW-09, MPA, MPD, and MPE have a potential for exceeding the 30 TAC 335 subchapter S, RRS 2 SAI-Ind standards. Accordingly, the containers which were labeled to contain these soils, approximately 4 drums, were sampled June 19, 1997, for proper waste characterization as specified in 30 TAC 335 Subchapter R. Results of TCLP analyses for the IDW soils identified no VOCs, and lead was present at 0.0754 mg/l. The drill cutting soils meet nonhazardous Class 2 criteria levels. The containerized soils were properly disposed of at an off-site landfill.