| If necessary and appropriate, the site layout (pilot test VEWs and VMPs) could easily be incorporated into a full-scale SVE system to remediate the site. | A total of six VEWs and six multi-depth VMPs were placed within the SWMU B-3 trench area. As shown on Figure 2.9, three VEWs and two VMPs (identified as VEW-01, VEW-02, VEW-03, MPA, and MPD) were placed within the main SWMU B-3 trench limits, one VMP (MPB) was installed in the transition zone between near surface trench soils and limestone on the edge of the main trench boundary, and one VMP (MPC) and two VEWs (VEW-04 and VEW-05) were installed in limestone material outside the trench limits. All of these VMPs and VEWs, except MPD, were installed in a line that crosses the portion of the landfill that exhibited the greatest VOC concentrations during the soil gas survey. MPD was constructed approximately 30 feet perpendicular from the main line of VEWs and VMPs which stretched from west to east across the trenches. This location was selected to assess the radius of influence from VEWs in the north-south direction inside the trench.
A second test system was installed northeast of the primary test line in an isolated area of soils that also exhibited high levels of contamination in soil gas. This system consists of one VEW (VEW-06) and two VMPs (MPE and MPF). This layout was consistent with the proposed locations identified in the work plan with one exception. Because the eastern edge of the west trench was encountered in a shorter distance to the dirt road than anticipated, the planned location for VEW-03 was converted to MPB. VEW-03 was relocated to the western side of MPA for use as a monitoring point or in a possible future full-scale SVE configuration.
Based on the types of fill material encountered inside the trench limits and shallow limestone encountered outside the trench during installation activities, the spacing between VMPs and VEWs proposed in the work plan was appropriate; therefore, 10 feet outside and 15 feet inside the trench limits were used for the system construction. The pilot test layout was designed to allow air permeability testing of soils within the main SWMU B-3 trench and outside the trench in the native soils.
A 2.5-horsepower (hp) regenerative blower was manifolded to the VEW which now operates in the SWMU B-3 area. An instrumentation diagram for the initial pilot test SVE system constructed at the site is presented on Figure 2.10. The system consists of a vacuum regenerative blower, a moisture separator (knock out pot), an air filter, flow control and air bleed valves, pressure and temperature gauges, a flow measurement port, sampling ports, and 2-inch polyvinyl chloride (PVC) pipe manifolded to the top of five of the six VEWs. The pilot test vacuum blower is a Gast Regenair R5 Series Model R5325A-2 (Parsons ES, 1996a).
After completion of all VEWs and VMPs, the VEWs were connected to the blower using 2-inch schedule 40 PVC with all connections and flow control valves placed above ground level to allow easy access. An electric fence was placed around the test site to protect the aboveground pipe from free-range cattle in the area surrounding SWMU B-3 and to prevent vehicles from traversing the site. CSSA electricians connected power from the electric service pole west of the site to a control box and power monitor for the operation of the electric fence and test blower.
As shown on Figure 2.10, the VEWs are manifolded together with individual control valves to turn on and off the vacuum applied to each VEW. Each VEW was also constructed with a pressure monitoring port to allow measurement of pressure responses in the VEW when not being utilized as an extraction well. This flexibility in the system design allowed extraction from any or all of the VEWs and collection of data from the disturbed landfill trenches and undisturbed soils outside the trenched areas using the same blower.
The 2.5-hp blower unit was mounted in a small shed on the west side of the main SWMU B-3 trench. The moisture separator and filter system with appropriate gauges and pressure relief controls for the blower system are located outside the blower shed. Electrical power is wired to the blower from an electric service pole located approximately 45 feet west of the blower shed.
Following completion of the SVE test activities, the valves of the five manifolded VEWs were opened during blower extraction to determine the air flow obtained from each of the VEWs and to adjust the flow so that the extraction rates from each VEW were relatively uniform. The pressure vacuum (resistance to flow) was also measured from each VEW to determine if flow would be limited from any VEW by tightness of the screened interval. Valves were adjusted to try to obtain uniform air flow from each VEW. Open flow extraction indicated an initial range of 2.1 cubic feet per minute (cfm) at VEW-02 to 41.4 cfm at VEW‑05. After adjusting the valves, the air flow extraction rates ranged from 2.6 cfm to 22.9 cfm. Air flows measured at the other three VEWs ranged from 17 to 21.8 cfm. The differences observed in extraction flow rates is a function of the permeability present in the screened intervals of each VEW. Although VEW-04 and VEW-05 were installed in limestone, the high flow rates suggest that the formation is more permeable than the screened intervals of VEWs installed inside the landfill limits. This finding is further described in Section 2.4.2.5.
Automated drainage of the moisture separator was not installed in the blower system because the original intent of the pilot study was not to run the blower over a long duration. Accumulated liquids in the moisture separator have been manually drained from the blower system on an as needed basis, when operating. An automated drainage system has been designed as part of the expanded operation of the blower system and is planned for installation prior to the completion of this project.
2.4.2.2 Soil Sampling Results from 1996 SVE Pilot Test
Volatile Organic Compounds
The analytical results of the boreholes sampled during the pilot test system installation activities are shown on Table 2.1. This table also includes the criteria for the detected compounds using the Texas Natural Resource Conservation Commission (TNRCC) risk reduction standard 2 (RRS 2) closures (groundwater protection for residential scenarios) and updated background concentration ranges for metals at CSSA for the Tarrant Association gently undulating soil type and the underlying Glen Rose Limestone recently determined under a separate delivery order (Parsons ES, 1997). Only five VOCs were detected in the fifteen samples analyzed by method SW-8260: chlorobenzene, cis-1,2-DCE, PCE, toluene, and TCE. The highest TVH concentrations were measured in VEW-01, VEW-02, MPA, and MPD, which are all located within the limits of the main trench. In borings with multiple sample depths, the greatest VOC concentrations were detected in samples collected from deeper depths. Samples collected from 13 to 15 feet bgs had the greatest levels of VOC contamination of all samples collected within the limits of the SWMU B-3 trench.
Samples collected from the soil borings drilled northeast of the main trench area had significantly less VOC contamination than those in the main trench. Drilling of VEW-06, MPE, and MPF encountered fill material to a depth of 6 feet with numerous discolorations and debris observed in the samples; however, the Micro-tip photo-ionization meter did not indicate significant TVH levels during screening of soil core samples. The concentrations of TCE and cis-1,2-DCE were at least an order of magnitude less than concentrations detected in samples collected from the main trench. However, PCE was detected in MPE at 0.650 milligrams per kilogram (mg/kg), but was not detected in any other soil boring sample.
The contaminant levels of TCE detected in soil samples exceeded the TNRCC RRS 2 Groundwater Protection criteria for residential use in nine of the fifteen samples tested. Five samples exceeded the criteria for cis-1,2-DCE contamination, and six samples exceeded the criteria for PCE. Chlorobenzene and toluene were below the TNRCC RRS 2 criteria in each sample in which they were detected. Except for the PCE detection in MPE, all of the RRS 2 exceedences occurred in samples collected within the limits of the main landfill trench. It is unknown if the RRS 2 standard was exceeded for PCE for samples VEW-01 (9-11), VEW-01 (13-14), VEW-02 (14-15), MPA (14-15), and MPD (14-15), and for TCE for sample VEW-01 (9-11) because the detection limit was greater than the standard.
Inorganic Compounds
The intent of the sampling and analytical program for the 1996 pilot test was to analyze one sample per borehole for metals and VOCs. Because the focus of this test was removal of VOCs, the supervising geologist was directed to collect additional samples from each borehole for VOC analysis if contamination was indicated. Therefore, only nine of the fifteen samples collected were analyzed for metals. Additionally, no samples were collected from some boreholes because no evidence of contamination was indicated during drilling and because air rotary drilling was used to penetrate the limestone to the planed depth.
The analytical results of the metals are also presented in Table 2.1, along with the metals results. Corresponding background concentration ranges at CSSA and criteria for TNRCC RRS 2 are also presented for each of the metals evaluated. For the purposes of comparison, Tarrant Association background levels are used for the fill soil, as it is likely that the fill was taken from the area of and around the site. Metals levels for the fill or Tarrant Association soil samples are within the normal range of background concentrations at CSSA in all samples collected during the 1996 pilot test except MPD (8 to 10 feet bgs) and VEW-06 (6 to 8 feet bgs). In VEW-06 (6 to 8 feet bgs), the detected concentrations of chromium, copper, cadmium, lead, nickel, and zinc were significantly greater than the calculated maximum background concentration for CSSA soils. Only copper, cadmium, lead, and zinc were detected at levels above background in MPD (8 to 10 feet bgs); however, these metals were detected at concentrations less than those detected in VEW-06 (6 to 8 feet bgs). The Glen Rose rock sample (MPF) analyzed for metals, was found to contain barium, chromium, copper, zinc, and lead in concentrations higher than background. Mercury was detected slightly above background in one sample, VEW-06 (6 to 8 feet bgs). Lead in VEW-01 (9 to 11 feet bgs) was also above background level. The natural concentrations of metals at CSSA are greater than the levels provided in the TNRCC risk reduction rules, so it is appropriate to use the background levels to determine whether metal concentrations detected are indicative of contaminated backfill material or naturally occurring conditions of the soils at SWMU B-3.
Miscellaneous and Physical Data
One sample was collected from each sampled boring for testing of geotechnical and other physical properties that are useful in evaluating site-specific SVE feasibility. The test parameters include soil moisture, bulk density (and porosity), total organic carbon (TOC), pH, permeability, and particle size distribution. Additionally, total kjeldahl nitrogen and total phosphates were also measured to assess the biological nutrient content of the fill material. Permeability was not measured in any of the samples because disaggregation of each sample prohibited its determination by the proposed method.
The results of the physical property testing are shown in Table 2.2. Two samples were collected from 13 to 15 feet bgs, four samples were collected between 8 to 11 feet bgs, and four samples were collected at depths less than 6 feet bgs. Based on the data from these samples, none of the physical parameters appear to limit the potential effectiveness of SVE in the SWMU B-3 landfill trench. The primary factors affecting soil gas movement, moisture content, porosity, and particle size distribution, are highly variable. Moisture contents range from 27.2 percent in VEW-03 (13 to 15 feet bgs) to 7.8 percent in VEW‑01 (13 to 14 feet bgs). With regard to viable use of SVE, high moisture contents (i.e., 27.2 percent), coupled with low porosity, could result in a situation that could be very limiting for air flow. However, the variability of subsurface materials encountered during drilling (lithologic logs), plus the aggregation of debris apparently mixed into the fill material, have resulted in generally favorable conditions for subsurface air flow.
The soil pH, TOC, phosphates and nitrogen values are adequate to support bioremediation in the fill material. Low oxygen levels measured in the initial soil gas indicate that a significant amount of natural biological activity has already been occurring at the site. The particle size distribution analysis indicates that silt makes up a significant portion of the fill material (greater than 39 percent in each sample) and that the quantity of sand and clay are highly variable. In general, the sand content is greater in the shallow soils, and the clay content is greater in the deeper soils. There are no observed soil physical characteristics that should result in restricted air flow, such that SVE is not a viable option.
2.4.2.3 1996 Delineation of Volatile Organic Compounds
Based on the pilot test results and soil gas and geophysical surveys, the area of the SWMU B-3 landfill trench that requires treatment for VOCs was estimated to be 15,000 square feet (150 feet by 100 feet). The estimated average thickness of this potentially contaminated soil was 15 feet (based on observations made during drilling), which totals 225,000 cubic feet (or 8333 cubic yards). The average porosity of the fill material in the trench was 30 percent, which is used to calculate a soil bulk density of 1.85 grams per cubic centimeter (g/cm3) (or 115 pounds per cubic foot [lb/ft3]). Based on these assumptions, the total mass of fill material in the trench requiring treatment was estimated to be approximately 25,875,000 pounds (11,747,250 kilograms) of solid material. Additional data was collected as part of the 1997 SVE treatability study expansion to obtain more accurate estimates of the soil mass required to be treated and the mass of contaminants present in the treatment zone of the trench. The updated treatment zone delineation and contaminant mass estimation are discussed in Section 4.
2.4.2.4 1996 Delineation of Metal Contamination
Metal contamination appears to be located randomly throughout the trench areas, based on the results of the metal analyses performed on the nine samples collected during the SVE pilot system installation. The highest metals concentrations at SWMU B-3 were detected in soil samples collected from VEW-06 (arsenic at 15 mg/kg, barium at 160 mg/kg, cadmium at 12 mg/kg, chromium at 120 mg/kg, copper at 580 mg/kg, lead at 8,700 mg/kg, mercury at 0.21 mg/kg, nickel at 100 mg/kg, and zinc at 850 mg/kg). All of these values are greater than the residential RRS 2 levels except barium, and all of the values except arsenic exceed the statistically calculated background for Tarrant soils. The small trench encountered at VEW-06 is located in an area geographically independent of either of the main trenches identified at the site.
Samples from MPD, VEW-01, and VEW-02, which were collected from the main trench limits, had detected metal concentrations greater than background levels for Tarrant soils. Sample MPD (8 to 10 feet bgs) had metal levels greater than background levels. VEW-01 (9 to 11 feet bgs) also had lead above background levels, and all the analyzed soil samples contained mercury slightly above background. The MPD borehole is located approximately 30 feet from VEW-01 and VEW-02, and is away from the main pilot study test line. No metals contamination was encountered in the line of five soil borings sampled during the pilot study.
Metal contamination does not likely extend from MPD to VEW-06, which are located approximately 90 feet apart. Two factors suggest that these two areas are separated. The first is the apparently normal metal levels found in MPF, which is a Glen Rose Limestone sample found only 20 feet south of VEW-06. The second is the apparent boundary of the main trench located between MPD and VEW-06. This boundary was identified based on results of the geophysical survey, soil gas survey, and the presence of the trench�s edge encountered during drilling at MPB. Additionally, soil gas extraction at VEW-06 resulted in no measurable response at MPD.
Previously collected data from shallow soil borings supports the spotty and limited presence of metal contamination at SWMU B-3. Twenty samples were collected from seven soil borings in 1995 to assess the lithology and potential contamination. Only one sample, collected from 2 to 4 feet bgs in B3-SB4, had any metals detected above the respective background levels. B3-SB4 is located approximately 60 feet south of MPB. Lead was the only metal detected in this boring above the calculated background levels for CSSA.
2.4.2.5 1996 Initial Pilot Testing Results
The initial SVE pilot test was performed in March 1996 and consisted of initial soil gas chemistry measurement, air permeability testing, radius of influence monitoring, and air emission/contaminant mass removal testing. The results of the testing are described in Section 7 of the �Ground Water Investigation and Associated Source Contamination� (GWIASC) report (Parsons ES, 1996c), and are briefly summarized below. In general, the results indicated that SVE would likely be an efficient method for remediating VOC contamination in the SWMU B-3 trench.
Initial Soil Gas Chemistry
Initial soil gas chemistry data collected after the installation of the SVE pilot test system indicated that VOCs (primarily TCE and cis-1,2-DCE) are present inside the main SWMU B-3 trench, with lower VOC concentrations measured in the northeast portion of the main (west) trench. The sample points were initially field screened for oxygen, carbon dioxide, and TVH to determine which points would be most suitable for collection of soil gas samples for analytical testing. Table 2.3 lists the results from the initial field screening. Also listed in Table 2.3 are measurements of the vacuum pressures exerted on each point during sample point purging. This pressure data provides information on the apparent tightness of the screened formation being tested.
Screening results indicated that anaerobic conditions had been produced at several of the tested subsurface locations, primarily within the limits of the main SWMU B-3 trench at depths of 10 feet or greater. These anaerobic conditions are indicative of biological activity, most likely the microbiological degradation of organic carbon (including organic contaminants) in the trench. Carbon dioxide, a byproduct of organic compound degradation, was also encountered at high levels in the anoxic soils to further support the presence of biological activity. The only low oxygen level encountered outside the trench was in MPC at a depth of 14 feet. This VMP was set in an interval of weathered limestone outside the main trench limits. This low oxygen reading suggests that subsurface pathways may exist that allow communication of the trench to some portions of natural limestone surrounding the trench. According to the results, the most depleted oxygen levels appear to be associated with deeper zones of the trench located at VEW-01, and extend west to MPA and VEW-03 and north to MPD. The oxygen levels increase to the east from VEW-01 to VEW-02 and MPB. The differences observed in MPC-14 and MPB suggest that little or no connection exists between these two VMPs.
TVH levels were measured to assess the approximate range of contamination present at different portions of the site. In general, high TVH readings were encountered in deeper soils within the landfill trench. However, high TVH readings were observed at each point where low oxygen levels were encountered. One of the key findings from the initial soil gas data was the low oxygen and high TVH readings measured in MPC-14, which is located outside the trench limits in an interval of weathered limestone. Moreover, MPB-13, which is located in similar material only 10 feet from MPC-13, exhibited high oxygen readings. These results suggest that subsurface pathways are present in the weathered limestone that allow communication of contaminated soil gas in the trench to some portions of the surrounding formation, while limiting communication to other portions of the native material. The samples were collected at four of the screened points to quantify and characterize the VOCs present in the soil gas. These samples were collected in Summa canisters for analytical laboratory testing. Selection of sampled points were based on the results of the field screening, and included VEW-01, MPA-15, MPD-15, and MPE-04. Samples from within the main trench area provide data for assessing contamination in fill material. The sample collected from MPE-04 provides data to assess the fill material encountered northeast of the main trench. These two areas do not appear to be interconnected by subsurface air pathways. The analytical results of the VOCs detected are shown on Table 2.4. The first sample collected (VEW-01) was analyzed for all TO-14 VOC parameters on a 24-hour laboratory turnaround schedule. The results of this analysis were used to specify the VOCs to be analyzed in the other samples and during subsequent sampling events, because it exhibited high TVH, low oxygen, and high carbon dioxide levels during the screening process. The primary contaminants encountered include TCE, cis-1,2-DCE, and vinyl chloride. Because no vinyl chloride is suspected to ever have been disposed of at SWMU B-3, its presence supports the occurrence of TCE bioremediation at the site. Cis-1,2-DCE and vinyl chloride are intermediary breakdown products of TCE.
Vacuum pressures were also observed during the purging activities to monitor the relative permeability of sampling intervals with each other. The magnitude of the vacuum measured during purging is inversely proportional to soil permeability. The lower the vacuum resistance, the greater the permeability of the formation being purged. Based on the measured vacuum pressures, the soils at the site have fairly high permeability. Only two of the intervals, VEW-04 and MPF-8.5, indicated vacuum pressures (or permeability) that may be limiting to air movement in soils. Both of these points are located completely or partially in the natural limestone material outside the trench. The high apparent permeability associated with some of the other VMP or VEW points screened in the natural limestone suggest that fractures are probably present in the formation. Based on these limited results, and supported by the air permeability results, fracturing of the natural limestone appears to be relatively variable at this site.
Air Permeability Testing
Two air permeability tests were attempted at the site. The first test was performed outside the main SWMU B-3 trench at VEW-06, and the second test was performed at VEW-01 inside the main trench. The pressure measured at VMPs during air extraction at VEW-06 reached steady state conditions within only 5 minutes after initiating the air permeability test. No response was observed at any of the adjacent VMPs or VEWs during air extraction at VEW-01. These results illustrate the heterogeneous nature of the subsurface conditions at the site. The results of the air permeability testing are briefly described below.
The final pressure responses measured at MPE and MPF during air extraction at VEW-06 were used to calculate air permeability values for this portion of the site. The screened interval thickness was 9 feet and the measured air extraction flow rate was 30.5 cubic feet per minute (cfm). The static method was used for calculating soil gas permeability because steady state pressure responses were obtained at each point in less than 5 minutes. Assumed radius of influence values used for the calculation ranged from 25 to 35 feet from the extraction well. The resulting air permeability values were calculated at approximately 200 darcys. Pressure responses were observed for all three depths measured, indicating relatively uniform lateral air flow toward VEW-06.
No pressure response was observed at adjacent VMPs or VEWs during the second air permeability test performed at VEW-01; therefore, no air permeability values were calculated. The second air permeability test was continued for 140 hours because it was coupled with an evaluation of the VOC emission/mass removal study and a radius of influence test. The VMPs measured during this test included MPA, VEW-02, VEW-03, and MPD. The findings of these studies are briefly described below.
Radius of Influence Monitoring
Three activities were performed on the installed pilot test system to assess the subsurface influence of air extraction on soil gas. The first activity was performance of two air permeability tests, which are briefly described above. The second activity was collection and direct comparison of soil gas samples after approximately 90 hours of air extraction at VEW-01 with initial soil gas sample results. The final activity was the manipulation of the VEW configuration following completion of the air permeability test. Pressure responses were monitored at various VMPs and VEWs to assess the influence created by air extraction from different configurations.
2.4.2.6 Air Permeability Test Results
Air permeability test results conducted in March 1996 did not allow determination of lateral influence because the site does not behave as typical unconsolidated soils. The sporadic fracturing of the weathered limestone, and the heterogeneous nature of the fill material have allowed for the creation of natural preferential pathways for air flow, which make interpretations of the results difficult. For instance, some points located directly adjacent (within 10 feet) to an actively pumping extraction well exhibited no response, whereas other points located up to 30 or 60 feet from the extraction well were significantly affected. The lack of response observed at the adjacent monitored points during air extraction at VEW-01 is direct evidence that air flow pathways interconnected with VEW-01 bypass the adjacent, monitored test points.
2.4.2.7 Soil Gas Chemistry Comparison
A comparison of the initial soil gas screening results with soil gas measurements collected after approximately 95 hours is presented in Table 2.5. Significant changes were observed in soil gas concentrations at numerous test points after 95 hours of extraction from VEW-01, including points where no pressure response was observed. The primary changes were oxygen level increases, although significant changes in carbon dioxide and TVH were also observed in affected points. The greatest increase in oxygen levels were observed in VEW-03, VEW-05, MPA-5, and MPD-15. However, little or no change was observed in several points, including VEW-02, VEW-04, MPC, and MPB. These results provide strong evidence of the interconnection between some of the screened intervals of the VMPs and VEWs, and the apparent isolation from other screened intervals. Since most of the monitored points form a line, the strongest evidence of interconnected air pathways is the soil gas changes observed in VEW-05, which is located farther (at least 60 feet to the east) from the extraction point than VEW-04, MPB, and MPC. Also unusual is that VEW-05 is located at least 30 feet from the edge of the trench in natural weathered limestone. It is conceivable that a fracture or fractures are present that connect the screened formation at VEW-05 with a permeable portion of the trench fill.
Decreases in the carbon dioxide and TVH levels were also observed in the measured points with increased oxygen. This is likely caused by the influx of fresher air from a less contaminated area (or from the atmosphere) to replace soil gas that has been evacuated in the soil formation caused by air extraction at VEW-01. The changes in oxygen, carbon dioxide, and TVH observed at MPA, MPD, VEW-03, and VEW-05 are comparable to soil gas chemistry changes resulting from air extraction in shallow soils. Thus, all of the observed changes are likely due to SVE applied to VEW-01.
Multiple Configuration Testing
Following completion of the second air permeability test and the air emission sampling, all five pilot VEWs were opened to assess pressure response from the five VEW operating systems and to measure the relative contribution of air flow from each VEW. The VEW valves were adjusted to even flow from each VEW, then pressure responses were measured at the VMPs. A pressure response was observed at depth intervals MPA(15) and MPD(15), but not in any of the other VMP screened intervals. Air flow was shut off from VEW-01 and VEW-02 to determine if either of these VEWs are causing the responses at MPA and MPD. No change in the pressure responses at MPA-15 and MPD-15 were observed. When flow was shut off at VEW-04, the pressure dropped markedly. As flow resumed at VEW-04, pressure responses were observed immediately at MPD-15 and MPA-15. Based on this observation, VEW-04 has been determined to be interconnected to the 15-foot depth of MPD and MPA inside the trench area, but not to any of the other VMPs or VEWs.
The multiple configuration testing was performed with the SVE pilot test system to identify interconnections in the subsurface environment in the SWMU B-3 trench. Complicating the influence issue is non-agreement of soil gas chemistry changes and pressure response observations. For instance, soil gas changes were observed in MPA and MPD, but not in VEW-04 during extraction from VEW-01. Conversely, pressure responses were observed in MPA and MPD when extracting from VEW-04, but no response was observed in VEW-01. This observation implies that a significant portion of the treatment area is interconnected, although the route of connection might be convoluted.
Soil gas contamination detected in native limestone outside trench limits in VEW-04 and VEW-05, and the interconnections identified by subsurface testing, suggest that migration of VOCs into limestone fractures is common. The multi-configuration testing performed on the expanded SVE system was aimed at studying the complexity of the subsurface trench environment, and determining the most appropriate VEWs to be used to maximize removal of VOCs from the entire SWMU B-3 trench and the surrounding limestone material. Results of additional testing are discussed in Section 4.
2.4.2.8 Air Emission/Contaminant Mass Removal Testing
The emission rate over the approximately six-day pilot test is graphically presented on Figure 2.11. The contaminant masses removed from SWMU B-3 during the 140-hour extraction period at VEW-01 are estimated at 43 pounds of TCE and 16 pounds of cis-1,2-DCE. At the end of the 140-hour test, approximately 0.24 lb/hr TCE were still being removed from extraction at VEW-01, and 0.1226 lb/hr cis-1,2-DCE. The removal rates were only slightly lower than the measured removal rate after 47 hours of extraction, suggesting that the rate of removal was stabilizing. The contaminant mass/removal calculations and assumptions are included in Section 7 of the GWIASC report (Parsons ES, 1996c).
The estimated emission rates were within the acceptable emission limits for TNRCC Standard Exemption 118(c), so no off-gas treatment of the existing SVE system was required. If the system is expanded to include more than six operating VEWs at a given time, and if new soils data indicated higher concentrations of constituents of concern, then the Standard Exemption would need to be modified accordingly.
Based on the limited 1996 characterization data, it appeared that the six-VEW pilot test SVE system was capable of removing significant masses of VOCs which are present in the trench. However, based on the complexities of the subsurface environment, multiple configurations were determined to be necessary to ensure that all of the contamination in the trench is being addressed. In December 1996, the SVE system was expanded to improve the contaminant removal capacity of the pilot test system and to collect additional characterization data for more accurately delineating the extent of the treatment area. Additional tests were also performed on the expanded system to develop a conceptual subsurface model which can be used to optimize the design of the full-scale SVE treatment to remediate the VOCs at SWMU B-3. These test activities and data evaluation are discussed in Sections 3 and Section 4 of the report, respectively.
Laboratory analysis of the treatability study soil boring samples were analyzed for VOCs, metals, and geotechnical parameters by ITS Laboratory in Richardson , Texas . In 1999, ITS came under scrutiny of the USEPA due to improper laboratory procedures. As a result, the USEPA determined that results of analyses performed by ITS are of questionable accuracy and only usable as screening or qualified data. Review of the ITS data indicated that only the VOC and metal analyses were suspect. The results of the geotechnical analyses performed by ITS is consistent with results from other laboratories and, therefore, no resampling was required for these parameters.
A resampling effort was preformed in April 2000 which consisted of collection of 12 soil samples collected from borings drilled near SVE wells VEW07 through VEW18. The rework soil borings were drilled within three to five feet of the original boring locations. The soil borings were advanced using solid stem augers to reduce the amount of investigation-derived waste generated requiring disposal. The depths for the collection of the samples approximately the same as the original sampling scheme at each boring. The borings were advanced to the desired sample depths and then a split spoon or shelby tube was driven through the desired sampling interval to collect the sample from the desired depth. Soil samples collected from the rework borings were analyzed for the parameters listed in Table 3.1. Due to the proximity of the rework borings to the original locations, boring logs for the rework sampling were not completed.