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Groundwater Investigation and Associated Source Characterization
Section 7 - SWMU B-3 Characterization
Chlorinated hydrocarbons were first detected in well 16 at concentrations above drinking water standards, prompting an investigation of the possible source areas that contribute to contamination of groundwater. Source characterization began with surface geophysical surveys performed during January through March 1995 at seven potential source areas. An anomaly location map of SWMU B-3 is shown on Figure 7.1-1. Two large anomalous areas were detected at SWMU B-3 during the EM survey as discussed in the technical memorandum (Parsons ES, 1995b). Based on this geophysical data, soil borings were drilled at potential areas including SWMU B-3 to investigate the portions of each area exhibiting geophysical anomalies. The analytical results of this sampling at B-3 are summarized in Table 7.1-1, and "hot spots" are shown on Figure 7.1-2. A subsequent soil gas survey of SWMU B-3 identified TCE and PCE associated with the geophysical anomalies, with occasional detection of 1,2-DCE. Soil gas results are summarized on Figure 7.1-3, Figure 7.1-4, and Figure 7.1-5 for TCE, PCE, and cis-1,2-DCE, respectively. Results of the soil gas survey performed at B-3 are also discussed in section 5 and appendix F. The presence of these chlorinated hydrocarbons have implicated B-3 as a likely source area for the contamination detected in well 16.
An SVE pilot test was recommended at SWMU B-3 for removal of soil VOC contamination. The primary objectives of the pilot test were to determine if SVE is a viable alternative and to collect data to design a full-scale SVE system. Borings drilled for construction of the SVE pilot test system could also be used to collect soil samples in the soil gas "hot spots" to determine VOC concentrations. These samples were also tested for total metals and soil physical characteristics to assist in evaluating potential remedial options. SVE was the primary treatment technology selected for the pilot study because of the volatility of TCE, PCE, and cis-1,2-DCE. Also, an in situ technology was favored over ex situ treatment because the excavation of SWMU B-3 would be significantly more expensive. It was understood that this treatment alternative would not address potential inorganic contamination at the site.
The basic theory of soil vapor extraction is to apply a negative pressure, or vacuum, to the subsurface to create a pressure gradient. This gradient produces advective air flow which will remove vapor-phase compounds and also promote continued volatilization of organic compounds adsorbed in soils. The vacuum is created by using blowers or vacuum pumps, and is applied to the subsurface soils through extraction wells. Specific objectives of the SVE pilot test were to estimate soil permeability to vapor flow, radius of influence of the extraction well, and vacuum variability with depth.
This section of the report was prepared to describe the results of the SVE pilot test activities and to discuss possible design implications for implementation of a full-scale SVE system to remediate the site. Details of the system construction, characterization, sampling, and pilot test activities are described in Section 3.5, Section 3.8, and Section 3.9 of this report. The activities performed are summarized below in the chronological order that they were performed.
| Six VEWs and six VMPs were constructed from February 19 to February 23, 1996. Fifteen soil samples were collected to characterize VOCs and moisture. Ten of the samples were analyzed for geotechnical parameters, such as grain size, total organic carbon, porosity, pH, and biological nutrient content. Ten of the samples were also analyzed for metals. The results are discussed in Section 7.2 of this report. | |
| The blower was manifolded to the VEWs on March 4-5, 1996 for the SVE pilot test activities. | |
| Initial soil gas samples were collected beginning on March 5 and were completed on March 7, 1996. Each VEW and VMP screened interval was sampled for oxygen, carbon dioxide, and TVH using field instruments. During the pre-sample purge, vacuum pressures were monitored and recorded to provide data on the tightness of the screened formations or fill materials at each screened interval. Soil gas samples were collected in Summa canisters for analytical laboratory testing from four screened intervals to determine the VOC constituents in the soil gas. The results of this testing are discussed in Section 7.4. | |
| As part of the second air permeability test, emissions from the blower exhaust were monitored to evaluate the quantity of VOCs being emitted to the atmosphere. Air samples were also collected over time for laboratory testing to assess the actual content of contaminants to determine off-gas treatment options. Results of this testing are discussed in Section 7.4. | |
| Soil gas samples were collected and measured for oxygen, carbon dioxide, and TVH using field instruments following 4 days of continueal air extraction from VEW-1 to determine the influent of extraction on soil gas chemistry throughout the landfill trench and subsurface soils at the site. | |
| On March 13, the flow control valves were set up in different configurations to assess the contributing flow from connected VEWs, and to evaluate possible preferential subsurface pathways for air flow. | |
| The blower was shut down on March 13 to determine the volume of condensed liquids collected in the moisture separator. Approximately 15-20 gallons of water were drained from the separator tank after 6 days of extraction. |
7.2 - Characterization Activities
Prior to installation of the SVE pilot test system, the majority of the characterization activities were geophysical and soil gas surveys that were performed to identify the probable locations of SWMUs, and if possible, to determine the extent of those SWMUs. The results of these surveys have been summarized in Section 5.1 and Section 5.3, respectively. Seven soil borings were also drilled and sampled in the former B-3 landfill area in March 1995 to partially characterize the waste constituents. The results of these borings and soil analysis are summarized in Section 5.2. Table 7.1-1 provides a summary of the compounds detected during this sampling event. The locations of contaminants detected in soil boring samples are depicted on Figure 7.1-2. The results of the above-mentioned testing, along with the soil gas survey results, were used to plan the appropriate placement of the SVE test system wells and VMPs.
Soil gas samples were collected during the pilot study to assess the volatility of VOCs in the subsurface material. Soil gas chemistry results can be compared to soils concentrations to obtain an indication of the relationship between VOCs present in soil gas to VOCs in soil. The results are summarized in Section 7.4 below.
The locations of the VEWs and VMPs are shown on Figure 7.3-1 relative to the SWMU boundary as determined by the geophysical surveys, and the locations of soil gas "hot spots." As discussed in Section 3.8, the site layout was modified because the transition zone of the landfill was encountered at the proposed location of VEW-3. A VMP was envisioned for the transition area of the landfill to natural material, so the VEW-3 borehole was completed as MPB instead. VEW-3 was relocated 15 feet to the west of MPA. A cross-section of the SVE pilot test layout across the main B-3 landfill is presented in Figure 7.3-2. Descriptions of the layers depicted in Figure 7.3-2 are summarized in Table 7.3-1.
Based on lithologic logs, MPA, VEW-1, VEW-2, VEW-3, and MPD are located within the limits of the main landfill trench at SWMU B-3. The approximate thickness of the fill material appears to extend to almost 20 feet bgl. MPB is located in a transitional area with natural limestone encountered at 9 feet bgl and no apparent indications of fill material or contamination in the clayey soils overlying this limestone. MPC, VEW-4, and VEW-5 were all placed in soils and limestone outside the mail pilot test trench of the B-3 landfill. VEW-6, MPE, and MPF were constructed in a location northwest of the presumed trench area to assess the possibility of an ancillary source area outside of the main trench. Fill material was encountered to a depth of approximately 9 feet bgl in this portion of the site. This fill material lies on relatively competent limestone, suggesting that a shallower trench may have been excavated in this portion of the site. This trench does not appear to be interconnected to the main trench described above.
7.4 - SVE Pilot Test and Characterization Results
The purpose of this section is to summarize results of the pilot testing and characterization activities performed at SWMU B-3. The results of soil and soil gas sampling and the results of air emissions sampling are described in the section below. The results of initial soil gas chemistry, air permeability testing, and subsurface influence testing are also discussed in this section.
Soils in the B-3 trench area consist of clay and silty clays with white caliche fragments near the surface, progressing with depth to competent limestone. The depth to limestone is variable across the site, as shown in Figure 7.3-2. Towards the eastern portion of the site, the limestone becomes shallow and is exposed at ground surface east of the pilot test layout. Lithologic descriptions of the soil borings used to prepare the B-3 cross-section are included in Appendix A. The fill material encountered in the main trench consisted of dark brown, reddish brown, and black material with fragments of limestone, plastic and metal debris, and charred wood mixed with poorly sorted coarse sand with nonplastic clay.
Outside the trench limits, the soils consisted of dark brown silty clay and clay from the surface to the top of limestone. The limestone appeared highly weathered, pale yellow to gray with occasional interbeds of hard massive limestone. Intervals of vugs, some with calcite growths, bedding, bioturbation, and fracturing were observed. Some interbedding of clay and weathered shale was also observed in the core samples evaluated. No saturated soils were encountered in any of the soil borings, and no moisture accumulated in any of the VEWs following construction.
Fifteen soil samples were collected from ten of the twelve soil borings drilled for VEW or VMP construction. Samples were not collected from the two VEW borings located outside the trench area (VEW-4 and VEW-5) because no evidence of contamination was indicated during drilling. Another factor in the decision to not collect samples from these two borings was the need to use air rotary coring to penetrate the limestone to the planned VEW depths. The test methods for soil samples are previously discussed. The locations of the sampled boreholes are shown on Figure 7.3-1. The analytical results of the soil samples are presented in Table 7.4-1. Table 7.4-1 also includes the criteria for the detected compounds using the Risk Reduction Rules for standard number 1 closures (groundwater protection for residential scenarios) and preliminary background concentrations ranges at CSSA recently determined under AL/OEB Order 126 (Parsons ES, 1996c). Geotechnical results are provided on Table 7.4-2. Data reporting sheets fro analytical and geotechnical samples analyses are on file with Parsons ES.
7.4.2.1 Volatile Organic Compounds
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-1, VEW-2, MPA, and MPD, which are all located within the limits of the main landfill 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 bgl had the greatest levels of VOC contamination of all samples collected within the limits of the B-3 trench.
Samples collected from the soil borings drilled northeast of the main test area had significantly less VOC contamination than those in the main trench area. Drilling of VEW-6, 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 instrument did no 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 B-3 trench. However, PCE was detected in MPE at 650 ug/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 eight of the fifteen samples tested. Three samples exceeded the criteria for DCE contamination, and one sample 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 exceedances occurred in samples collected within the limits of the main landfill trench.
Soil samples collected during the SVE pilot system installation were also analyzed for metals. The analytical results of the metals are presented in Table 7.4-1, along with the organic chemical results. Nine of the fifteen samples collected were analyzed for metals. Corresponding background concentration ranges at CSSA and criteria for TNRCC Risk Reduction Rules standard number 2 are also presented for each of the metals evaluated. Metals levels are within the normal range of background concentrations at CSSA in all samples except MPD (8 to 10 feet bgl) and VEW-6 (6 to 8 feet bgl). In VEW-6 (6-8 feet bgl), 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 (8-10 ft bgl); however, these metals were detected at concentrations less than those detected in VEW-6 (6-8 ft bgl). 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 B-3.
The presence of elevated metals concentrations in only two of the nine samples analyzed is indicative of conditions typical of landfilled materials. numerous metallic fragments and burned debris were encountered sporadically throughout the drilling activities, so it can be speculated that the presence of metals contamination is probably not common in the B-3 trench. Metals contamination within the limits of the landfill are likely limited to "hot spots" scattered throughout subsurface material. The actual extent and location of metals contamination is difficult to predict given the inconsistent nature of material disposed in the trenches, and the results of the site analyses.
7.4.2.3 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 laboratory was not capable of performing an alternative method using a remolded sample.
The results of the physical property testing are shown in Table 7.4-2. Two of the samples were collected from 13 to 15 feet, four samples were collected between 8 to 11 feet, and four samples were collected at depths less than 6 feet. Based on the data from these samples, none of the physical parameters or nutrients appear to limit the potential effectiveness of SVE in the 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-3 (13-15 feet) to 7.8 percent in VEW-1 (13-14 feet). With regard to viable use of SVE, high moisture content (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 suggest that a significant amount of natural biological activity is already occurring at the site. This is discussed later in Section 7.4.3. 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 soil physical characteristics that should result in restricted air flow, such that SVE is not a viable option.
7.4.2.4 Delineation of Volatile Organic Compounds
Based on the results of the soil gas and geophysical survey, the estimated area of the B-3 landfill trench that requires treatment for VOCs is 15,000 square feet (150 feet by 100 feet). The estimated average thickness of this potentially contaminated soil is 15 feet (based on observations made during drilling), which totals 225,000 cubic feet (or 8,333 cubic yards). The average porosity of the fill material in the trench was 30 percent, which converts into a bulk density of 1.85 g/cm3 (or 115 lb/ft3). Based on these assumptions, the total mass of fill material in the trench requiring treatment is approximately 25,875,000 pounds (11,747,250 kilograms) of solid material.
The two VOCs requiring treatment based on the comparison to TNRCC risk reduction rule criteria are TCE and DCE. The average concentrations of these compounds are 32.9 mg/kg and 6.6 mg/kg, respectively. Based on these estimates, the total quantities of TCE and DCE needed to be extracted from the main trench area are 386 kilograms (850 pounds) TCE and 78 kilograms (170 pounds) DCE. These estimates are based on the limited characterization data that was collected. Additional characterization data may be necessary to obtain more accurate estimates of the mass of contamination and the required treatment duration.
7.4.2.5 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 most significant contamination was encountered in VEW-6, which is not located within the defined limits of the main B-3 landfill trench. Only one sample, MPD (8-10 ft bgl), collected from the main trench limits had metal levels greater than background levels. The MPD borehole is located approximately 30 feet from VEW-1 and VEW-2, and is away from the main pilot study test line. No metals contamination was encountered in the line of five soil borings drilled on the southern portion of the VOC treatment area, as defined by the soil gas and geophysical survey results.
Metal contamination does not likely extend from MPD to VEW-6, 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, only 20 feet south of VEW-6. The second is the apparent boundary of the main trench located between MPD and VEW-6. 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-6 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 bgl 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.
7.4.3 Initial Soil Gas Chemistry
Soil gas samples were collected from the six VMPs and six VEWs. Each VMP has three screened sampling intervals, whereas each VEW has only one screened interval. The VMPs have screened intervals of less than 1 foot, so they provide more discrete sampling points with regard to depth than the VEWs. Each VEW is screened across approximately 1o feet of formation or fill material. The sample points were initially field screened for oxygen, carbon dioxide, carbon dioxide, and TVH to determine which points would be most suitable for collection of soil gas samples for analytical laboratory testing. Table 7.4-3 lists the results from the initial field screening. Also listed in Table 7.4-3 are measurements of the vacuum pressures exerted on each sample point during sample point purging. This pressure data provides information on the apparent tightness of the screened formation being tested.
The screening results indicate that anaerobic conditions exist at several of the tested subsurface locations. The majority of these locations are within the limits of the main landfill trench at depths of 10 feet or greater. These anaerobic conditions are indicative of biological activity, most likely caused by the biodegradation activities 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 trench limits. The low oxygen level suggests that subsurface pathways may exist that allow communication of the trench to some portions of the natural limestone surrounding the trench. According to the results, the most depleted oxygen levels appear to be associated with the deeper zones of the trench located at VEW-1, and extend west to MPA and VEW-3 and north to MPD. The differences observed in MPC-14 and MPB-13 suggest that little or no communication 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. High TVH readings were observed at each point where low oxygen levels were encountered. In addition, high TVH readings were also measured at VEW-2 and MPB. Both of these points had relatively high oxygen levels.
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 the points 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 7.4-4. The first samples 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 subsequent sampling events. The primary contaminants encountered include TCE, cis-1,2-DCE, and vinyl chloride. Because no vinyl chloride is suspected to even have been disposed of at 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 observed during the purging activities by attaching a vacuum gauge to a tee in the vacuum side of the system. 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 permeabilities. Only two screened intervals indicated vacuum pressures that may be limiting to air movement in subsurface soils. Both of these intervals, MPF-8.5 and VEW-04, are located completely or partially in the natural limestone material outside the trench. The other intervals that exhibited tightness in the formation are MPF-12.5 and MPD-15. The high permeabilities associated with some VMP and VEW screened intervals located in the natural limestone suggest probable fractures in the formation. Based on these limited results, fracturing of the natural limestone appears to be relatively variable at this site. These is insufficient data to determine whether some fractures are short circuiting to the surface.
7.4.4 Air Permeability Test Results
Two air permeability tests were attempted at the site, as described in Section 3.8.3. One test was performed outside the main trench at CEW-6 and the second test was performed at VEW-1 inside the trench. The pressure measured at VMPs during air extraction at VEW-6 reached steady state within 5 minutes after initiating the air permeability test. No response was observed at any of the adjacent VMPs or VEWs during air extraction at CEW-1. These results illustrate the heterogeneous nature of the subsurface conditions at the site. The results of the air permeability testing are briefly discussed below.
The final pressure responses measured at MPE and MPF during air extraction at VEW-6 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 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 venting well. The air permeability results are high, especially considering the tightness of the deep VMP interval encountered at MPF. The values were calculated at approximately 200 darcys.
Pressure responses were observed for all three depths measured, indicating relatively uniform lateral air flow toward VEW-6. At 10 feet from VEW-6 (MPE), 4- and 8-foot depth intervals had a vacuum, of approximately -0.2 inches of water and the 10-foot interval had a measured vacuum of -0.13 inches of water. At a horizontal distance of 20 feet from air extraction, the 4-foot interval had a vacuum response of -0.2 inches of water, and the 8.5- and 12.5-foot intervals had vacuum responses of -0.1 to -0.14 inches of water, respectively.
As mentioned above, no pressure response was observed at adjacent VMPs or VEWs during air extraction at VEW-1. Air extraction was performed continuously from VEW-1 for 140 hours. Pressure response was measured at MPA, VEW-2, VEW-3, and MPD. This air permeability test was continued because it was coupled with an evaluation of the VOC emission/mass removal study to estimate the quantity of VOCs that the SVE system is capable of removing, and whether the SVE system can maintain those removal rates over time. The results of the air emission/mass removal study are discussed in Section 7.4.6.
7.4.5 Subsurface Influence Findings
Three activities were performed to assess the subsurface influence of air extraction on soil gas. The first activity was performance of two air permeability tests. The second activity was collection and direct comparison of soil gas samples after approximately 90 hours of air extraction at VEW-1 with initial soil gas sample results. The final activity was the manipulation of the VEW extraction 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.
7.4.5.1 Findings From Air Permeability Test Results
The air permeability tests were not successful in determining 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. However, the lack of response observed at the adjacent monitored points during air extraction at VEW-1 is direct evidence that air flow pathways interconnected with VEW-1 bypass the adjacent test points. Based on the air permeability test results, VEW-1 is directly connected to VEW-2, VEW-3, MPA, or MPD. The probable interconnections were investigated using different air extraction configurations. These results are discussed later in this section.
7.4.5.2 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 7.4-5. Significant changes were observed in soil gas concentrations at numerous test points, 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-3, VEW-5, MPA-5, and MPD-15. Little or no change was observed in several points, including VEW-2, VEW-4, MPC, and MPB. These results provide strong evidence of the subsurface interconnection between some 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 the interconnected air pathways is the soil gas changes observed in VEW-5, which is located farther (at least 60 feet to the east) from the extraction point than VEW-4, MPB, and MPC. Also unusual is that VEW-5 is located at least 30 feet from the edge of the trench in natural weathered limestone. It is conceivable that a fracture is present that connects the screened formation at VEW-5 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-1. The changes in oxygen, carbon dioxide, and TVH observed at MPA, MPD, VEW-3, and VEW-5 are comparable to soil gas chemistry changes resulting from air extraction in shallow soils. Thus, all of the observed changes are probably due to SVE applied to VEW-1.
7.4.5.3 Multiple Configuration Testing
Following completion of the second air permeability test and the air emission sampling, all five 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 response 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 interconected to the 15-foot depth of MPD and MPA inside the trench area, but not to any of the other VMPs or VEWs.
Results of the different subsurface influence tests illustrate the complexity of the subsurface environment in and outside the landfill trench area. 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 implies that a significant portion of the treatment area in the B-3 landfill is interconnected, although the route of connection might be convoluted. Soil gas contamination detected in native limestone outside the landfill in VEW-04 and VEW-05, and the interconnections identified by subsurface influence testing, suggest that migration of VOCs into limestone fractures is common, and would need to be addressed in a full-scale remedial design.
7.4.6 Air Emission/Mass Removal Summary
7.4.6.1 Contaminant Mass Removal Estimates
The estimated contaminant mass removed from SWMU B-3 is presented in Figure 7.4-1. The figure shows the contaminant removal rates over time for TCE and cis-1,2-DCE from extraction at VEW-01. The mass removal estimates are based on analytical test results of air samples collected from the blower exhaust. The emission rates are calculated by multiplying the concentration detected in a sample with the measured flow rate from the blower exhaust, as discussed below. The area under the curves in Figure 7.4-1 represents the total mass of each contaminant removed by the blower extraction system during the 140 hour test. The total TCE mass removed in the 140 hour test was estimated at 43 pounds, and the estimated total mass of cis-1,2-DCE removed was 16.88 pounds.
Figure 7.4-1 depicts the predictable drop in the rate of contaminant removal as the pore volumes of soil gas in the influenced areas are evacuated from the subsurface soils. VOCs present in the soil gas at the start of the extraction are readily removed. After the first pore volume is removed, the concentrations of VOCs measured in the blower emissions begins to drop off rapidly. This results because the only VOCs available for removal are volatilized VOCs in the influenced treatment area, and VOCs that have diffused or migrated from outside the treatment area in response to the soil air pressure gradient. Both volatilization and diffusion dynamics result in significantly slower introduction of VOCs into the soil gas of the treatment zone. As VOC concentrations in the soil decrease, the rates of volatilization and diffusion will decrease causing further reductions in the rate of contaminant removal.
At the end of the 140 hour test, approximately 0.24 lbs/hr TCE were still being removed from extraction at VEW-1. This is only a slight reduction from the 47 hour measurement of 0.28 lbs/hr. Assuming an average removal rate of 0.2 lbs/hr from continuous operation of the system, approximately 1,750 pounds of TCE could be extracted from the subsurface soils in 1 year of operation. This also assumes that TCE concentrations in soils will not be appreciably depleted, which could significantly reduce its removal rate. Only a slight reduction in the rate of cis-1,2-DCE was also realized from the 47 hour measurement of 0.1226 lbs/hr to the 141 hour measurement of 0.0956 lbs/hr. Assuming an average removal rate of 0.07 lbs/hr from the continuous operation of the system, approximately 600 pounds of cis-1,2-DCE would be removed from the B-3 SVE pilot study treatment area in 1 year of operation. If the existing SVE pilot test system is capable of treating all subsurface soils in the B-3 trench, and assuming that the VOC delineation estimates are accurate, then 1 year of operation should be more than sufficient to remove the 850 pounds of TCE and 170 pounds of cis-1,2-DCE estimated to remain in B-3.
Vinyl chloride was not detected in any of the emission samples. It is probable that little vinyl chloride remains in the soils. The presence of vinyl chloride can be attributed to the degradation of TCE, as vinyl chloride is a breakdown product of TCE biodegradation. Vinyl chloride's volatility accounted for its presence in the soil gas samples collected to assess initial soil gas chemistry. Initial evacuation activities of the treatment zone likely resulted in the significant removal of vinyl chloride.
In order to compare the worst case scenario with the emission limits established in the applicable Standard Exemption, the maximum for the SVE system soil gas concentrations will be used in the calculations. The Standard Exemption is promulgated by 30 TAC 116.211 Standard Exemption 68 and 118. 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).
Chemical constituent emission limits (E) for Standard Exemption 118(c) are calculated by using the equation E = L/K, where L is the value as listed in Standard Exemption 118(a) and K is the value from Standard Exemption 118(c). The calculated emission rates versus the exempted emission rates are summarized in Table 7.4-6.
7.4.7 Investigation Derived Waste Characterization
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, approximately ten containers were generated. For characterization purposes a composite sample was analyzed for TCLP VOCs and TCLP RCRA eight metals. Results of analyses for the IDW soils are presented in Table 7.4-7. The IDW was characterized as nonhazardous per 30 TAC 335 subchapter R.