2.1 - Overview

Chlorinated hydrocarbons were first detected in well 16 in 1991 at concentrations above drinking water standards, prompting an investigation of the possible source areas that contribute to the contamination of groundwater. Source characterization began with surface geophysical surveys performed during January through March 1995 at seven potential source areas. A geophysical anomaly location map of SWMU B-3 is shown on Figure 2.1. Two large anomalous areas were detected at SWMU B-3 during the EM survey. Based on this geophysical data, soil borings were drilled at potential areas near SWMU B-3 to investigate the portions of each area exhibiting apparent geophysical anomalies. The analytical results of this sampling at B-3 are summarized in Table 2.1, and "hot spots" are shown on Figure 2.2. 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 2.3, Figure 2.4, and Figure 2.5 for TCE, PCE, and cis-1,2-DCE, respectively. The presence of these chlorinated hydrocarbons have implicated B-3 as a likely source area for the contamination detected in well 16.

A limited SVE pilot test was performed at SWMU B-3 to evaluate removal of VOC contamination from vadose soils. The primary objectives of the pilot test were to determine if SVE is a viable remediation alternative and to collect data to design a full-scale SVE system. Borings drilled for construction of the SVE pilot test system were also 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 characterizing subsurface soil conditions.

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 addendum summarizes the results of the Spring 1996 SVE pilot test activities and discusses 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 this section. The activities performed are summarized below in the chronological order that they were performed.

2.2 - Geology and Hydrogeology

Based on the initial exploratory borings and the initial pilot test boring logs, 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 locations of the initially sampled borings are shown on Figure 2.2. The locations of the pilot test borings are shown on Figure 2.6. Representative cross sections of the initial borings and the pilot test borings are shown on Figure 2.7 and Figure 2.8. The orientations of the cross sections are shown on Figure 2.6. The depth to limestone appears to be is variable across the site. Towards the eastern portion of the trench, the limestone becomes shallow and is exposed at ground surface east of the pilot test layout. 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 non-plastic clay. Lithologic logs of all the boreholes drilled at this site are provided as an attachment to this addendum. The limits of the main B-3 trench used to design the configuration of the additional 12 wells are estimated based on soil gas and geophysical test results.

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, and 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.

2.3 - Soil/Rock Analytical Results

Twenty-two soil/rock samples were collected during drilling of the seven preliminary soil borings for analysis (Table 2.1). Chloroform was detected at a depth of 2-4 feet in SB4 and m,p-xylene was detected in SB3 from 11 to 13 feet. Cis-1,2-dichloroethene (DCE) was detected in four soil borings at sample depths ranging from 2 to 19 feet. Trans-1,2-DCE was also detected in the same samples as cis-1,2-DCE in borings SB4 and SB5. Tetrachloroethene (PCE) was detected in three samples collected from two borings at depths ranging from 5 to 28 feet. Trichloroethene (TCE) was detected in six samples collected from four borings at depths of 1 to 19 feet. Di-n-butylphthalate, a common laboratory contaminant, was detected in thirteen samples. Figure 2.1 shows the distribution of volatile organic and metal contaminants detected at the seven sampled locations. The distribution of VOCs in the initial borings drilled at B-3 is shown on Figure 2.8.

Fifteen additional soil samples were collected from 10 of the 12 borings drilled for VEW or VMP construction in February 1996. Samples were not collected from two of the VEW borings located outside the trench area (VEW-04 and VEW-05) because no evidence of contamination was observed during drilling. The distribution of VOCs in the SVE pilot test borings is marked on the cross section presented in Figure 2.7. Analytical results from the 1996 sampling are presented on Table 2.2.

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 below ground surface (bgs) 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 hydrocarbon 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 B-3 trench. However, PCE was detected in MPE at 650 m g/kg but was not detected in any other soil boring samples.

The contaminant levels of TCE detected in soil samples exceeded the TNRCC risk reduction standard 2 (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.

Inorganic data were also collected to help characterize the fill material, but this data is not critical for development of this addendum. The analytical results for metals are more fully discussed in Section 7 of the Groundwater Investigation and Associated Source Characterizations (GWIASC) report prepared for CSSA (Parsons ES, 1996c).

Miscellaneous and Physical Data

One sample was collected from each sampled boring during the SVE pilot test construction 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 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 2.3. 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. indicate that the primary factors affecting soil gas movement are highly variable. These factors include moisture content, porosity, and particle size distribution. The variability of subsurface materials encountered during drilling, plus the aggregation of debris apparently mixed into the fill material, have resulted in generally favorable conditions for subsurface air flow in the trench. The results of the physical characteristics testing indicate that no conditions are present in the subsurface soils that would restrict air flow. These results are also more fully discussed in Section 7 of the GWIASC report (Parsons ES, 1996c).

Soil pH, total organic carbon (TOC), phosphates, and nitrogen values reported 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. Some of the testing planned in this addendum will further evaluate the contribution of bioremediation in the removal of contaminants from the subsurface soils.

Delineation of Volatile Organic Compounds

Based on the results of the soil gas and geotechnical survey, the estimated area of B-3 landfill trench that requires treatment for VOCs is approximately 7,500 square feet (150 feet by 50 feet). The approximate extent of the VOC plumes identified during the soil gas survey, and the edge of the trench as determined by geophysical testing and lithologic descriptions at two soil borings is delineated on Figure 2.9. The estimated average thickness of this potentially contaminated soils is 15 feet based on observations made during drilling, which totals 112,500 cubic feet (or 4166 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 12,937,500 pounds (5,880,681 kilograms) of solid material.

Two VOCs, TCE and cis-1,2-DCE, exceed the TNRCC RRS2 criteria, and thus require treatment. 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 193 kilograms of TCE and 39 kilograms of cis-1,2-DCE. These estimates are based on the limited characterization data that was collected. Additional data will be collected as part of the SVE study expansion to obtain more accurate estimates of the mass of contamination present in the treatment zone of the trench to evaluate the required treatment duration.

The soils data supports the finding that VOC contamination is limited to the general area defined by the soil gas readings inside the trench area. VEW-3 is presumably located just outside the plume defined by the soil gas and is only 15 feet west of MPA. The concentrations of VOCs at VEW-3 were below detection limits of 10-25 parts per billion, which is less than the levels detected at all three sampled depths at MPA. The soil gas survey indicated the edge of the contaminant plume extends between VEW-3 and MPA. However, soil gas chemistry at VEW-3 reveals low oxygen and elevated TVH levels indicative of lingering contaminant residuals. As part of the expanded SVE study, additional characterization data will be collected to evaluate the need for the final installed SVE system to treat low residual contaminant concentrations that may be present at the perimeter of the target treatment area.

2.4 - Groundwater Analytical Results

No groundwater was encountered during the installation and testing of the SVE pilot test system.

2.5 - Geophysical Testing Results

No changes.

2.6 - Soil Gas Testing Results

A soil gas survey was initially conducted at B-3 in 1995 to identify potential contaminant source areas. The location for the existing SVE pilot test system was selected based on the results of the initial soil gas survey, coupled with geophysical survey and soil boring sampling results. The results of the initial soil gas survey are presented in Section 5 of the GWIASC Report (Parsons ES, 1996c) and were summarized in the Work Plan for SVE Pilot Test System (Parsons ES, 1996a). Figure 2.3, Figure 2.4, and Figure 2.5 show soil gas contaminant concentration contours for TCE, PCE, and cis-1,2-DCE, respectively.

2.7 - 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 were described in Section 7 in the GWIASC Report (Parsons ES, 1996c), and are briefly outlined below. In general, the results indicated that SVE would likely be an efficient method for remediating VOC contamination in the B-3 trench.

Initial soil gas chemistry data collected after the installation of the existing SVE test system indicated that significant VOCs (primarily TCE and cis-1,2-DCE) are present inside the main B-3 trench, with much lower concentrations in a minor trench located northeast of the main SVE test VEWs and VMPs. 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 2.4 lists the results of from the initial field screening. Also listed in Table 2.4 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 exist at several of the tested subsurface locations, primarily within the limits of the main B-3 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. This low oxygen reading 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 oxygen levels increase to the east from VEW-1 to VEW-2 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. 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. Meanwhile, 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.

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^TM 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 2.5. The first sample collected (VEW-1) 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. 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 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 permeabilities. Only two of the intervals, VEW-04 and MPF-8.5, indicated vacuum pressures (or permeabilities) 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 permeabilities 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.

Two air permeability tests were attempted at the site. The first test was performed outside the main B-3 trench at VEW-06, and the second test was performed at VEW-1 inside the main trench. The pressure measured at VMPs during air extraction at VEW-6 reached steady state conditions were achieved in the monitoring points 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-1. 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-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 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 extraction well. The resulting air permeability values were high, especially considering the tightness of the deep VMP intervals encountered during drilling 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-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-2, VEW-3, and MPD. The findings of these studies are briefly described below.

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, 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 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.

Air Permeability Test

Air permeability testing and 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 observed soil gas changes during the initial pilot test activities suggest the subsurface environment in and around the landfill trench is very complex and contains preferential pathways. 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-1 is direct evidence that air flow pathways interconnected with VEW-1 bypass the adjacent, monitored test points.

A comparison of the initial soil gas screening results with soil gas measurements collected after approximately 95 hours is presented in Table 2.6. 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-3, VEW-5, MPA-15, and MPD-15. However, little or no change was observed in several points, including VEW-02, VEW-04, MPC, and MPB. These combined results provide results from the three radius of influence tests 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 the interconnected air 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 the interconnected air pathway 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-05 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.

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 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 existing SVE system to identify interconnections in the subsurface environment in the B-3 trench. . Based on limited testing, it was determined that VEW-04 is interconnected to the 15-foot depths of MPD and MPA inside the trench area, but not to and of the other VMPs or VEWs. Also 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, and would need to be addressed to design a full-scale SVE treatment system. The multi-configuration testing planned in this addendum is aimed at studying the complexity of the subsurface trench environment, and determining the most appropriate VEWs that should be used to maximize removal of VOCs from the entire B-3 trench and the surrounding limestone material.

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 16.88 pounds of 1,2-DCE. At the end of the 140-hour test, approximately 0.24 lb.lbs/hr TCE were still being removed from extraction at VEW-01, and 0.1226 lb.lbs/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 above to include more than six operating VEWs at a given time, and if new soils data indicate higher concentrations of constituents of concern, then the Standard Exemption would need to be modified accordingly.

Based on the limited characterization data, it appears that the existing SVE system is capable of removing almost the entire mass of VOCs estimated to be present in the trench. However, based on the complexities of the subsurface environment, multiple configurations may be necessary to assure that all of the contamination in the trench is being addressed. The additional pilot test data planned in this addendum should provide sufficient data for developing a conceptual subsurface model which can be used to optimize the design of the full-scale SVE treatment to remediate the VOCs at B-3, and to estimate the time required to achieve VOC target concentrations in soil. These target concentrations have not yet been defined, but they will be based on those levels that can remain in the trench area without appreciable loss through leaching. To assess this, more data is needed on the factors that affect contaminant migration, such as local climatology, subsurface partitioning factors (soil temperature, moisture, organic carbon, clay content, etc.), and vacuum pressures that can be applied to particular portions of the subsurface trench soils.

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