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Groundwater Investigation and Associated Source Characterization
Section 3 - Methodology
The groundwater investigation and source characterization involved many field procedures. This section describes methods for each of those procedures, including geophysical investigative techniques, drilling and associated well actions, soil gas surveys, SVE test procedures, sampling of various media; decontamination and management of investigation-derived waste (IDW), analytical testing, quality assurance/quality control (QA/QC), and data validation.
3.1 - Surface Geophysical Surveys
3.1.1 Electromagnetic Induction
3.1.1.1 Determination of Data Grids
Prior to collecting EM or GPR data, a grid system was established at each site which encompassed the areas of suspected ground disturbance. These grids consisted of staked locations separated by intervals ranging from 25 to 100 feet, depending on the size of the area and the amount of obstructions, if any. The grid system and spacing used are shown on individual site base maps.
EM data were collected at 2-foot intervals along transects that were separated by 20 to 50 feet using the established geophysical survey grid. EM measurements were taken using a Geonics EM31-DL ground conductivity meter, and recorded with a Polycorder data logger. The conductivity meter consists of transmitter and receiver coils that are separated by 12 feet. The instrument has a nominal depth of penetration of approximately 16 feet when operated in the vertical-dipole mode. The instrument measures both quadrature- and in-phase components of an induced magnetic field. The quadrature-phase component is a measure of apparent ground conductivity while the in-phase component is more sensitive to the presence of ferromagnetic metal. A lateral variation in apparent ground conductivity indicates a lateral change in subsurface physical properties (i.e., related to degree of disturbance). Apparent ground conductivity is measured with a precision of approximately �2 percent of the full-scale meter reading which corresponds to approximately 2 milliSiemens per meter (mS/m). The in-phase component of the EM-31 is the response of the secondary to primary magnetic field measured in units of parts per thousand (ppt). The primary magnetic field is due to the current source from the EM-31. The secondary magnetic field is due to induced currents within conductive material in the subsurface.
Data were collected by setting the instrument to record in an automatic vertical dipole mode. Readings were taken at 0.6 to 0.8 second intervals which corresponded to a reading every 2 feet along a given transect. Both apparent ground conductivity (i.e., quadrature phase) and in-phase data were recorded. The operator aligned himself along a transect and, with the instrument parallel to the transect, paced between marked or staked stations separated by 20 to 50 feet. The variation in transect footage was related to the size of the site and the number of obstructions.
During each field day, data were transferred from the data logger to computer diskettes. The data were processed using DAT31 software (Geonics, LTD) and contoured using Surfer software. For EM data that was not collected using the data logger, values were recorded on a log sheet, manually entered into a computer file, and contoured using Surfer software. Contour maps of apparent conductivity and in-phase data for each site were created. Cool colors (blue) were used for contours of lower conductivity and in-phase values, and warm colors (red) were used for contours of higher conductivity and in-phase values.
3.1.2 Ground Penetrating Radar Survey
3.1.2.1 Equipment Specifications
The GPR survey was performed using a Geophysical Survey Systems, Inc. (GSSI) SIR-3 system equipped with a Model SR-8304 graphic recorder, and 300 and 500 megahertz (MHz) antennas. In addition, a Sensors and Software, Inc. Pulse EKKO IV system with 100 MHz antennas was used. Acquistion parameters for the Pulse EKKO IV consisted of 4-foot separation between transmittter and receiver electrode, station interval of 1 foot, sampling rate of 0.8 nanoseconds (nS), and a 250-fold vertical stack.
To obtain deeper penetration, a second survey used 25 MHz antennas. GPR profile transects, equipment specifications and procedures used during subcontracted GPR work are described in the GPR profiling report (Young, 1996).
GPR is a surface geophysical technique which uses high-frequency electromagnetic energy. Pulses of short-duration electromagnetic energy are transmitted into the subsurface from the radar antenna that is moved across the ground surface at a slow and uniform pace. The radiated energy encounters heterogeneities or anomalies in electrical properties of the subsurface which causes some energy to be reflected back to the receiving antenna and some to be transmitted downward to deeper material. The amplitude or strength of the electromagnetic energy reflected from subsurface materials depends on contrasts in the electrical properties (conductivity and dielectric constants) of those materials. The reflected signal is amplified, transformed to the audio-frequency ranges, recorded, processed, and displayed. Recorded data displays the two-way travel time for a signal to pass through the subsurface, reflect, and return to the surface.
The observed time for the reflected signal to return to the antenna from a subsurface feature is an indication of the depth to the reflector. The two-way reflection time can be converted to depth if the electromagnetic wave velocity of the subsurface material is known. In the absence of such information, an approximate time to depth conversion can be estimated by using published values of material velocity for different soil types.
The interpretation of subsurface conditions is based on analysis of the recorded sections. Buried objects such as pipes and tanks are usually evident as prominent hyperbolic reflections on the GPR records. Subsurface soil changes can be difficult to interpret, but often can be discerned as a lateral change in the texture or reflection character of the GPR signal. Optimal subsurface conditions for use of GPR are dry sandy soils. The presence of even minor amounts of clay may effectively limit depth of investigation to less than a few feet due to absorption and reflection of the electromagnetic energy. Stratigraphic changes are often very prominent and may affect the GPR readings. The use of GPR to determine landfill boundaries and buried waste disposal trenches can be at times very successful due to contrasts in reflection character between the natural stratigraphy outside the trench boundaries and the disturbed soils within the disposal areas.
The objectives of the seismic reflection survey was to determine locations and amounts of offset due to faulting in the Bexar Shale and to delineate any stratigraphic units within the Lower Glen Rose Formation which might be seismic reflectors. To accomplish these goals, acoustic waves were generated using an elastic wave generator (EWG). These seismic waves passed through the ground were reflected back up at the same angle to the Mark Products 40 Hz geophones (receivers). A geophone spacing of 10 feet was selected following a walk away noise test to ensure sufficient sampling and minimize exposure of the recorded data to noise. An end-on array was used, with the near trace positioned at 80 feet from the EWG source and the far trace 550 feet from the source. Shots were taken at each receiver point to maximize fold.
3.1.3.2 Downhole Velocity Survey
A downhole velocity survey was conducted in well 16 to obtain one-way travel times at increasing depths in the well. The signal was created with a sledgehammer and the receivers were a string of hydrophones suspended in the well water column at 1 foot intervals. Using the times recorded for a surface-generated wave to travel between the source and downhole receivers, velocities were calculated. From the veolocities and one-way travel times, the two-way travel times and depths could be calculated for the various stratigraphic units of interest.
Seismic reflection works in much the same way as GPR. The main difference is that seismic reflection uses an acoustic source rather than an electromagnetic source to create a signal. Displacements of the stratigraphic units are determined by offsets of the seismic reflectors. The amount of displacement between reflectors is measured in time and converted to feet.
An in-depth explanation of seismic reflection survey techniques is further discussed in the seismic reflection survey report (Blackhawk Geoscience, 1996).
Soil borings were drilled at seven potential source areas regarding contamination in well 16 and to install the SVE system at SWMU B-3. Soil borings were located based on geophysical anomalies. When an anomaly was not present, topographic lows or surface features, such as metal debris or man-made mounds, were chosen as locations. When an anomaly was present, locations were selected near or within the anomaly. The locations of the SVE borings were based on analytical results from the source characterization drilling and geophysical anomalies at B-3.
Drilling was performed using hollow-stem augers (HSA) where soil was present and air-coring when rock was encountered. Cores were continuously collected every 5 feet for classification and potential sampling. Monitoring of the breathing zone was performed with an HNU� and a MINIMRAM�. Due to the heat and moisture created from air coring, the HNU sometimes indicated false positives while scanning the cores. While drilling with HSAs, no difficulties were noted in taking readings with the HNU, because heat and moisture were not being generated by the samples. Because the HNU appeared to be reading accurately when not exposed to high moisture, exchanging the instrument did not seem to be an improved method for air coring.
The soil and rock core were described by a qualified geologist. The core was described using the Unified Soil Classification System (USCS). Core samples were described according to lithology, Munsell color charts, fossil content, textural features (e.g., bedding, bioturbation), structural features (e.g., fractures, solution cavities), hardness, and moisture content. Lithologic descriptions were recorded on soil boring logs which can be found in Appendix A. These logs also contain sample recovery intervals, analytical sample intervals, a graphic log and comments concerning drilling and sampling.
The soil boring locations have not been surveyed at the present time. CSSA may survey these boring locations for approximate horizontal locations after they purchase GPS units for the post.
Boreholes were grouted within 24 hours or less of being drilled, with the exception of those not completed within one day. A bentonite-cement grout was used. When fractures and/or solution cavities were encountered, bentonite pellets were used to plug the void so that grout could be emplaced to surface level. The bentonite pellets were installed, hydrated and allowed to expand before emplacing additional grout.
3.3 - Geophysical Borehole Logging
3.3.1 Logging of Existing Water Wells
Borehole geophysical logging was performed on CSSA wells 2, 16, and D. Only three wells were selected for logging based on proximity to one another (i.e., wells 2, 3, and 4 are located within a few hundred feet of each other) and the levels of contamination found in each well.
The tools used in the logging procedure were the caliper, natural gamma, gamma-gamma, and neutron. These tools yield information on borehole size changes, shale content, bulk density/porosity, and moisture content, respectively.
Parsons ES contracted Century Geophysical Corporation of Tulsa, Oklahoma, to perform the geophysical services. Logging was performed on August 22 and 23, 1994. Two downhole runs were made in each well with the selected tool setups. Before being lowered in each borehole, each tool and its entire cable length were decontaminated with a high-pressure steam cleaner to prevent cross-contamination. Direct readouts were obtained on site during the logging process. Results of the logging were then used to determine the depth to and thickness of potential perched water bearing zones above the water table. This data was subsequently evaluated for the purposes of discrete interval well sampling and determination of casing setting intervals during well upgrades (Section 3.4). In addition, information on lithology and stratigraphy was obtained from the geophysical data.
3.3.2 Logging of Lower Glen Rose Pilot Holes
Each pilot hole was geophysically logged by the EUWD. The plan was to log each hole with natural gamma, caliper, sonics, and resistivity tools to provide information about water-bearing zones, the contact between the lower Glen Rose and Bexar Shale, and any large voids in the limestone detected by the caliper log. Only natural gamma and caliper tools were used due to equipment malfunctions and schedule constraints. The EUWD performed these services free of charge. Appendix B contains copies of the geophysical logs.
During the preliminary assessment performed in late 1992, all known CSSA water wells were surveyed for location, depth, and condition of surface completion and surface casing settings. based on this information, Parsons ES determined the need for well upgrade work.
All of the wells scheduled for plugging (CSSA wells 5, 6, B, and F) were blocked by rock/debris at various depths. Well 6 (which was scheduled to be upgraded with surface casing) was ultimately plugged when attempts to remove the blockage and upgrade the well failed. During the setting of the annular seal for the 120-foot surface casing, the grout leaked around the plug set the previous day and migrated downhole to the blocked bottom at 190 feet. Well 5 was plugged from its blockage depth of 158 feet bgl to the surface and wells B and F were plugged from 8 and 20 feet bgl, respectively.
Well upgrades were performed on wells 2, 3, 4, 16, and D. The upgrading of the wells consisted of setting approximately 200 feet of surface casing in each well based on the measured water levels and the geophysical logging. Two centralizers were set in each well., One was set near the bottom of the casing and one was set near the top.
3.5 - Well Installation Methodology
Three different types of wells were constructed during the groundwater evaluation studies, including vapor monitoring points (VMPs), vapor extraction wells (VEWs), and monitoring wells. The VMPs are multiple depth screened intervals constructed in the same borehole for collection of soil gas samples from the vadose soils. VMPs were constructed at the B-3 site during the SVE pilot test to monitor changes in soil gas chemistry and to measure pressure response from air extraction. VEWs were constructed at the B-3 site to assess the extraction efficiency of contaminants from the subsurface soils during the SVE pilot test. Groundwater monitoring wells were also constructed at two locations downgradient of well 16 to assess potential fault zones and to obtain additional information on the nature of groundwater chemistry and movement.
Six multi-depth VMPs were installed at SWMU B-3 to measure pressure response and to collect soil gas samples during the SVE pilot test. VMPs were installed at three depths in 8-inch diameter boreholes. Six-inch-long VMP screened intervals were set at either 4, 8, and 12 feet bgl or at 4.5, 9.5, and 14.5 feet bgl. The supervising geologist selected the actual VMP screened intervals based on the lithology encountered. The VMP boreholes were advance by continuous-flight, hollow-stem auger drilling with continuous split-spoon sampling until reaching the hard limestone bedrock underlying the surface soils and fill material. Air rotary coring was used to advance the borehole through bedrock material to the desired construction depth. The VMP screen and casing was installed in the open 8-inch boreholes after reaming the hole and removing the drill stem.
Construction details of a typical pressure-monitoring point is illustrated in Figure 3.5-1. In general, the 6-inch screened intervals were set in the center of a 1-foot thick interval of number 6-9 sandpack. Each interval was separated by at least 3 feet of hydrated Benseal and bentonite chips. The VMP screened intervals are completed to the surface with 1/4-inch inside diameter schedule 80 PVC. Ball valves with hose nipple adaptors are connected to the top of the 1/4-inch riser to enable surface access for sampling. Thermocouple wire was attached to two VMP screens at monitoring point (MP) D set at 10 and 15 feet bgl for measurement of soil temperature. Boring logs for the six VMPs constructed are presented in Appendix A.
The VEW borings were drilled to either 17 or 20 feet bgl, and the VEW screen was placed from the total borehole depth to 7 to 9 feet bgl. The supervising geologist determined the length of screened interval for each VEW based on the lithology encountered. Similar to the VMPs, the VEW boreholes were advanced by continuous-flight, hollow-stem auger drilling with continuous split spoon sampling until reaching the hard limestone bedrock underlying the surface oils and fill material. Air rotary coring was used to advance the borehole to the desired VEW construction depth. The VEW casing was installed in the open 8-inch boreholes after reaming the hole and removing the drill stem.
The VEWs were constructed using 4-inch diameter, schedule 40 threaded-and-coupled polyvinyl chloride (PVC) casing. The VEWs were screened with 0.040-inch factory-slotted PVC screen. The well annulus was gravel-packed from the bottom of the screen to approximately 1 foot above the top of the screen with number 6-9 silica sand. Approximately 2 to 3 feet of bentonite pellets were placed above the gravel pack in each VEW, with tap water poured into teh annulus to hydrate the bentonite. A typical VEW construction diagram is presented in Figure 3.5-2. Soil boring logs are included in Appendix A.
3.5.3 Groundwater Monitoring Wells
Two groundwater monitoring wells were drilled by air rotary and completed as open hole wells. Prior to completion of the monitoring wells, two pilot holes were drilled into the Bexar Shale to obtain information on water-producing intervals and subsurface geologic structure. The following paragraphs describe the methods used to perform this work.
3.5.3.1 Drilling of Pilot Holes and Well Boreholes
Pilot holes (PH) and groundwater monitoring wells (MS) were air rotary drilled by Geoprojects International, Inc., under supervision of the Parsons ES geologist. Drilling was performed using a Gardner Denver 1500 rig outfitted with a mud/air package, a Leroy 620 cubic feet per minute (cfm) compressor which can maintain a pressure of 256 pounds per square inch (psi), and a coalescing filter system that ensures air blown downhole is free of contaminants. The drilling rig was in excellent mechanical condition and had been thoroughly steam-cleaned prior to the start of work. No fluids leaked from the rig's mechanical, hydraulic, or pneumatic systems. A 6-inch tricone rock bit was used to drill the pilot holes and the open hole completion interval of MW2. A 6-inch tricone bit with tungsten steel buttons was used to drill the open hole completion interval of MW1. The boreholes drilled for installation of surface casings were reamed with a 9-7/8-inch tricone rock bit.
In air rotary drilling, cuttings are lifted from the borehole by compressed air that is piped from the compressor to the swivel hose connected to the top of the kelly. The air is forced down the drill pipe and escapes through small ports at the bottom of the drill bit. The cuttings are blown out of the top of the hole and collect at the surface around the borehole. Cuttings were collected in 5-foot batches as the boreholes were advanced for examination and logging by the geologist. Description of the cuttings noted on the drilling log included lithology, color, grain-size, relative competence, moisture content, and degree of weathering. Air returns were monitored for indications of water. Other observations included rig speed, indications of rock hardness, and depth of driller's breaks (intervals where the bit drops indicating a vug or fracture). Rock core was collected from the saturated basal unit of the lower Glen Rose limestone at PH2 using a 10-foot long 2 1/2-inch inner diameter (ID) split-core barrels with a positive latch system. The drilling rig and all downhole equipment were decontaminated using a steam cleaner prior to use and each time the equipment was moved to another hole.
The plan for drilling the pilot holes was to first drill through the shallow zone where perched water might be present. If present, perched groundwater would be sampled and the zone cased off before drilling deeper. Prior to installing a temporary surface casing, the borehole would be logged with geophysical tools by the EUWD and the borehole reamed to a diameter of 7 7/8 inches. The installation of temporary casing was unnecessary since no perched water was present in the pilot holes.
3.5.3.2 Monitoring Well Construction
After the pilot holes were drilled into the Bexar Shale and geophysically logged, CSSA and Parsons ES decided to complete both PH1 and PH2 as monitoring wells since groundwater was found in both. Monitoring well construction began with filling the saturated portion of the pilot holes with clean, silica sand to protect the aquifer from drill cuttings and make it easier to develop the well. Then the hole was reamed to a final diameter of 9-7/8 inches down to a depth of 140 feet. Six-inch diameter, low-carbon steel casing with welded joints was installed in MW1. The MW1 surface casing was equipped with drillable casing shoe made of concrete, and steel centralizers were set approximately every 50 feet at depths of 135, 80, and 50 feet bgl. The surface casing for MW2 was constructed of 6-inch-diameter Schedule 40 PVC with a drillable casing shoe made of plaster of paris. PVC casing collar joints were joined with stainless steel sheet metal screws. Casing was new and decontaminated prior to emplacement. No glue was used to join casing segments. The annulus between the boreholes and surface casings was pressure-grouted to the ground surface via a tremie pipe. Grout was mixed using Type I Portland cement (Alamo Cement Company, San Antonio, Texas), 7.5 gallons of potable water per sack of cement and 5% bentonite by weight (Quik-Gel, Baroid Drillings Fluids, Inc., Houston, Texas). Mud scale measurements of grout weight ranged from 14.2 to 14.4 pounds per gallon. The casing grout cured for at least 24 hours before drilling deeper.
When the MW2 casing grout had cured, the casing shoe was drilled out, and the borehole advanced to the completion depth. Sand used to fill the lower section of the hole (the tail hole) and cuttings from reaming the shallow interval were removed using a 6-inch diameter rock bit. The monitoring well was completed as an open hole. Figure 3.5-3 is a diagram of the lower Glen Rose monitoring well completion.
Monitoring wells were completed above ground level. Surface casings extend 2 to 2 1/2 feet above ground surface. Protective casings, 8-inch-diameter carbon steel in 5-foot lengths, were installed around the surface casings to add structural strength to the surface completion and prevent bonding of the concrete slab to the surface casing. A two-piece aluminum locking cap was installed on both monitoring wells. These caps are comprised of a ring installed on the top of the protective casing with set screws and removable cap that, when locked in place, is secured on two sides. Concrete pads measuring 5 feet by 5 feet by 4 inches thick were constructed centered on the protective casings. A 2-inch-diameter brass monument permanently marked with the monitoring well identification was set into the concrete. The concrete pad was finished out with a broom to create a non-slip surface. The steel protective casings were painted white, the color selected by CSSA. Guard posts were not installed around the monitoring wells since the wells are located in areas of low vehicular traffic.
Because the well intended for installation in PH1 was plugged (see Section 3.6.2), MW1 was installed 13 feet east of PH1. The borehole was reamed from the surface to 140 feet bgl and the surface casing was installed and grouted into place as described above. After the grout had cured for more than 24 hours, the well was drilled down to a total depth of 320 feet with a 6-inch diameter, tricone, tungsten steel button bit. Lithologies encountered were the same as in PH1 but certain drilling features were not the same, probably because of the different bit.
3.5.3.4 Monitoring Well Development
Monitoring well development of MW1 and MW2 was performed by air lifting and pumping. Immediately after completion of each monitoring well, loose material was removed from well completion intervals by air lifting. Compressed air was blown downhole to within 20 feet of the total depth of the well via the drill pipe string. At intervals, the driller jetted the well by allowing pressure to build up and then let loose in an instant to remove a slug of sediments. Airlifting removed most of the sediment. Well development was completed by pumping each well with a 1-horsepower Grundfos submersible pump rated at 5 gallons per minute (gpm). Water removed from the wells was clear and temperature, conductivity, and pH measurements collected every 10 minutes had stabilized to within 10 percent. Water levels were measured every 10 minutes during pumping with an electric water level indicator until the instrument's cable became hung on ties running from the pump to the surface (electric wire, discharge line, and cable).
Plugging of inactive water and monitoring wells was performed in accordance with and 30 Texas Administrative Code (TAC) 338.48-49. The following describes procedures for each type of plugging performed.
3.6.1 Plugging of CSSA Inactive Water Wells
Four inactive water wells at CSSA were scheduled for plugging. Layne Environmental Services, Inc. of Houston, Texas, performed the plugging under supervision of a Parsons ES geologist. An attempt was made to remove any obstruction in each well; however, based on recommendations from the Texas Water Development Board (TWDB), if an obstruction could not be easily removed or pushed through, the obstruction was left in place. Cement/bentonite grout (4 percent bentonite) was pressure grouted with a tremie pipe. The grout was emplaced from the blockage depth to ground level and allowed to settle for 24 hours. After this period of time, the grout level was checked and grout added if the level had settled below ground.
3.6.2 Plugging of Tank Monitoring Wells
Shallow monitoring wells for removed CSSA underground storage tanks were plugged as part of the groundwater investigation. The TNRCC Petroleum Storage Tank Division required plugging of the wells as the final step for closure of the CSSA tank monitoring program.
Plugging was performed by first pulling the PVC casing from the 20- to 28-foot boreholes. The PVC casing was triple-rinsed with a steam cleaner and saved for CSSA use as part of the DoD Guidelines that encourage reuse of materials. The surface completions could not be saved due to the pressure of pulling casing from the surface, but the well locks were saved for CSSA use.
The boreholes were pressure-grouted with cement/bentonite grout (4 percent bentonite) from the borehole bottom to ground level and allowed to settle over 24 hours. After this period of time, the grout levels were checked and grout added if the level had settled below ground.
3.6.3 Plugging of Pilot Hole 1
PH1 was abandoned. During installation of the PVC surface casing in PH1, the drillable shoe failed and grout rose up inside the casing to a level of 20 feet above the bottom. Failure of the plaster of paris drillable shoe was likely due to the pressure exerted on it by pressure grouting. An attempt to drill through this 20 foot thickness of grout was unsuccessful; the bit was walking off the very hard cured grout and chewing into less dense PVC casing. The PH1 tail hole had been filled with clean, silica sand from total depth up to 229 feet bgl prior to reaming the surface casing interval and installing the PVC casing. Cuttings that fell into the tail hole when the hole was reamed are present from a depth of 229 feet bgl to an unknown point between 229 and 140 feet bgl. The interval between ground surface and 140 feet bgl was pressure grouted with cement/bentonite grout with the PVC casing in place. The plugged pilot hole was permanently marked with a brass monument set in concrete.
All CSSA wells were surveyed for top of casing and natural ground (NG) elevations, as well as northing and easting coordinates. Surveying was performed by a licensed surveyor. Two companies, Northstar Land Surveying and Macias and Associates, Inc., performed the surveying for this project. Northstar Land Surveying performed the original survey of all CSSA wells, including abandoned wells. Macias and Associates measured the top of casing and NG elevations for two monitoring wells, MW1 and MW2, and the seven CSSA water wells.
Control was brought in from the state benchmark BM1167 located near the intersection of Interstate 10 and State Highway 3351 (Ralph Fair Road). All points required to control the survey were occupied as stations within a closed and adjusted traverse. The controls meet or exceed third-order accuracy standards. Further control was brought in by Macias and Associates. Three GPS control points were established for all future survey work at CSSA. Figure 3.7-1 shows the locations of these control points.
Two vertical and horizontal controls were referenced by the surveyors. Northstar Land Surveying referenced vertical control to National Vertical Geodetic Datum (NVGD) 1983 and horizontal control to North American Datum (NAD) 1983. Macias and Associates reported data in NVGD 1927 and NAD 1929.
3.8 - Soil Vapor Extraction Test Procedures
This section summarizes the methodology used to install and test an SVE pilot test system at SWMU B-3 during February and March 1996. The primary activities performed were siting and construction of the SVE system, soil sampling, soil gas sampling, air permeability testing, air emission sampling, and blower optimization. VEW and VMP construction is described in Section 3.5 of this report. General soil, soil gas, and air emission sampling procedures are described in Section 3.9. The SVE system layout, the vacuum test blower installation and specifications, and the air permeability test procedures are described below.
The layout of the installed SVE pilot test system is shown on Figure 3.8-1. The locations of the VEWs and VMPs were selected using the following criteria:
| Based on gas chromatograph (GC) headspace analysis and soil gas survey results, soil vapor concentrations are highest at these locations; | |
| Boring log descriptions from borings completed in this area indicated the depth to competent limestone is at least 10 feet and that subsurface soils are representative of the landfilled trenches and surrounding lithologies; | |
| Surface geophysical survey data from this area indicate the probable boundary of a primary landfill trench is identifiable in this area, which allows testing to be performed on subsurface soils inside and outside the trench limits; | |
| 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 B-3 landfill area. As shown on Figure 3.8-1, three VEWs and two VMPs (identified as VEW-1, VEW-2, VEW-3, MPA, and MPD) were placed within the main B-3 landfill trench limits, one VMP (MPB) was installed in the transition zone on the edge of the main trench boundary, and one VMP (MPC) and two VEWs (VEW-4 and VEW-5) were installed in limestone material outside the landfill limits. All of these VMPs and VEWs, except MPD, were installed in a line that crosscrosses the portion of the landfill that exhibited the greatest volatile organic compound (VOC) concentrations during the soil gas survey. MPD was constructed approximately 30 feet north of the line. A second test system was installed northeast of the primary test line in an isolated area that also exhibited high levels of contamination in soil gas. This system consists of one VEW (VEW-6) 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 landfill was encountered sooner than anticipated, the planned location for VEW-3 was converted to MPB. VEW-3 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 landfill limits and shallow limestone encountered outside the landfill during installation activities, the spacing between VMPs and VEWs proposed in the work plan was appropriate. A spacing between the VEWs and VMPs of 10 feet outside and 15 feet inside the landfill limits was used for the system construction. The pilot test layout is designed to allow air permeability testing of soils within the landfill trench and outside the trench in the native soils.
3.8.2 Construction of Test System
This section describes construction of a 2.5 horsepower (hp) blower soil vapor extraction system which now operates in the SWMU B-3 area. An instrumentation diagram for the SVE system constructed at the site is presented on Figure 3.8-2. 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 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. Blower specifications and performance curves are included in Appendix B of the SVE pilot test work plan (Parsons ES, 1996a).
In general, construction activities were performed in accordance with the work plan (Parsons ES, 1996a). Soil sampling and well installation activities were completed as described in Section 3.5 and Section 3.9 of this report. 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 aboveground pipe from fee 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.
As shown on Figure 3.8-1, 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 SWMU B-3. 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. CSSA electricians connected power and a control box for the blower.
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 cfm at VEW-2 to 41.4 cfm at VEW-5. 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. 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 has been manually drained from the blower system on a weekly basis, when operating. An automated drainage system has been designed and is planned to be installed in a future project.
The pilot test layout is designed, in part, to allow air permeability testing of soils within the landfill trench and outside the trench in native material (limestone formation). The primary objectives of the air permeability testing were to provide data for determining the potential vapor flow rates and radius of influence for soils inside and outside the trench limits. Other key objectives include the determination of the quantity of extracted VOCs from soils and the rate of contaminant discharge into the atmosphere.
Two air permeability tests were attempted. The static pressure and vacuum at the blower and VMPs was monitored to evaluate when static conditions were achieved. The vacuum at each VMP was also measured following startup of the extraction test. Measurements were made at each screened interval to evaluate the relationship between vacuum at different distances and depths.
The first test was performed on March 6, 1996 at VEW-6. The 2.5-hp pilot test blower was used to extract air from VEW-6, and pressure response was measured at all three screened intervals at MPE and MPF. Air was extracted at a flow rate of approximately 30.5 cfm. Steady state conditions were achieved within 5 minutes after initiating air extraction. The monitored VMPs were located 10 and 20 feet from the extraction well, respectively.
The second air permeability test was initiated on March 7, 1996. Air was extracted at VEW-1 using the 2.5-hp pilot test blower at approximately 10 cfm. Pressure response measurements were made over a period of 6 days rom nearby VMPs and no response was observed from any of the monitored intervals. The monitored points included: VEW-2, located 15 feet east of VEW-1; VEW-3, located 15 feet west of VEW-1; MPD, located approximately 30 feet north of VEW-1; and MPB, located 30 feet west of VEW-1 (Figure 3.8-1).
During the second air permeability test, gas samples were collected over time from the blower emission outlet pipe for VOC analysis. These samples were collected directly into a Summa� canister attached to a fitting on the outlet pipe. Air flow was also measured following sample collection so the total quantity of VOCs being discharged could be calculated. After initiating air extraction from VEW-1, air emission samples were collected after 30 minutes, 2 hours, 5 hours, 11 hours, 23 hours, 47 hours, 95 hours, and 140 hours. Field screening instruments were used to take screening measurements at a more frequent rate. After the second air permeability test, extraction was attempted using several different air extraction configurations (e.g., extract from VEW-1 and VEW-2 simultaneously, extract from VEW-2 only, etc.) to determine if a pressure response could be observed by extracting air from different wells.
Lastly, initial soil gas chemistry was measured at each VEW's or VMP's screened interval for total volatile hydrocarbons (TVH), oxygen, and carbon dioxide using field instruments to establish baseline conditions prior to performing any air extraction at the site. Following 95 hours of air extraction at VEW-1, soil gas chemistry was measured again at the same locations to assess the potential radius of influence created by extracting air from VEW-1.
3.9.1 General Sample Handling Procedures
Sampling was conducted in accordance with the SAP (ES, 1993b). The following describes general handling procedures.
All samples were placed in precleaned (to EPA level 3) glass and plastic bottles for shipment to an appropriate laboratory. All bottles had Teflon�-lined lids. The precleaned bottles were obtained from the subcontracted analytical firm or other suitable vendor.
Individual sample bottles were wrapped in bubble pack and placed in sealed plastic bags to prevent breakage during shipment. The bags were placed into insulated shipping coolers with ice. A chain-of-custody record describing the contents of the cooler was placed in a sealed plastic bag and taped to the upper lid of the cooler. The shipping coolers were sealed with security labels taped over opposite ends of the lid. The coolers were shipped by overnight delivery to the laboratory.
Sample containers were selected to ensure compatibility with the suspected contaminants and to minimize breakage during transportation. Sample labels will be affixed to each container to identify the collector's name, date and time of collection, project site name, sample number, analysis to be performed, and preservatives added. The sample identification number was printed in a legible manner which enabled cross-reference with the field logbook.
A sample numbering system was used to identify each sample collected during the field investigation and for all blank samples. The numbering system had a tracking mechanism to allow retrieval of information about a particular location and to ensure that each sample is uniquely numbered. A listing of sample numbers was maintained by the field team leader. Each sample assumed the format described below.
There was an alphanumeric identification code unique to each sampling location. Equipment rinsate and trip blanks was also identified using an alphanumeric identification code. The field team leader noted in the field logbook which volatile samples were associated with each trip blank during shipment to the laboratory. Each sample number consisted of a location identification code and a consecutive sample number. Samples collected from soil borings had the depth at which the sample was collected in parentheses followed by the sample number.
3.9.2 Soil Gas Field Screening
There were two types of soil gas screening performed to support identification and characterization of potential source areas. These included an extensive soil gas survey of seven SWMUs and six open areas near well 16, and a focused study of the soil gas chemistry in the B-3 SWMU during the SVE pilot test. The methodologies for both of these screening activities are described below.
The soil gas surveys performed on multiple sites were carried out in two phases. Initially, a reconnaissance soil gas survey was performed at the sites to identify the potential sources of contaminants present in groundwater from well 16 and other CSSA wells. A follow-up survey was performed to define the extent of contamination and provide more information on the source areas. Soil gas sampling grids were set up based on the results of the geophysical investigation performed at each site. These grids were established on intervals of 25 to 100 feet, depending on the size of the area to be surveyed. The individual site maps are presented in the soil gas technical memorandum (Appendix F).
Soil profiles were attempted to determined the optimum sampling depth at each site. However, sampling at a uniform depth was not practical because of the variable nature of the soil cover and depth to limestone. Consequently, probes were generally driven to the bedrock-soil interface or until refusal. An electric hammer was used to drive the probes with detachable tips into the soils to the sample depths. The probes were driven into the soil on the end of decontaminated 5/8-inch diameter hollow steel rods to provide access for sample collection.
A four-step process was then used to collect the soil gas sample; the process is depicted on Figure 3.9-1. First, the probes were pulled up slightly to detach the driving tip from the probe and thus allow soil gas to enter the probe. Next, a sampling adapter was paced on top of the probe and a polyethylene tube was run from the adapter to a vacuum pump which was used to withdraw soil gases from the ground. Third, the system was purged with at least three probe volumes prior to sampling to insure that a representative sample of soil gas was obtained from the surrounding formation. After purging the sampling apparatus, the tubing was attached to a Tedlar� bag inside a desiccator. A vacuum was exerted on the Tedlar bag in the desiccator using a vacuum pump, which draws the soil gas sample from the sampling zone. A schematic of this process is shown in Figure 3.9-1. The samples were then transported to the field GC for analysis. Samples were usually analyzed within 4 hours of collection. The hole created by driving the soil gas probe was abandoned with granulated bentonite.
In addition, an initial screening was performed in the field prior to collection of the GC sample. This was accomplished by scanning the exhaust from the vacuum pump with an explosimeter (Industrial Scientific Corporation, Model HMX 271) that measured the levels of oxygen and explosive gases. The vacuum pump was a rotary vane, oil-less, 1/6 horsepower model equipped with a vacuum regulator.
The explosimeter was calibrated daily for oxygen readings by setting the readout to 20.9 percent oxygen when held in ambient air. For oxygen and lower explosive limit (LEL) measurements, the explosimeter had a stated accuracy of �1.2 percent oxygen by volume in the range of 5-30 percent and �10 percent of the actual concentrations in the range of 30-100 percent of the LEL.
3.9.2.2 SWMU B-3 Soil Gas Sampling
During the performance of the SVE pilot test, initial soil gas was screened at each VMP interval and VEW location for oxygen, carbon dioxide, and TVH. Soil gas screening was also performed following approximately 96 hours of system operation to assess the changes in soil gas chemistry. Soil gas samples were collected by attaching a vacuum pump to the sampling point using flexible tubing and pulling soil gas from the soil formation through the screened intervals of the VMPs and VEWs. Before collecting each sample, the sample point was purged to remove at least the casing and gravel pack volume to ensure a representative and consistent sample of soil gas from the surrounding formation was obtained. A 30-second purge was used for each VMP interval, and the VEWs were purged for 5 minutes.
After purging the VMP, the flexible tubing was attached to a vacuum sampling chamber (desiccator). An air sampling Tedlar bag was connected to the sampling tube within the sampling chamber. A vacuum pump was attached to the vacuum sampling chamber to create a low pressure system within the desiccator, causing air to be drawn up from the screened interval into the sample bag. Once full, the sample in the Tedlar bag was connected to field instruments to measure oxygen, carbon dioxide, and TVH. The oxygen and carbon dioxide were measured using a GasTech 3252 OX meter and the TVH was measured with a Microtip meter.
Based on the screening results, soil gas sample points were selected for analytical testing to confirm the actual contaminant concentrations present in the soil gas. The analytical sample collection procedures are similar to the screening sampling procedures except that the full Tedlar bag was transferred to a 6-liter Summa canister. This transfer was accomplished by connecting the bag to a special fitting on the top of the canister. The canister, which possesses a vacuum of up to 6 liters, is opened, and the vacuum inside the canister pulls the sample from the bag. The samples were analyzed for VOCs.
3.9.3 Soil Gas Emission Testing
Samples were collected for VOC analysis from a sampling port located on the exhaust of the blower to estimate the rate and volume of VOC removal and to assess the quantity of contaminants that may be discharged to the atmosphere during normal operation of a full-scale SVE system. To quantify the mass removal rate during the pilot test, eight air samples were collected at the blower outlet for laboratory VOC analysis. After initiating air injection, emission samples from the operating system were collected at 30 minutes, 2 hours, 5 hours, 11 hours, 23 hours, 47 hours, 95 hours, and 140 hours of continuous operation. VOC measurements from the exhaust port were coupled with air flow rate measurements to estimate the cumulative mass of VOC removal taking place over time. Periodic monitoring with a hydrocarbon meter was also conducted at the vapor outlet port to evaluate changes in emissions.
Vapor emission samples were collected by attaching the Summa canister directly to a sample port located on the exhaust pipe with flexible tubing and opening the canister valve. The vacuum inside the canister pulls the sample from the exhaust stream directly into the canister. For direct reading instruments, the instrument probes were attached directly to a sample port on the exhaust pipe. Target analytes included TCE, PCE, and DCE, and were measured by EPA Method TO-14.
All soil and rock sampling occurred during characterization of potential source areas (Section 5.2) or additional characterizations for SWMUs O-1 (Section 6) and B-3 (Section 7). Core samples were collected from one pilot hole (Section 8.4) for lithologic descriptions only.
Soil and rock samples were collected from soil borings with a split spoon or continuous core barrel. Typically, two to three samples were collected from each boring, depending on analytical objectives for the site. All sampling equipment and downhole equipment was decontaminated prior to sample collection. The soil/rock samples were described uding Unified SOil Classification System (USCS) terminology with respect to lithology, color using a Munsell color chart, moisture content, hardness, plasticity, and other pertinent observations such as fractures and fossils. The soil samples were placed in clean laboratory jars with Teflon lids, labeled, and placed on ice to await transport to the laboratory within 24 hours. Rock samples were broken with a hammer and then placed in clean laboratory jars, labeled, and placed on ice before being transported to the laboratory.
Composite soil samples were collected at SWMU O-1 (the oxidation pond) during the liner integrity study (Section 6.2). The top few inches of soil were scraped away from the area to be sampled and the soil collected in clean laboratory jars with Teflon lined lids. Because the samples were to be analyzed for VOCs, the soil was collected directly into the jars instead of a bowl for mixing, so as to minimize volatilization of VOCs that might be present in the soil.
3.9.5 Groundwater Monitoring and Sampling
3.9.5.1 Groundwater Monitoring Actions
The following describes the activities performed during quarterly groundwater monitoring events.
Water Level Measurements
Measurements were taken at CSSA wells every quarter. An electric line indicator was used at all wells except CSSA wells 10 and 11. Water levels were measured by airline at wells 10 and 11 prior to 14 December 1995, after which airline measurements were taken at well 10 only. Well H is obstructed approximately 20 feet below surface, and thus water level measurements have not been obtainable for this well. Measurements were taken as close as practicable during sampling event. CSSA ceases pumping from wells a minimum of 48 hours prior to water level measurements except when post water requirements dictate otherwise. In December 1994, a series of measurements were taken following pumping to determine if water levels attain equilibrium within a 48-hour period. The results indicate that more time may be necessary between pumping and water level measurements to establish water table equilibrium.
Purging
Purging was performed only to purge water from the pump rods, and wells sampled by bailer were not purged prior to sampling. Conventional purging is required to removed stagnant water from the filter pack around a well screen. However, the CSSA wells are not cased to depth and thus experience continuous, fairly rapid, open hole lateral flow that is assumed to not stagnant within the open borehole. Wells with pumps were typically purged for ten minutes prior to sampling. Information used in the decision to not purge the wells in a conventional manner is summarized below:
| The CSSA water wells are open borehole from the bottom of the surface casing to total depth. The wells are completed within stable rock formations (limestone or shale) and do not require slotted casing. | |
| The boreholes are open to the Glen Rose Formation (limestone), and in some cases, to the Bexar Shale and the underlying Cow Creek Limestone. These formations form the middle Trinity aquifer. | |
| Downhole camera surveys have shown that the limestone formations are permeable and fractured in some areas of each surveyed borehole. Water therefore moves freely through the limestone and is most probably not stagnant as found in screened portions of wells completed in shallow, unconsolidated aquifers. because the fractured and vuggy limestone should allow fairly free groundwater flow, it has not been necessary to remove stagnant water from the open boreholes. | |
| Total well depths range from 252 to 451 feet bgl. Footage of total water columns has ranged from 168 to 407 feet in the deepest well (no. 10) and from 1.16 to 116 feet in the shallowest well (no. 4). |
Conventional purging would require removal of 3 well bore volumes from each well, resulting in removal of 1 gallon from the shallowest well during a drought, and up to 3,187 gallons from the deepest well during rainy seasons. Removal, characterization, and disposal of any waters containing PCE from open borehole wells that do not appear to contain any stagnant water would cause a significant and unnecessary increase in quarterly groundwater monitoring costs. Therefore, purging has not been a part of the CSSA quarterly monitoring program.
Sampling
Water samples were taken at CSSA wells every quarter. Sampling occurred through use of clean Teflon bailers or sampling via the well's existing pump as noted below.
| Wells 2, 3, 4, 16, D, MW1, and MW2 were sampled by Teflon bailer. These are the only wells to contain PCE concentrations since quarterly monitoring began. | |
| Wells 1, 9, 10, 11, G, H, and I are sampled via their existing water production pump. As of December 14, 1995, the well 11 pump was removed, and this well has been sampled by Teflon bailer since that time. |
All equipment was scrubbed and decontaminated with laboratory grade detergent (Alconox), isopropyl alcohol, distilled water, and drinking water. Disposable gloves and paper towels are bagged and discarded as standard waste.
3.9.5.2 Discrete Interval Sampling During Packer Tests
The sampling and analysis of groundwater from perched zones was conducted in wells 2, 3, 4, 16, and D. Perched water was sampled by sealing off the well with a packer below successive perched water layers as defined by borehole geophysical logging. Samples collected from subsequent layers contained water from the above layers, and the data were used to estimate possible paths and depths of contaminant migration.
An inflatable packer attached to the end of a drill rod string was lowered down the borehole via the drilling rig to the desired depth. When the packer had been positioned below the first identified perched water zone, it was inflated through an air compressor line attached to the packer. After the packer had been inflated, an electronic water-level indicator was used to measure depth to water and rate of water-level rise above the packer. Depth-to-water measurements of the water above the packer was taken periodically while water was accumulating to assess the flow rate of perched water into the well.
A sample of the accumulated water was collected using a decontaminated Teflon� bailer lowered down the borehole annulus using a nylon or polypropylene cord. For each sampled well, a new nylon or polypropylene cord was used. Sample containers were filled directly from the bailer. Sample containers for volatile organic analyses were filled such that no headspace or air bubbles remained in the bottle.
After the first perched water layer was sampled and the sampling equipment had been decontaminated, the packer assembly was lowered to below the next perched water layer. Water level measurements and groundwater samples were sampled in the same manner as in the first. Each subsequent layer was sampled in this manner until the water table was reached.
Solid wastes generated and characterized for disposal included contaminated oils, purge and decontamination waters from groundwater wells and liner materials from the oxidation pond. The solid wastes were containerized and characterized by results of analyses from either discreet interval sampling or composite sampling. This section provides a discussion on the containers requiring characterization by composite sampling of the containers.
Composite sampling was conducted on containers generated from drilling on pilot hole purging which could not be properly characterized from the discreet sampling conducted. For example, during the SVE installation drill cuttings were containerized and discreet interval samples collected and analyzed. However, the results of analysis did not provide sufficient data to fully characterize the material. Therefore, a composite sample was taken to provide characterization. An aliquot from each container, limited to a maximum of 10 containers, was composited into one sample to be analyzed for TCLP VOC and TCLP RCRA eight metals. The composite sample provided data sufficient data to characterize the IDW as either hazardous or nonhazardous waste. Section 3.11 provides a discussion on the management of IDW for the investigative efforts.
3.10 - Decontamination Procedures
3.10.1 Downhole Decontamination
All drilling, geophysical tools, and sampling equipment were decontaminated before use. The drilling rig, augers, core barrel, and all downhole sampling and logging equipment were decontaminated with a high-pressure steam cleaner. The decontaminated equipment was stored on plastic sheeting or on racks until use.
3.10.2 Decontamination of Sampling Equipment
All sampling equipment, including Teflon� bailers, split spoons, trowels, and chisels were decontaminated prior to use with an Alconox� soap scrub wash, potable water rinse, isopropyl alcohol rinse, and distilled water rinse. Spilt sections of PVC pipe used to hold the core samples were also decontaminated in this manner. Decontaminated equipment that was not used immediately was allowed to air dry and then wrapped with aluminum foil for storage or transport.
3.11 - Management of Investigation-Derived Waste
For this project, IDW consisted of:
| Drill cuttings from air/water rotary drilling and coring operations, | |
| Purge and development water, | |
| Water and detergents used to decontaminated field and sampling equipment, | |
| O-1 liner material, | |
| Personal protective equipment (PPE), and | |
| Miscellaneous trash associated with sampling activities (e.g., paper sacks, plastic, etc.). |
All drill cuttings removed from the boreholes during boring activities were placed into 55-gal containers temporarily located at CSSA. Drill cuttings were generated and from drilling activities associated with the installation of a SVE system within B-3 and the potential source investigation of well 16 contamination. Each container was properly labeled with the boring ID from which the waste was derived and the date generated. Drill cuttings were screened for VOCs using a photoionization detector (PID) and discreet samples taken at various intervals for characterization purposes. If results of analyses from drill cuttings did not provide sufficient data to characterize IDW, then an additional composite sample was taken, from the containers associated with an investigation, and analyzed for TCLP VOC and TCLP RCRA metals.
Purge and development waters, along with decontamination waters, used for decontamination of equipment (including steam cleaning equipment), were stored in a temporary modular tank of 55-gallon containers. The waters were sampled and analyzed for total VOCs, and total metals to characterize for disposal or discharged to the ground within the surrounding area.
Ancillary wastes such as plastic sheeting, PPE, and disposable sampling equipment were segregated from other IDW and disposed of as nonhazardous plant production waste. Liner materials associated with the O-1 liner investigation were placed into 55-gallon containers and characterized for waste disposal. A composite sample of the liner materials was collected and analyzed for TCLP VOCs and TCLP chromium.
Laboratory analysis of various media (soil gas, soil, rock, and water) was conducted during the project. Because many of the analytical reports have been presented in previous reports, and the project analytical reports currently fill a 3-foot-long box, the analytical reports are not included in this report. However, the reports are available on file with CSSA and Parsons ES.
Soil-gas samples were analyzed on a HNu model 321 GC equipped with an electron-capture detector (ECD) and a PID with a 10.2 eV light source. A Spectra-Physics model 4400 dual-channel integrator was used to plot chromatograms, to measure the size of the peaks, and to compute compound concentrations.
The chromatographic column used for analysis was a 12-foot long, 1/8-inch diameter stainless steel packed column containing 3 percent OV-101 Chromosorb W-HP packing material with a 100/120 mesh particle size. The OV-101 Chromosorb W-HP is the column packing material that performs the actual separation of compounds. This column was selected for use since it is able to separate the compounds targeted for analysis and allows for a relatively rapid analysis time.
3.12.1.2 Target Compounds and Calibration
Soil gas samples analyzed for select volatile organic compounds including PCE, TCE, and DCE because these compounds have been detected in well 16 and other CSSA wells. In the initial phase, soil gas samples were also analyzed for benzene, toluene, ethylbenzene, and total xylenes, together referred to as BTEX to test for fuel contamination. Carbon tetrachloride was added in the second phase for the area between B-3 and the oxidation pond because its presence in this area was indicated during the initial reconnaissance of the area.
In addition to the above compounds, the total volatile hydrocarbon concentration was reported. The TVH concentration is defined as the sum of all peak areas on the chromatogram through ortho-xylene minus any halocarbon peak areas, divided by the toluene response factor and the injection volume. Halocarbons are defined as chlorine, fluorine-, or bromine-substiutued hydrocarbons and include compounds like TCE and PCE.
Calibration standards used to calibrate the PID and the ECD were prepared in a two-step procedure. First, a primary standard was prepared by diluting certified pure chemicals into methanol. A quantity of each primary standard was then injected into a 40-mL vial to make the day-to-day working standards. Calibration compounds with certified purities typically greater than 99 percent were obtained from Chem Service, Inc. Individual samples of chemicals were lot numbered for quality control.
Calibration standards were run at the beginning of each day to determine the response factor and retention time for each of the target compounds. The standards were injected directly into the gas chromatograph in the same manner as the soil gas samples.
3.12.1.3 Target Compound Identification and Quantification
Soil-gas samples were analyzed after the GC had been calibrated and an ambient air sample had been analyzed. The quantity of a target compound in a soil-gas sample was determined by dividing the area of the peak registered on the chromatogram by the injection volume and the response factor. The compounds PCE and TCE were quantified and identified using the ECD, while the compounds cis-1,2-DCE and BTEX were quantified and identified using the PID. If the concentration of target compounds in a soil gas sample was much beyond the linear range of the detector, the injection volume was decreased or the sample diluted to bring the sample concentration within or near to the linear response range of the detector.
Identification of the target compounds in soil-gas samples was based on a single column analysis and a peak on the chromatogram being within 3 percent of the retention sample, the sample would need to be analyzed on a second column and the results compared to that of the first column. In the case of complex mixtures of hydrocarbons such as weathered fuel vapors, a second column confirmation is usually needed for positive identification of the compounds benzene, toluene, and ortho-xylene due to chromatograph interferences from the numerous other hydrocarbons often present in fuel vapor samples.
Whenever a compound is detectable on both detectors, the respone on the second detector can be used to confirm the presence of the compound in the sample. For example, the compounds PCE, TCE, and cis-1,2-DCE respond to both detectors and, therefore, would be confirmable. A negative response for these compounds on either detector when they would be above the detection limits of both detectors constitutes a nonconfirmed response. In the case of a nonconfirmed response, the compound would be reported as not detected.
Detection limits for compounds in soil gas samples can vary depending on the volume injected and chromatographic interferences from the adjacent peaks. Detection limits are defined as the minimum discernible peak divided by the maximum injection volume and the response factor for the particular compound of interest.
Under optimum operating conditions detection limits were on the order of 0.005 ug/L for halocarbons and 0.2 ug/L for hydrocarbons. Optimum conditions are defined as an injection volume of 1,000 microliters (uL), a smooth baseline, and no interferences from other peaks.
Typically, 500 uL of each soil gas sample was injected into the GC giving detection limits of approximately 0.01 to 0.02 ug/L for halocarbons such as PCE and TCE, and approximately 0.2 ug/L for aromatic hydrocarbons such as benzene. Approximate sample detection limits are listed in the results for each sampling location.
Soil samples collected for source characterization efforts were analyzed for total volatile organic compounds, semivolatile organic compounds, and metals (ES, 1993b). All analytical methods utilized for soil/rock sampling efforts were approved SW-846 methods.
Groundwater samples were analyzed for total volatile organic compounds, semivolatile organic compounds, and metals associated with materials suspected at potential source area SWMUs (ES, 1993b). All analytical methods utilized for groundwater sampling efforts were approved SW-846 methods.
Waste characterization samples collected were analyzed using TCLP (SW-846 method 1311) for VOCs (SW-846 method 8260A) and RCRA eight metals (SW-846 method 6010A and 7000 series). The RCRA eight metals included: arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver.
In addition to samples taken for chemical analysis, SWMU B-3 soil and rock core samples were taken for geotechnical characterization. One soil sample was collected from each boring at a significant lithologic change to better understand physical properties of the unsaturated zone. The samples were tested for soil moisture, bulk density (porosity), total organic carbon (TOC), pH, permeability, particle size distribution, total kjeldahl nitrogen, and phosphates.
3.14 - Quality Assurance/Quality Control
Quality assurance/quality control (QA/QC) was performed in accordance with requirements of the SAP (ES, 1993b). These procedures included sample handling (Section 3.9), laboratory analysis, and data validation (Section 3.15). The laboratory was contractually required to follow the SAP, particularly with regard to the QC replicates, surrogate spike analyses, matrix spike/matrix spike duplicates, and method blanks. Data validation was performed on the chemical analyses throughout the project.
3.14.2 Soil Gas Quality Control Procedures
3.14.2.1 Sampling Quality Control Procedures
Probes were decontaminated with an Alconox and tap water wash, a final rinse of water and air dried before use. Syringes used to analyze samples were decontaminated prior to each use by washing in Alconox and hot tap water, and then rinsing with methanol or alternatively baking them in the GC oven for a couple of hours. To verify the effectiveness of the decontamination procedure, syringe blank and system blank air samples were analyzed. To aid in the identification of contamination problems in the sampling system, syringes were numbered. The syringe blank samples were collected in the same room as the GC.
3.14.2.2 Analytical Quality Control Procedures
Precision is demonstrated by the internal consistency of the analytical system. Internal consistency is demonstrated in two ways: (1) the consistent response of the detectors over the course of the day coupled with the consistency of the daily calibration for various compounds, and (2) results for duplicate samples being with 25 percent of the average of the two analyses.
Approximately 12 percent of all analyses performed over the course of the soil-gas investigations were for QC documentation. This QC included analyzing both duplicate and blank samples. Duplicate samples consisted of approximately 10 percent of the QC, samples and were analyzed by repeating the analysis performed on a particular sample.
Determination of Linear Response Range
Three-point curves were constructed to demonstrate the linear response range of the PID and ECD. A measure of the linear response of the detector is the correlation coefficient. The closer the correlation coefficient is to 1, the more linear the detector response. Single point calibration checks performed on a daily basis during the course of soil-gas survey were within the linear response range of the detector as determined by the three-point calibration curve. If the concentrations of compounds in the sample were beyond the linear response range of the 3-point curve, the injection volume was decreased or the sample diluted to bring the sample within or near the linear response range of the detector.
Linear response range curves for PCE and TCE were determined using the ECD. The correlation coefficient was greater than 0.99 for both compounds. Linear response range curves for the compounds cis-1,2-DCE, benzene, toluene, meta- and ortho-xylenes, and ethylbenzene were determined using the PID. The correlation coefficient was greater than 0.99 for all the above compounds.
Daily Calibration Results
At the beginning of each day, standards containing the compounds targeted for analysis were run through the gas chromatograph to determine a response factor for each compound. The response factor is defined as the area units of the standard peak divided by the concentration in the standard. Continuing calibration checks were performed at the end of the day and were within QC criteria.
Results of Duplicate Sample Analyses
Duplicate sample results within 20 percent of the mean of the results from the two analyses were regarded as within the necessary precision. This criteria was met in all cases.
Air Sample and System Blank Analytical Results
After the instrument had been calibrated, an ambient air blank sample was analyzed. An ambient air sample was collected at the beginning of each day by filling a decontaminated syringe with air from the GC room. The sample was analyzed to aid in identifying any contaminants present in the analysis room and to check the effectiveness of the syringe decontamination procedure.
System blanks were also collected by taking a sample of ambient air through the sampling appartus. Usually, only trace amounts, 0.02 ug/L or less, or PCE were detected in the system blank. On 1 December 1996, TCE was detected at 0.55 ug/L in the system blank. Consequently, the trace amounts of TCE, less than 0.1 ug/L, detected at two locations were flagged as not detected.
Trace amounts up to 0.02 ug/L PCE were detected in air samples collected from the GC room. The occurrence of PCE in soil-gas samples at trace amounts, approximately 0.02 ug/L or less, may not be representative of site conditions.
Data validation to EPA level 3 was performed on all laboratory report packages. The data validator used the SAP to determine project requirements (ES, 1993b). Due to the length of the validation reports (a report plus all laboratory packages), and that the majority of the validation reports have been presented in quarterly groundwater monitoring reports and technical memoranda, these reports have not been included in this report. However, the reports are available on file with CSSA and Parsons ES. The only data flagged as unusable was reported in a technical memorandum of soil boring characterization (Parsons ES, 1995d). The boring locations whose samples were affected by VOCs not run within holding times were redrilled at the analytical firm's expense and the data reported in an addendum to the technical memorandum (Parsons ES,1995e).