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Final SWMU B-15/16 RCRA Facility Investigation Report
Section 2 - Field Investigation
2.1 - Field ActionsAs outlined in the Site-Specific Work Plan (Volume 1-2, SWMU B-15/16), the objectives of the site assessment were to conduct an additional EM geophysical survey to identify the northern/western extents of anomalies B and C. After the completion of the EM survey, a soil gas survey was conducted and nine soil borings were advanced to characterize the surface and subsurface soils. The soil boring locations were biased towards the results of the EM geophysical survey. Soil samples were submitted to an analytical laboratory and analyzed for VOCs, metals, and explosives. All field activities conducted were in accordance with the Field Sampling and Analysis Plan (Volume 1-4).
Additional EM and GPR geophysical data was collected at SWMU B-15/16 in August 1999 during the site investigation of AOC-47 and AOC-48. The investigation grid for AOC-48, which lies directly north of SWMU B-15/16, was extended an additional 100 feet south to define the extent of the anomalies that were previously identified at SWMU B-15/16. Prior to collecting EM data, a grid system was established at the site, which encompassed the areas of suspected ground disturbance. This grid consisted of staked locations separated by intervals of 50 feet (Figure B15/16-5).
EM data were collected at 2-foot intervals along transects that were separated by 10 feet using the established geophysical survey grid (50-foot intervals). EM measurements were taken using a Geonics EM-31 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, which is 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 that corresponds to approximately 2 mS/m. The in-phase component of the EM-31 is the response of the secondary to primary magnetic field measured in units of 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 0.5 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 10 feet.
The EM-31 survey was completed according to the procedures described in Volume 1-4, Sampling and Analysis Plan, Section 1.1.2. Prior to the survey, an area near the site that was determined to be free from disturbances and anomalies was selected and marked to perform background checks and calibration. The background checks were also performed after the survey. All calibration and before and after background readings were recorded in the field logbook.
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. Contour maps for both apparent conductivity and in-phase data were created for this site.
GPR is a surface geophysical technique that 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 heterogeneity’s 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.
GPR surveys were conducted with a GSSI SIR-2 instrument to verify the information obtained by the EM survey. Three GPR profiles were created in generally an east-west direction (Figure B15/16-5). A 300 megahertz (MHz) antenna with a range setting of 90 nanoseconds was used for all profiles. The individual GPR survey profiles were conducted over anomalies that were detected during the EM-31 survey. Additional surveys were also conducted at the site to provide background information. If no anomalies were identified during the EM-31 survey, the GPR was used to gather background information for the site.
GPR profiles were sequentially numbered as they were created throughout the day at multiple sites within CSSA. The GPR profile number is not related to the number of profiles created at the site. GPR profile for number 18 is included in this report as Figure B15/16-8.
Soil gas sampling at B-15/16 took place between August 20 and 22, 1996. A total of 54 samples were collected from 46 locations in the SWMU and in the areas directly north and south of the SWMU (Figure B15/16-4). Of these samples, 18 were collected from 16 locations inside the SWMU boundary. One location (A,2) had analytical samples representing two different depths and a duplicate sample was collected from location D,2. The samples were collected at grid intervals of 100 feet.
The samples were analyzed for benzene, toluene, ethylbenzene, total xylenes, cis-1,2-dichloroethene (cis-1,2-DCE), trichloroethene (TCE), 1,1,1-trichloroethane, and tetrachloroethene (PCE). None of the target analytes were detected in excess of the system control limits, which includes system blank and ambient air samples.
Surface soil samples were collected during the sampling of the soil borings. Samples collected between the ground surface and one foot below ground surface (bgs) are considered surface soil samples. Discussions related to results of the surface soil sampling program are presented in Section 2.1.4.
Nine soil borings were completed at SWMU B-15/16. The boring locations were based on the results of the EM geophysical survey (conducted in August 1999), which revealed three anomalies that suggested former waste management activities occurred at SWMU B-15/16. Soil borings B15/16-SB01 and SB02 are located immediately west of anomaly C which trends north-south (Figure B15/16-4). Soil boring B15/16-SB03 is located at the intersection of anomaly C and B (approximately 85 feet south of soil boring B15/16-SB02). Soil boring B15/16-SB04 is located along the western, downgradient edge of the SWMU boundary. Soil boring B15/16-SB05 is located approximately 45 feet south of boring B15/16-SB03, within anomaly B. Soil borings B15/16-SB06 and SB07 are situated on either side of the western portion of linear anomaly A, which trends northeast-southwest. Soil borings B15/16-SB08 and SB09 are situated on either side of a trench which is part of anomaly A.
The borings were drilled in areas that were considered most likely to characterize the nature of the geophysical anomalies present (Figure B15/16-4) and any potential contamination, without actually drilling into the anomalies. Samples collected above the bedrock were obtained using a decontaminated hollow-stem auger and split-spoon sampler. Rock samples were obtained by air core rotary drilling using a decontaminated core barrel. All decontamination, sample preparation, and sample handling followed those protocols established in the Field Sampling and Analysis Plan (Volume 1-5, Quality Assurance Project Plan, and RL83 Addendum). Environmental sampling also included the collection and submittal of quality assurance (QA) and quality control (QC) at those frequencies outlined in the AFCEE Quality Assurance Project Plan (QAPP) (Volume 1-4).
All samples were collected and submitted to APPL Laboratories in Fresno, California, between March 6 and 8, 2000. Samples were analyzed using U.S. Environmental Protection Agency (EPA) methods SW8260B (VOCs), SW6010B (barium, chromium, copper, nickel, and zinc), SW7060A (arsenic), SW7131A (cadmium), SW7421A (lead), and SW7471A (mercury). Samples were also submitted to DataChem Laboratories in Salt Lake City, Utah, to analyze samples for explosives (SW8330). Samples were obtained by those methods previously described. A total of 27 environmental samples, four field duplicates, one equipment blank, three trip blanks, two matrix spikes, and two spike duplicates were submitted for analyses.
In general, surface soils were hard, damp, plastic, silty clays with various amounts of clasts. At the time of sampling, no evidence of contamination was detected with the photoionization detector (PID). In addition, no visual or olfactory evidence of contamination was noted during sampling activities.
All soil boring locations were surveyed by Parsons using a Trimble Asset-grade global positioning system (GPS). All soil boring locations and analytical data have been incorporated into the CSSA geographic information system (GIS).
No groundwater samples were collected at SWMU B-15/16. Groundwater was not encountered in any of the borings.
The objective of the EM and GPR surveys at SWMU B-15/16 was to identify the northern and western extent of anomalies that were identified during the previous geophysical investigation in 1995. Figure B15/16-4 shows the three anomalies, which are labeled A, B, and C. The survey was conducted during the investigation of neighboring AOC-48.
Figure B15/16-6 and Figure B15/16-7 show the in-phase and quadrature phase data, respectively, collected at SWMU B-15/16 and AOC-48. The northern extent of anomaly C was found to converge with the approximate north-south trending linear anomaly in AOC-48. The survey indicated that the remaining northern portion of the site had no evidence of waste management activities.
The GPR surveys were conducted to verify the information obtained by the EM survey. Unlike the EM survey, the GPR revealed no evidence of subsurface anomalies. The GPR profile included in this report (Figure B15/16-8) represents the typical 300 MHz antenna survey profiles that were produced at SWMU B-15/16 and AOC-48. Resolution of the profiles was poor due to the homogeneous nature of the soil and underlying bedrock. The vertical scale on the profile, time (in nanoseconds), can be converted into feet using the following formula:
Range = Depth x Time (ns) x 1.5
Where Range = 90 ns for profiles with a 300 MHz antenna.
Depth = depth below ground surface in feet.
Time = 4.5 ns per foot. (The value given for dry limestone in the GSSI SIR-2 instruction manual).
According to this equation, the depth that represents 90 ns is 13.3 feet. The 2-way travel time is only an estimate and can vary somewhat from site to site and also within the profile itself.
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.
Eighteen soil gas samples were collected from 16 sample locations throughout SWMU B-15/16. Sample location A,2 had samples collected from two different intervals and location D,2 had a duplicate sample collected for quality assurance purposes. In addition to the soil gas samples that were analyzed, system blank samples and ambient air samples were analyzed as well. Soil gas sample results are provided in Table B15/16-1.
Thirty-six additional samples were collected from the areas directly north and south of SWMU B-15/16 as part of this investigation. Sample location maps and analytical results, can be found in the closure reports for AOC-47 and AOC-48 which are located south and north of SWMU B-15/16 respectively.
None of the samples collected at this site had detectable concentrations of the target analytes. This suggests that there is not a significant source of volatile organic compounds within the survey area of SWMU B-15/16.
Samples collected between the ground surface and one foot bgs are considered surface soil samples. A total of nine surface soil samples were collected as part of the soil boring effort conducted at SWMU B-15/16. However, to ease discussion, surface soil sample results are presented in Section 2.2.4.
Twenty-seven soil samples were collected from nine borings throughout SWMU B-15/16 and analyzed for VOCs, explosives, and metals. No explosives were detected above the laboratory reporting limits. Laboratory analytical results are presented in Appendix A, soil boring logs are included in Appendix B, and data verification reports are included in Appendix C. The results of the soil samples analysis are summarized on Table B15/16-2, and sample locations are shown on Figure B15/16-4.
RRS1 closure criteria for VOCs require a comparison between the RL of the sample and the reported value. Toluene was detected in ten samples; however, all concentrations were below the RL of 0.005 milligrams per kilogram (mg/kg). Only one VOC (methylene chloride) was detected at concentrations exceeding the laboratory RL of 0.005 mg/kg. Methylene chloride was detected in nine samples originating from six different borings. The reported levels ranged from a high of 0.0222 mg/kg in sample B15/16-SB03 (0.5-1.0), to a low of 0.0061 mg/kg in B15/16-SB01 (10.0-10.5).
Methylene chloride is a common lab contaminant throughout the environmental laboratory industry. It is highly unlikely that methylene chloride was detected as a consequence of the soil matrix since methylene chloride was the single VOC analyte detected. If methylene chloride contamination were actually present in the site subsurface soils, other VOC compounds would have also been detected at levels exceeding the laboratory RL. Therefore, the reported concentrations of methylene chloride are believed to be a result of the laboratory.
Although several metals exceeded background soil or rock levels, the exceedances were very slight. Barium was detected above the Glen Rose background level of 10 mg/kg at SB01 (10-10.5) with a concentration of 25.81 mg/kg, at SB02 (14.5-15) with 10.1 mg/kg, and at SB02 (19.5-20) with 11.25 mg/kg. The Texas-specific median background concentration (30 Texas Administrative Code [TAC] 351.51(m)) is 300 mg/kg.
Chromium exceeded the Glen Rose background concentration of 8.1 mg/kg in SB01 (10-10.5) which had a concentration of 8.8 mg/kg. Copper exceeded the CSSA soils background level of 23.2 mg/kg in SB01 (0.5-1) with a concentration of 28.02 mg/kg and in SB09 (0.5-1) with a concentration of 23.94 mg/kg.
Nickel was detected above the Glen Rose background concentration of 6.8 mg/kg at SB01 (10-10.5), where 11.93 mg/kg was detected. The Texas-specific median background concentration is 10 mg/kg.
Finally, zinc was detected above the Glen Rose background level of 11.3 mg/kg at SB01 (10-10.5) where 12.04 mg/kg was detected, and at SB04 (14.5-15) where 11.37 mg/kg zinc was detected. The Texas-specific median background concentration for zinc is 30 mg/kg.
No groundwater samples were analyzed at SWMU B-15/16.