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AOC 48 RCRA Facility Investigation Report
Section 2 - Field Investigation
As outlined in the Environmental Encyclopedia site-specific work plan (Volume 1-3, AOC 48), the objectives of the RFI were to conduct a geophysical survey and to collect surface soil samples. After identifying any geophysical anomalies, three grab surface soil samples were collected at a depth of six inches below surface grade based upon the locations of the geophysical anomalies. Each sample was analyzed for VOCs, SVOCs, explosives, and metals. All field activities were conducted in accordance with the Field Sampling and Analysis Plan (Volume 1-5, Field Sampling Plan, Quality Assurance Project Plan, and RL83 Addendum).
Electromagnetic and ground penetrating radar geophysical surveys were conducted at AOC 48 between July 26 and August 10, 1999. Prior to collecting EM or GPR 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 20-foot intervals, depending on the size of the area and the amount of obstructions, if any. Figure AOC48-4 illustrates the layout of the geophysical survey grid located at AOC 48 as well as the GPR survey profile locations.
EM data were collected at 2-foot intervals along transects that were separated by 10 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 two 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 0.5 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 variation in transect footage was related to the size of the site and the number of obstructions.
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, a site near AOC 48 that was determined to be free of 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 quadrature phase (apparent conductivity) and in-phase data were created for each 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 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.
GPR surveys were conducted with a GSSI SIR-2 instrument to verify and further substantiate the results of the EM survey. Fifteen profiles were created with a 300 MHz antenna, thirteen in the east-west direction, and two in the north-south direction (Figure AOC48-4 ). The individual GPR survey profiles were conducted over anomalies that were detected during the EM31 survey. Additional surveys were also conducted at the site to provide background information. If no anomalies were identified during the EM31 survey, the GPR was used to gather background information for the site.
GPR profiles are sequentially numbered as they are created throughout the day. Multiple sites are generally surveyed in any one-day and test profiles are created for each site during the investigation. The GPR profile number is not related to the number of profiles created at the site.
Soil gas sampling at neighboring SWMU 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 AOC48-4 ). Most of these soil gas samples were collected within the AOC 48 boundary and its surrounding areas. As shown in Figure AOC48-4 , eight soil gas samples were collected within the boundary, and fourteen samples were collected north and east of the site.
Soil gas probes were driven to the bedrock-soil interface or until refusal. Samples were collected by manually driving a decontaminated ¾-inch stainless steel hollow sampling rod to the selected depth with a pneumatic hammer. The sampling rod was then backed a few inches out of the ground allowing the detachable point to drop off the sampling probe and exposing a void space of the formation. Soil vapors were then pulled from the soil through the probe into a Tedlar bag using a portable vacuum pump. The soil formation around the sample rod was purged for at least three probe volumes prior to sample collection. The samples were then transported to the field GC temporarily located at CSSA for analysis. Samples were analyzed within four hours of collection. After sampling, probes were decontaminated for use at another location. Decontamination procedures consist of washing off the probes with Alconox and water, rinsing and allowing the probes to air dry.
An initial screening of the soil gas samples was performed in the field by scanning the exhaust from the vacuum pump with an HMX-271 explosimeter for oxygen content. Each sample was analyzed for benzene, toluene, ethylbenzene, total xylenes, cis-1,2-DCE, TCE, and PCE using an HNu model 321 GC equipped with an electron-captor detector and a photoionization detector with a 10.2 eV light source. A Spectra-Physics model 4400 dual-channel integrator was used to plot the chromatograms, to measure the size of the peaks, and to compute compound concentrations. The chromatographic column used for analysis is 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.
Three surface soil samples were collected at AOC 48 in June 2000. The samples were collected at locations (Figure AOC48-4) that were based on the results of the geophysical surveys. Sample AOC48-SS01 was collected at the location of a strong geophysical anomaly, AOC48-SS02 was collected on top of the central north-south trending soil mound, and AOC48-SS03 was collected in the large shallow depression located between the linear soil mounds on the western side of the AOC.
All samples were collected on June 14, 2000 and submitted to APPL Laboratories in Fresno, California and DataChem Laboratory in Salt Lake City, Utah. Samples were obtained by using a decontaminated hand trowel to obtain soil from the first 0.5 feet of the soil column. A total of three environmental samples, one equipment blank, and one trip blank were submitted for analyses. Environmental sampling also included the collection and submittal of QA/QC samples at those frequencies outlined in the AFCEE QAPP (Volume 1-5, Quality Assurance Project Plan). All samples (including QA/QC) were collected in a collaborative effort from AOCs 47 and 48. A field duplicate, matrix spike, and matrix spike duplicate were collected from AOC 48. Sample chain-of-custody documentation is provided in Appendix B.
Samples were analyzed using EPA methods SW-8260B (VOCs), SW-8270C (SVOCs), SW-8330 (explosives), SW-6010B (barium, chromium, copper, nickel, and zinc) SW-7060A (arsenic), SW-7131A (cadmium), SW-7421A (lead), and SW-7471A (mercury). APPL conducted all analyses, except explosives analyses, which were conducted by DataChem.
The AOC 48 soil samples originated from the Trinity and Frio soil type. In general, the soils were stiff, moist, calcareous clays with a measurable gravelly component (less than 25 percent). At the time of sampling, no discernable evidence of contamination was noted.
All decontamination, sample preparation and handling followed those protocols established in the Field Sampling and Analysis Plan (Volume 1-4, Field Sampling Plan, and RL83 Addendum). All sampling points were surveyed by Parsons ES using a Trimble Asset-grade GPS. Surveying methodology is described in the Amendment to the Field Sampling Plan (Parsons ES, 2001b). All sample locations and analytical data will be incorporated into the CSSA GIS, after it has been approved by AFCEE and CSSA.
In accordance with the approved work plan, subsurface soil samples were not collected in association with the current AOC 47 investigation.
In accordance with the approved work plan, groundwater samples were not collected in association with the current AOC 48 investigation.
The geophysical surveys revealed evidence of a subsurface anomaly that is probably not related to past waste disposal activities. When contoured, the variation in the data that were recorded during the EM survey show the linear trending dirt mounds and the linear anomaly located on the western side of the AOC. The anomaly is labeled A on Figure AOC48-5 and Figure AOC48-6. Anomaly A originates in SWMU B-15/16 and has a pipeline type signature. In-phase readings during the EM survey ranged from a minimum of –2.53 ppt, to a maximum of 3.81 ppt. Quadrature-phase readings ranged from a low of –9.46 mS/m, to a high of 81.33 mS/m.
The GPR surveys were conducted to further investigate the data obtained by the EM survey. Like the EM survey, the GPR also revealed evidence of a subsurface disturbance or anomaly. The GPR profile included in this report (Figure AOC48-7) represents the typical 300 MHz antenna survey profiles that were produced at AOC 48. Anomaly A is visible in GPR profile 30 as a hyperbolic reflection located approximately at grid coordinate 0, 130. 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 (ns), 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, this is 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 two-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.
As described in Section 2.1.2, several soil gas samples collected as part of the SWMU B-15/16 investigation were located near or within AOC 48. Each sample was analyzed for BTEX compounds, cis-1,2-DCE, TCE, and PCE. None of these analytes were detected in any of the soil gas samples collected at SWMU B-15/16 or AOC 48. Results are included in Table B-15/16-1 in the SWMU B-15/16 RCRA Facility Investigation Report (Parsons ES, 2001).
As described in Section 2.1.3, three surface samples collected from AOC 48 were analyzed for VOCs, SVOCs, explosives, and metals. None of the VOCs, SVOCs, or explosives were detected at concentrations exceeding RLs, and none of the metals levels exceeded background levels for CSSA soils. Results are summarized in Table AOC48-1, and a complete list of results is provided in Appendix A.
Very low concentrations of common laboratory contaminants methylene chloride and di-n-butylphthalate were detected in the duplicate of AOC48-SS03. These detections are not considered to be associated with any possible waste management activities at AOC 48.
In accordance with the approved work plan, subsurface soil samples were not collected in association with the current investigation conducted for AOC 48.
In accordance with the approved work plan, groundwater samples were not collected in association with the current investigation conducted for AOC 48.