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Final AOC 56 RCRA Facility Investigation Report
Section 2 - Field Investigation
As outlined in the Environmental Encyclopedia site-specific work plan (Volume 1-3, AOC-56), 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 anomalies. Each sample was analyzed for VOCs, SVOCs, 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-56 in August 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 intervals ranging from 20 to 50 feet, depending on the size of the area and the amount of obstructions, if any. Figure AOC56-4 illustrates the layout of the geophysical survey grid located at AOC-56 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 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 0.5 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 the AOC 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. Figure AOC56-5 and Figure AOC56-6 show the results of the EM survey which are discussed in Section 2.2.1.
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 further investigate the information obtained by the EM survey. Four GPR profiles were created in the northeast-southwest direction and eight were created in the northwest-southeast direction (Figure AOC56-4). A 300 mega-hertz (MHz) antenna with a range setting of 90 ns was used for all profiles. 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. GPR profile numbers 7, 8, 9, and 12 are included in this report.
In accordance with the approved work plan, soil gas samples were not collected in association with the current AOC-56 site investigation.
Three surface soil samples were collected as part of the AOC-56 site investigation. Sample locations were biased towards any anomalies discovered in the geophysical surveys. AOC56-SS01 is located in the northwest portion of the AOC (Figure AOC56-4). AOC56-SS02 is located approximately 140 feet southeast of AOC56-SS01. AOC56-SS03 is located just west of East Outer Road in the southeastern portion of the site.
All samples were collected and submitted to APPL Laboratories in Fresno, California on January 14, 2000. 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 3 environmental samples, one equipment blank, and one trip blank were submitted for analyses. Environmental sampling also included the collection and submittal quality assurance/quality control (QA/QC) samples at those frequencies outlined in the AFCEE QAPP (Volume 1-4, Quality Assurance Project Plan). All samples (including QA/QC) were collected in a collaborative effort from AOC-56, AOC-58, and AOC-61. Therefore, matrix spikes and matrix spike duplicates were collected from one of the other sites for the analytical batch of AOC-56. Sample chain-of-custody documentation is provided in Appendix B.
Samples were analyzed using EPA methods SW-8260B (VOCs), SW-8270 (SVOCs), SW-6010B (barium, chromium, copper, nickel, and zinc) SW-7060A (arsenic), SW-7131A (cadmium), SW-7421A (lead), and SW-7471A (mercury). APPL Laboratories did not analyze the five-gram dry weight limit as required by the AFCEE QAPP for VOCs (EPA method SW-8260B). Therefore, a resampling event was completed on February 11, 2000 to replace VOC results that were deemed unusable.
The AOC-56 soil samples originated from the Tarrant complex 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 sampling points were surveyed by Parsons using a Trimble Asset-grade geographic positioning system (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 geographic information system (GIS).
In accordance with the approved work plan, subsurface soil samples were not collected in association with the current investigation conducted for AOC 56.
In accordance with the approved work plan, groundwater samples were not collected in association with the current investigation conducted for AOC 56.
Both the in-phase component and the quadrature phase component of the EM survey revealed anomalies in the northwestern and central portions of the AOC. Two anomalies were also noted near the southeastern portion of the site. The in-phase portion revealed several anomalous areas which may be related to waste management activities. Figure AOC56-5 shows these anomalies which are located in areas mentioned above and labeled A through D. Anomalies A and B are thought to be related to waste management activities because of the magnitude and aerial extent of the anomalies. In-phase readings ranged from 0 to 18 ppt and quadrature phase readings ranged from –80 to 20 mS/m in these areas. Many smaller, less conspicuous anomalies exist between anomalies A and B on the in-phase contour map (Figure AOC56-5). These anomalies are also thought to be related to waste management activities because they coincide with areas of disturbed subsurface as revealed in the quadrature phase portion of the survey (Figure AOC56-6).
Anomaly C which is located on both Figure AOC56-5 and Figure AOC56-6 is possibly related to an old rail line that may have crossed the site at this location and not to waste management activities. Evidence of the rail line is the linear mound that trends in a nearly north-south direction across the southeastern portion of the site, an abundance of non-indigenous river gravel that may have been used as ballast, and several rail spikes found during the initial visual reconnaissance survey. All rail spikes observed while preparing the geophysical survey grid were removed before conducting the actual survey. In-phase readings near anomaly C ranged from 0 to -10 ppt while quadrature phase readings ranged from 10 to 70 mS/m.
Anomaly D, which
is located in the northern portion of the AOC and visible in both the in-phase
and the quadrature phase portions of the survey, could be related to road
construction activities and not waste management activities.
The location of the anomaly places it nearly on the road and away from the
excavated interior portion of the AOC where the other geophysical anomalies
were detected. In-phase readings
near anomaly D ranged from 0 to -16 ppt while quadrature phase readings ranged
from –70 to 30 mS/m.
The GPR surveys were conducted to further investigate the information obtained by the EM survey. The four profiles included in this report (Figure AOC56-7, Figure AOC56-8, Figure AOC56-9, and Figure AOC56-10) represent the typical 300 MHz antenna survey profiles that were produced at AOC 56. The vertical scale on the profiles, Time (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, 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 and the depth representing 45 ns is 6.6 feet. The two-way travel time is only an estimate and can vary somewhat from site to site and also within the scan itself.
GPR profile 7, which trends southeast-northwest, indicated multiple reflectors (approximately 1-3 feet bgs) in the western portion of the profile (Figure AOC56-7). These multiple reflectors coincide with the location of EM anomaly A and indicate that past waste management activities (possible buried trash) may have been carried out at AOC 56. GPR profile 8, which trends northwest-southeast, shows where the bedrock has been excavated in the western portion of the profile (Figure AOC56-8). Profile 8 also shows disturbed subsurface conditions in the vicinity of EM anomaly A and clearly shows a subsurface reflector 1 to 3 feet in depth in the vicinity of EM anomaly C. GPR profile 9, which trends from the southeast to the northwest, shows the contrast between the relatively undisturbed subsurface conditions in the southeastern portion of the site and the disturbed area in the northwestern portion (Figure AOC56-9). GPR profile 12, which trends northeast-southwest, indicates multiple subsurface reflectors at an approximate depth of 1 to 3 feet bgs. The disturbed subsurface area corresponds with the area around anomaly A (Figure AOC56-10).
In accordance with the approved work plan, soil gas samples were not collected in association with the current investigation conducted for AOC-56.
All samples reported VOC and SVOC concentrations below RLs, therefore RRS1 criteria for VOCs and SVOCs have been met (Table AOC56-1). The following compounds were reported in the soil samples at concentrations below the RLs: chloroform, toluene, benzo(a)anthracene, benzo(a)pyrene, benzo(b)fluoranthene, benzo(g,h,i) perylene, bis(2-ethylhexyl)phthalate, chrysene, fluoranthene, indeno(1,2,3-cd)pyrene, phenanthrene, and pyrene. A complete list of results is provided in Appendix A.
RRS1 closure criteria for metals require the reported values for metals be compared to the soil background values. Zinc and lead concentrations were detected at concentrations slightly above the background levels in samples AOC56-SS01 (0-0.5 ft). Lead was detected at a concentration of 88.32 milligrams pre kilogram (mg/kg) and zinc at 97.75 mg/kg (Table AOC56-1). Background levels for lead and zinc are 84.5 mg/kg and 73.2 mg/kg, respectively.
In accordance with the approved work plan, subsurface soil samples were not collected in association with the current investigation conducted for AOC-56.
2.2.5 Groundwater SamplesIn accordance with the approved work plan, groundwater samples were not collected in association with the current investigation conducted for AOC-56.