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Final AOC 61-RCRA Facility Investigation/Closure Report
Section 2 - Field Investigation
2.1 - Field Actions
As outlined in the Environmental Encyclopedia site-specific work-plan (Volume 1-3, AOC 61), the objective of the site assessment was to complete activities required to permit closure of the site. Data needs at AOC 61 included; a map of the site area based on visual reconnaissance, a geophysical survey to determine locations of anomalies and subsurface disturbances, if any, and analytical data for surface soils collected from the site. Contaminants of potential concern (COPCs) are 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). Sample locations are indicated on Figure AOC61-6.
Electromagnetic and ground penetrating radar geophysical surveys were conducted at AOC 61 in August 1999. 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 10 to 50 feet, depending on the size of the area and the amount of obstructions, if any. The grid system and spacing used are shown on Figure AOC61-7.
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 EM31 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 this site and are discussed in Section 2.2.1 of this report.
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 at 25 foot intervals in the north-south direction (Figure AOC61-7). 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 number 33 is included in this report and discussed in Section 2.2.1.
In accordance with the approved work plan, soil gas samples were not collected at AOC 61 as part of the current investigation.
A total of three environmental samples, one matrix spike, one spike duplicate, one equipment blank, and one trip blank were collected on January 14, 2000. All samples were submitted to APPL Laboratories in Fresno, California for analysis. Since the geophysical surveys did not detect any subsurface anomalies, the soil samples were collected near the proposed locations with one exception. Sample AOC61-SS02, which was originally proposed to be collected from the center of the site, was collected closer to the eastern edge because the soil profile was too thin near the center (Figure AOC61-6). The samples were analyzed using EPA methods SW-8260B (VOCs), SW-8270C (SVOCs), SW-6010B (barium, chromium, copper, nickel, zinc), SW-7060A (arsenic), SW-7131A (cadmium), SW-7421A (lead), and SW-7471A (mercury). A collaborative field effort was initiated to conduct sampling events for AOCs 56, 58, and 61. APPL Laboratories did not analyze the VOC samples in accordance with the five-gram dry weight limit required by AFCEE guidelines. A resampling event was completed on February 11, 2000, to replace VOC results that were deemed unusable. A total of three environmental samples, one field duplicate, one matrix spike, one spike duplicate, one trip blank, and one equipment blank were submitted for VOC analysis.
The AOC 61 soil samples originated from the Brackett soil type. At the time of sampling, no discernable evidence of contamination was noted. All samples were obtained using a decontaminated hand trowel. All decontamination, sample preparation and handling followed those protocols established in the Field Sampling and Analysis Plan (Volume 1-5, Field Sampling Plan, Quality Assurance Project Plan). 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, 2001b).
In accordance with the approved work plan, subsurface soil samples were not collected at AOC 61 as part of the current investigation.
In accordance with the approved work plan, groundwater samples were not collected at AOC 61 as part of the current investigation.
The geophysical surveys revealed no evidence of subsurface anomalies related to past waste disposal activities. There was little variation in the data that were recorded during the EM survey, which can be interpreted as homogenous and consistent soil and bedrock profiles throughout the AOC (Figure AOC61-8 and Figure AOC61-9). In-phase readings during the EM survey ranged from a minimum of –0.512 parts per thousand (ppt), to a maximum of 0.042 ppt. Quadrature-phase readings ranged from a low of 16.48 milliSiemens per meter (mS/m), to a high of 21.85 mS/m.
The GPR surveys were conducted to further investigate the information obtained by the EM survey. Like the EM survey, the GPR also revealed no evidence of subsurface anomalies. The GPR profile included in this report (Figure AOC61-10) represents the typical 300 MHz antenna survey profiles that were produced at AOC 61. Resolution of the profiles was poor due to the homogeneous nature of the soil and underlying bedrock however; the shallow bedrock can be seen throughout the entire GPR profile. 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 2-way travel time is only an estimate and can vary somewhat from site to site and also within the profile itself.
In accordance with the approved work plan, soil gas samples were not collected at AOC 61 as part of the current investigation.2.2.3 Surface Soil Samples VOC and SVOC concentrations were reported below RLs for samples collected from the three sampling locations; therefore, RRS1 criteria for VOCs and SVOCs have been met (Table AOC61-1). Chloroform and bis(2-ethylhexyl)phthalate were both reported to have concentrations below the RLs in all three of the samples submitted for analysis. RRS1 criteria include soils background concentrations for metals. Since none of the surface soil sample metals results exceeded soils background levels and since no VOCs or SVOCs were detected above RLs, the site is considered to meet RRS1 criteria.
In accordance with the approved work plan, subsurface soil samples were not collected at AOC 61 as part of the current investigation.
In accordance with the approved work plan, groundwater samples were not collected at AOC 61 as part of the current investigation.