[Home] [Master Table of Contents]
[Table of Contents] [Next Section]
AOC 43 Final RCRA Facility Investigation/Closure Report
Section 2 - Field Investigation
Past waste management activities at AOC-43 may have included munitions disposal. Three-inch fragments of munitions were located at the surface on AOC-43. Between November 1999 and March 2001, investigation activities at AOC-43 included a geophysical survey and collection of surface soil samples (Figure AOC43-4). All samples were collected in accordance with the Field Sampling and Analysis Plan (Volume 1-4, Field Sampling Plan, Sections 1-3).
A summary of the analytical results of surface soil sampling performed is included in Appendix A. All sampling points were surveyed by Parsons ES using a Trimble Asset-grade GPS tool. All sample and corresponding analytical data are being incorporated into the CSSA GIS database.
The field investigation for AOC-43 began with a geophysical survey conducted in December 1999. The site was measured and divided into 50-foot intervals that were marked with 2-foot wooden stakes forming a surface grid. The grid dimensions were 190 feet at the widest in the east-west direction and 200 feet in the north-south direction (Figure AOC43-5 and Figure AOC43-6.)
The EM-31 survey was completed according to the procedures described in Volume 1-4, Sampling and Analysis Plan, Section 1.1.2. Additional information regarding EMI can be found in Volume 5, Groundwater Investigation and Associated Source Characterization, Section 3.1. Prior to the survey, a site near the AOC 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 data before and after background readings were recorded in the field logbook. The data were recorded in a DL720 Polycorder and downloaded into a computer. The data were then processed and evaluated using DAT31 software and later contoured using SURFER software.
A ground penetrating radar (GPR) survey was conducted at the site using a Geophysical Survey Systems, Inc. (GSSI) SIR-2 GPR instrument equipped with a 300 megahertz (MHz) antenna. The survey was conducted in accordance with procedures outlined in Volume 1-4, Sampling and Analysis Plan, DO5068 Addendum, Section 2.4. 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. A total of six survey profiles were created during the field effort. Three profiles were generated in the east-west direction and three profiles were created in the north-south direction (Figure AOC43-5).
In accordance with the approved work plan, a soil gas survey was not performed in association with the current investigation conducted for AOC-43.
Three surface soil samples were collected at AOC-43 in March 2001. The samples were collected at pre-determined locations that were based on the results of the geophysical surveys. The samples were collected at a depth of 6-inches below the surface and were analyzed for explosives and metals. Sample AOC43-SS01 was collected in the northwest area, AOC43-SS02 was collected in the central area, and AOC43-SS03 was collected in the southeastern area (Figure AOC43-4). At the time of sampling, no visible evidence of contamination was noted.
All samples were collected using a decontaminated stainless steel hand trowel. All decontamination, sample preparation and handling followed those protocols established in the Field Sampling and Analysis Plan (Volume 1-4, Field Sampling Plan, and DO5068 Addendum). Environmental sampling also included the collection and submittal of quality assurance (QA) and quality control (QC) at those frequencies outlined in the AFCEE QAPP (Volume 1-4, Quality Assurance Project Plan).
In accordance with the approved work plan, subsurface soil samples were not collected in association with the current investigation conducted for AOC-43.
In accordance with the approved work plan, groundwater samples were not collected in association with the current investigation conducted at AOC-43. Since subsurface borings were not performed, groundwater was not encountered during the site investigation.
The geophysical surveys revealed no evidence of subsurface anomalies related to past waste disposal activities. Very little variations were recorded during the EM survey, which can be interpreted as homogenous and consistent soil and bedrock profiles throughout the AOC (Figure AOC43-5 and Figure AOC43-6). In-phase readings during the EM survey ranged from a minimum of –1.26 parts per thousand (ppt), to a maximum of 1.163 ppt. Quadrature-phase readings ranged from a low of 18.676 milliSiemens per meter (mS/m), to a high of 29.052 mS/m.
The GPR surveys were conducted to verify 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 AOC43-7) represents the typical 300 MHz antenna survey profiles that were produced at AOC-43. Resolution of the profiles was poor due to the homogeneous nature of the soil and underlying bedrock however, the shallow eastward dip of the bedrock can be seen on the western portion of the 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 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 profile itself.
In accordance with the approved work plan, a soil gas survey was not performed in association with the current investigation conducted for AOC-43.
As described in Section 2.1.3, three surface samples collected from AOC-43 were analyzed for explosives and metals. No explosives were detected at concentrations exceeding RLs, and no metals were detected at concentrations above the background levels for Trinity and Frio soils. Results are summarized in Table AOC43-1, and a complete list of results is provided in Appendix A. None of the analyte concentrations exceeded RLs or background levels; therefore, all samples meet RRS1 criteria.
All chemistry data obtained during the course of this RFI were evaluated for usability by those processes discussed in Section 4.1 of Volume 1-1, Risk Assessment Technical Approach Document. All data is evaluated against a stringent set of requirements as put forth by the AFCEE QAPP version 3 (Volume 1-5, Quality Assurance Project Plan). The data validation report for AOC-43 is included in Appendix B.
In accordance with the approved work plan, subsurface soil samples were not collected in association with the current investigation conducted for AOC-43.
In accordance with the approved work plan, groundwater samples were not collected in association with the current investigation conducted for AOC-43.