[Home] [Master Table of Contents]

[AOC 48 Links]

AOC 48

TO0019 Work Plan Addendum

Site Description and History

AOC-48 is a 2.7-acre tract located near the southeastern corner of CSSA, in the southernmost portion of the Outer Cantonment Area. The general shape of the site is 500 ft long (north-south) by 250 wide (east-west). The site consists of three north-south trending soil mounds and a construction debris pile. Two large shallow depressions are present between the three soil mounds. The site was visually identified during the field efforts of the SWMU B-15/16 investigation. Salado Creek is located on the western side of AOC-48 (Figure AOC48-2).

Former activities at AOC-48 include the suspected use of the site as a four-wheel drive/all-terrain vehicle (ATV) training area and as a waste disposal area. Aerial photographs were reviewed in an effort to better determine when the site was used. In a 1957 aerial photograph, bare soil is visible in a rectangular shaped area in the center of the site. This area is the location of one of the depressions at the site. No activities are currently practiced at the site.

Summary of Previous Investigation Results

Soil Gas Survey

Soil gas sampling was conducted at neighboring SWMU B-15/16 during August 1996. A total of 54 samples were collected from 46 locations within SWMU B-15/16 and in areas directly north and south of the SWMU (Figure AOC48-2). Eight soil gas samples were collected within AOC-48 and fourteen samples were collected directly north and east of AOC-48.

Each sample was analyzed for BTEX compounds, 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).

Geophysical Survey

EM and GPR geophysical surveys were conducted at AOC-48 during July and 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 20-foot intervals, depending on the size of the area and the amount of obstructions, if any. Figure AOC48-2 illustrates the layout of the geophysical survey grid located at AOC-48 as well as the GPR survey profile locations.

The EM survey revealed evidence of a subsurface anomaly that is likely unrelated to past waste disposal activities. The data from the EM survey picked up the soil mounds and a linear anomaly located on the western side of the AOC. The linear anomaly is labeled A on Figure AOC48-2. 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.

A GPR survey was conducted to further investigate data obtained by the EM survey. The GPR survey also revealed evidence of a linear anomaly in the western portion of the site. The GPR profile included in this report (Figure AOC48-2) represents a typical 300 MHz antenna survey profile collected at AOC-48. Anomaly A is visible as a hyperbolic reflection located approximately at grid coordinate 0, 130. Resolution of GPR profiles collected at the site was poor, due to the homogeneous nature of the soil and underlying bedrock.

Surface Soil Samples

Three surface soil samples were collected at AOC-48 in June 2000. Sample locations were based on the geophysical survey results. 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 site. Samples were analyzed for VOCs, SVOCs, explosives, and metals (barium, chromium, copper, nickel, zinc, arsenic, cadmium, lead, and mercury).

No VOCs, SVOCs, or explosives were detected at concentrations exceeding RLs (RRS1) and none of the metals concentrations exceeded background (RRS1) for CSSA soils. Very low concentrations of methylene chloride and di-n-butylphthalate were detected in the duplicate of AOC48-SS03. These detections are common laboratory contaminants and are not considered to be associated with any possible waste management activities at AOC-48.

Closure Plan and Sampling Recommendations

Surface soils meet RRS1 criteria for VOCs, SVOCs, explosives, and metals. However, EM and GPR surveys indicate an anomaly, which could be caused by buried waste. Although the geophysical survey results show a pipeline-like signature, there are no records or surface evidence at the site to confirm the presence of a pipeline in this area. It is suspected that this anomaly is a water line formerly used to fill depressions for off-road vehicle training; however, there is no conclusive evidence that this is a pipeline. It is recommended that test pits be excavated at Anomaly A and the soil mounds to determine if buried waste is causing the geophysical anomalies.

If buried waste is found, it and any remaining waste residue or contaminated soils should be excavated, characterized, and properly disposed of. Following excavation, confirmation samples should be collected from the trench bottoms and sidewalls to verify that waste residue has been removed. If waste is encountered, samples should be analyzed for VOCs (SW-8260B), SVOCs (SW-8270C), explosives (SW-8330), arsenic (SW-7060A), cadmium (SW-7131), lead (SW-7421), mercury (SW-7471), and barium, chromium, copper, nickel, and zinc (SW-6010B). Samples will be collected at a frequency of one per 100 linear ft of sidewall, with a minimum of one sample per sidewall. If the bottom of the excavation is bedrock, bottom samples will be collected at a frequency of one per 20,000 ft2 of bottom surface area, with a minimum of two samples per excavation area. If the bottom of the excavation is soil, bottom samples will be collected at a frequency of one per 100 ft (100-ft grids). If waste is encountered, excavated material will require collection of waste characterization samples. Waste characterization samples will be analyzed for TPH and TCLP Texas Eleven metals.

Following excavation of waste and waste residue, any resulting trenches will be backfilled with clean fill and the site restored.

If no waste is found at Anomaly A or the soil mounds, it is recommended that closure in accordance with RRS1 be pursued after characterization, removal, and disposal of the construction debris pile has been completed. If water or other utility lines are identified during excavation, CSSA will locate the line with GPS and add it to their utility maps.

Summary Of Planned Work

  1. Excavate test pits at the anomaly to determine if there is any buried waste. Remove all waste for disposal, and if practical, separate metal debris for recycling. If no waste is encountered, backfill excavation.

  2. If waste is encountered, collect samples of the excavated material. The landfill requires one sample (consisting of up to five aliquots) per 1,000 CY of material. Dispose of any waste encountered and soils exceeding CSSA background levels at Covel Gardens Landfill under waste profile CG‑25591. This profile will be amended, if necessary, to include waste from AOC-48.

  3. If waste is encountered, collect samples of the excavation sidewalls and bottom. Samples will be analyzed for VOCs, SVOCs, explosives, arsenic, barium, cadmium, chromium, copper, lead, mercury, nickel, and zinc.

  4. Continue excavation until RRS1 soil criteria or competent bedrock, whichever comes first, is reached.

  5. Re-grade the site with clean fill and reseed.

  6. Validate data and prepare Closure Report for AOC-48.