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TO0019 Work Plan Addendum

Area of Concern 38

Site Description and History

AOC-38 consists of a 35-acre open pasture area south of North Outer Drive and immediately southwest of Well CS-16, near the approximate center of CSSA (Figure AOC38-2). The general shape of the site is 1,400 ft long (east-west) by 1,100 ft wide (north-south). The site is bounded by a fence line and paved road to the north and east, trees to the west, and Salado Creek to the south. The area that AOC-38 lies in is generally referred to as the “South Pasture.”

One livestock well (Well CS-4) is located in the southwest portion of AOC-38. The livestock well is inactive, but it is included in the quarterly groundwater monitoring program at CSSA. A livestock corral exists in the northeast corner of AOC-38, and the entire site is used occasionally for cattle grazing. Some agricultural equipment, including metal cones, is stored on the site, just to the west of the corral. A six-inch underground water supply line runs through the northwestern portion of the site. This water line has apparently been capped.

During the installation of a fiber optic communications line in July 2000, a trench containing expended munitions was encountered in the northwest corner of the site. This area, approximately one acre, of AOC-38 was separated from the previously defined area of AOC-38 and designated SWMU B-71. The area of SWMU B-71 is now excluded from investigation for AOC-38. A previous report (Technical Memorandum on Surface Geophysical Surveys) indicated that a bermed burn area containing metal wire, wood, rock, and ashes was situated in the north-central portion of AOC-38. However, this bermed area could not be located during field investigations in 2000. It is possible that this area was part of what is now SWMU B-71. No formal waste management activities were known to occur at AOC-38, with the exception of the area now designated SWMU B-71.

In 1995, due to the proximity of the northern boundary of AOC-38 to Well 16, investigations were performed in an effort to identify potential source areas for contamination at Well CS-16. Based on the site’s vicinity to Well CS-16, chemicals of potential concern are VOCs. The survey in spring 1995 included soil gas sampling and an EM geophysical survey.

Summary of Previous Investigation Results

Geophysical Survey

A geophysical survey was conducted at the site in March 1995 using an EM-31 geophysical instrument. Grid points were staked at 100-foot intervals with data transects conducted at 50-foot intervals in both north-south and east-west directions. Both apparent conductivity and in-phase EM readings were collected at 2-foot intervals along each transect.

Five anomalies were detected during the survey; however, all but one were attributed to underground/overhead utilities or surface metal debris. The one unexplained anomaly is shown as “A” on Figure AOC38-2. A 1.5-inch diameter metal pipe was located on the southwestern end of Anomaly A, which may have affected the EM readings in this area.

In an effort to further identify the source of this anomaly, field personnel walked the area in August 2001. At the north end of the anomalous area, a portion of a barbed wire fence, presumably washed down during a precipitation event, was found. If this fence part was on the ground during the 1995 geophysical survey, it may have been the cause of Anomaly A along with the 1.5-inch diameter pipe which remains at the site today.

Soil Gas Survey

During July 1995, soil gas samples were collected at 26 points within AOC-38. Sampling locations are shown in Figure AOC38-2. Each sample was analyzed for BTEX, TPH, DCE, TCE, and PCE. PCE was detected in every sample, at concentrations ranging from 0.02-3.1 μg/L. The maximum concentration was detected at the location identified as “South Pasture 110,40,” which was situated in the northeast corner of AOC‑38. No other analytes were detected.

During November 1995, 95 additional soil gas samples were collected in the South Pasture area and analyzed for DCE, TCE, PCE, and carbon tetrachloride. These additional samples were collected to provide additional information within the area surveyed in June 1995, as well as to extend the survey area to the south and west. During the second survey, TCE was detected at 10 locations at concentrations ranging from 0.02-0.21 μg/L. PCE was detected at 81 locations at concentrations ranging from 0.01-2.6 μg/L. The maximum PCE and TCE concentrations were detected at sample location “South Pasture 80,-40,” in the southeast corner of the site. A PCE soil gas plume map is provided as Figure AOC38-2.

Surface Soil Sampling

Sixty surface soil samples (AOC38-SS01 through AOC38-SS60) were collected at AOC-38 during January 2000. Samples were analyzed for the following selected VOCs: 1,1‑DCE, DCE, trans-1,2-DCE, PCE, and TCE. Samples were collected within the top two ft of soil, adjacent to soil gas survey grid points that had detected concentrations of applicable COCs (Figure AOC38-2). None of the analytes were detected above their respective RLs; therefore, the site meets RRS1 criteria for VOCs.

Closure Plan and Sampling Recommendations

There are no records indicating that the AOC-38 area was ever used for waste disposal, nor are there any surface indications of waste or trenches. Soil gas and surface soil samples were collected at AOC-38 to whether or not chlorinated hydrocarbons were present at the site, due to the presence of a soil gas plume near Well 16. Soil gas and surface soil samples show minor concentrations of VOCs, all of which are below RRS1 criteria.

The geophysical anomaly identified in the northeast corner of the site will be investigated further to confirm that there is no buried waste at the site. Although surface soil samples collected in the vicinity of this anomaly were not contaminated, there is a possibility that buried waste could exist in this area. The anomaly will be excavated and any waste that is found will be disposed of properly.

If waste is present at the anomaly location, it is estimated that approximately 125 CY of material will be excavated. If waste is present, confirmation sampling will be conducted along the sidewalls and base of the excavation. Samples will be collected every 100 ft along the excavation sidewalls and at the base of the excavation. If waste is recovered, samples will be analyzed for VOCs (SW-8260B). Excavated material should be characterized by TPH and TCLP metals analyses.

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

Summary Of Planned Work

  1. Re-locate anomaly location using GIS data and GPS.

  2. Excavate test pit at the anomaly location to determine what is causing the anomaly. If waste is present, it is estimated that up to 125 CY of material will be removed.

  3. If waste is encountered, collect samples of excavated soils for waste characterization purposes. One composite sample consisting of five aliquots will be collected per 1,000 CY of soil. Analyze samples for TPH and TCLP antimony, arsenic, barium, beryllium, cadmium, chromium, lead, mercury, nickel, selenium, and silver.

  4. Dispose of all waste and soils containing metals concentrations above CSSA background levels at Covel Gardens Landfill under waste profile CG-25591. This profile will be amended to include waste from AOC-38, if necessary.

  5. If waste is found, collect samples of the excavation sidewalls and bottom. Samples will be analyzed for VOCs, based on the results of samples collected at the site in 2000.

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

  7. Re-grade the site with clean fill.

  8. Validate data and prepare Closure Report for AOC-38.