TO0019 Work Plan Addendum

Site Description and History

AOC-37 is located in the extreme southern portion of the North Pasture, northwest of Well 16 and north of Well D (Figure AOC37-2). The site consists of a fenced livestock area, approximately 0.4 acres in size. It was designated an AOC due to its proximity to high PCE concentrations in groundwater at well CS-MW16.

Summary of Previous Investigation Results

Geophysical Survey

A geophysical survey was conducted in May 1995 at all “open” areas within a 2,000-foot radius of Well 16. This radius includes the entire extent of AOC-37. No anomalies were identified within AOC-37.

Soil Gas Survey

A soil gas survey was performed at AOC-37 during December 1999. This work was performed in conjunction with adjacent soil gas surveys at AOC-41 and AOC-66. The entire survey area encompassed approximately 25 acres situated north of Well 16. Sampling locations at AOC-37 are shown on (Figure AOC37-2). Six samples were collected within AOC-37. The sampling depths varied from 2 ft to 6 ft, and were usually determined by the depth to probe refusal. The average soil thickness at the site was 4.2 ft. Samples were analyzed for BTEX compounds, PCE, TCE, and DCE at an on-site field laboratory.

PCE was the only target compound detected in soil gas samples from AOC-37. PCE was detected at location AOC37-6 at a concentration of 0.01 mg/L, a value that is very close to the detection limit. Although there was a single occurrence of PCE at AOC-37, the detection appears to be related to PCE detections reported at AOC-66 or in the well CS-MW16 area. Overall, the data suggests that a source of VOCs is not present at AOC-37. The results of this soil gas survey combined with results from soil gas surveys performed near AOC-37 suggest that the PCE plume present at Well 16 terminates as it approaches the southern boundary of the North Pasture.

Closure Plan

There are no records indicating that the AOC-37 area was ever used for waste disposal, nor are there any surface indications of waste or trenches. Soil gas samples were collected at AOC-37 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 samples show minor concentrations of VOCs, all of which are below RRS1 criteria. Since the geophysical survey did not reveal any subsurface anomalies and a soil gas survey indicated that contamination is not present in the subsurface at AOC-37, a technical/closure report will be prepared in accordance with Texas Commission on Environmental Quality (TCEQ) RRS1 closure requirements.