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AOC-66 is located in the southern portion of the North Pasture, just north of Well 16 (Figure AOC66-2). The site is approximately 15 acres in size. Soil gas surveys were conducted in surrounding areas in 1995 to determine the extent of the soil gas plume around Well 16, but no data was collected within AOC-66.
Soil Gas Survey
The soil gas survey at AOC-66 was performed in conjunction with adjacent soil gas surveys at AOC-37 and AOC-41. The entire survey area encompassed approximately 25 acres. Samples were collected between during December 1999 and analyzed on-site at a field laboratory. A total of 100 samples were collected within AOC-66 and analyzed for benzene, toluene, ethylbenzene, and xylenes (BTEX), PCE, TCE, and DCE. Sampling depths varied from 1.5 ft to 6 ft and were usually determined by probe refusal. The average soil thickness at the site was 4.1 ft.
PCE and TCE were the only target compounds detected in soil gas samples from AOC-66 (Figure AOC66-2). TCE was detected in one sample, AOC66-81, at a concentration of 0.04 mg/L. The TCE detection came from the same sampling location as the highest PCE detection. PCE was detected at 19 locations and detections ranged in concentration from 0.01 to 0.16 mg/L. The highest concentration occurred in the duplicate sample analysis from AOC66-81. PCE was detected at or above 0.10 mg/L at three locations.
The concentrations of PCE detected in the soil gas were generally spotty, with more than a third of the reported detections occurring along the boundary with AOC-35. At the outer cantonment fenceline, the PCE concentrations in soil gas appear to be consistent with previous results that indicated low concentrations of PCE extending from AOC-35 into the North Pasture. Overall, the concentration and frequency of PCE detections in the North Pasture were not as pervasive as in the Inner Cantonment Area, and suggests that the suspected groundwater contamination source does not extend into the North Pasture. The isolated occurrence of PCE at locations AOC66-61 and AOC66-81 could be related to gas migration focused along fractures or faults to these locations.
There are no records indicating that the AOC-66 area was ever used for waste disposal, nor are there any surface indications of waste or trenches. The site was identified as an area of concern in 1995 due only to its proximity to known contamination at well CS-MW16, not due to any records of waste management in the area, indications of ground disturbance in historic aerial photographs, nor visible evidence of waste management at the site. The recent and previous data collected at this site suggest that the site does not contain a source of VOC contamination. The results of this soil gas survey combined with soil gas survey results in areas adjacent to AOC-66 suggest that the PCE plume terminates as it approaches the southern boundary of the north pasture.
Since the soil gas survey indicated that contamination is not present in the subsurface at AOC-66, a technical/closure report will be prepared in accordance with TCEQ RRS1 closure requirements.