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Area of Concern 41

TO0019 Work Plan Addendum

Site Description and History

AOC-41 is located in the southern portion of the North Pasture, near the gate area (north and west of Gate 6) and east of Well 16. The site is approximately 5.9 acres in size (Figure AOC41-2). AOC-41 originated under work completed for Order 67, which required a geophysical survey to be completed in all “open” areas within a 2,000-foot radius of Well 16. The geophysical survey was conducted in May 1995 and an area encompassing three discovered anomalies was designated AOC-41.

Based on the site’s vicinity to Well CS-16, chemicals of potential concern include VOCs and metals (arsenic, barium, cadmium, chromium, copper, lead, mercury, nickel, and zinc). Metals are included due to the possibility of burned metal in Anomaly B. VOCs analysis is included in the analytical program based on soil gas survey results. SVOCs and explosives are not included at this based on the soil gas and geophysical survey results.

Summary of Previous Investigation Results

Geophysical Survey

A geophysical survey was conducted at the site in May 1995, during which three geophysical anomalies were identified. The three anomalies were designated A, B, and C. Anomaly A was subtle and was not related to known surface/subsurface features. Anomaly B was the largest and thought to be related to additional underground metal piping or fragments. Anomaly C was associated with a burn pit at the site (Figure AOC41-2). In addition, an abandoned underground 4-inch diameter metal pipe is present in the western portion of the site. The pipe trends northeast-southwest across the site and travels toward Well 16. Exposed portions of the pipe were observed during the field investigation.

Soil Gas Survey

In June and July of 1995, a soil gas survey was conducted in the vicinity of Well 16. Twelve soil gas survey samples were collected and analyzed for chlorinated and aromatic volatile organics. PCE was detected at AOC-41. The highest detected concentration was 0.07 ug/L.

In December 1999, another soil gas survey was conducted that focused specifically on AOC‑41. Forty samples were collected. The sampling depths varied from 1.5 ft to 6 ft, and were usually determined by the refusal of the probe advancement. The average soil thickness was 4.5 ft. PCE was the only target compound detected.

PCE was detected at the southwestern corner of the site at locations AOC41-1 and AOC41-11 (Figure AOC41-2). Both samples were reported with a concentration of 0.02 mg/L. In the prior soil gas survey (June-July 1995), PCE had been detected at concentrations of 0.01 - 0.07 mg/L in the vicinity of locations AOC41-20, AOC41-25, AOC41-28, and AOC41-33. However, these results were flagged based on the detection of PCE in blanks.

Investigations at Neighboring SWMU B-8

In the spring of 2003, additional work was done at neighboring SWMU B-8 to define the horizontal extent of contamination at the site. Additional surface soil samples showed that surface soil is contaminated beyond the original SWMU B-8 boundary, and this contamination reaches into the current AOC-41 boundary.

In May 2003, a geophysical survey was conducted in the area with a Geonics EM‑61 Mark II time domain metal detector, after three ordnance items were identified on the ground surface in neighboring SWMU B-8. This survey was conducted to determine if buried UXO or other waste may be present in the area surrounding SWMU B-8, including the AOC-41 area.

A number of small anomalies were identified, and excavation of selected anomalies indicate that they were caused by wire, nails, bolts, metal scrap, and spent small arms ammunition. Two pieces of ordnance scrap were also identified. The results of this investigation are described in the SWMU B-8 RFI Addendum Report.

Planned Work

The recent SWMU B-8 geophysical survey and intrusive UXO results indicate that AOC-41 may also have the potential for containing UXO items, mostly in the northern half of the site, where past waste management activities are known to have occurred. Although some of the anomalies have already been investigated, and no waste trenches were encountered, further investigation of the cause of these anomalies by UXO specialists will be conducted. Results of the additional excavations will be amended to the RFI Report Addendum for SWMU B-8.

Further, because metals concentrations in excess of RRS1 closure standards at neighboring SWMU B-8 extend into the AOC-41 boundary, the boundaries of these two sites will be revised. SWMU B-8 will become a larger site, and AOC-41 a smaller site.

Summary Of Planned Work

  1. Identify coordinates of remaining anomalies using the CSSA GIS.

  2. Reacquire locations of anomalies in the field using GPS and hand-held Schonstedt magnetometer.

  3. Excavate anomalies to determine if any UXO or additional ordnance-related items may be present at the site.

  4. Backfill excavated soil and re-grade.

  5. Prepare AOC-41 Closure Report.