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AOC-42 was discovered during a geophysical investigation conducted in May 1995 of the “open” areas within a 2,000-foot radius of Well CS-16-LGR. Both in-phase and quadrature-phase EM data from the 1995 geophysical survey identified four geophysical anomalies as areas possibly associated with past waste disposal activities. Each linear anomaly is approximately 200 feet in length. The open area surveyed as the “Salado Creek” area was divided into AOC-62, AOC-52, and AOC-42 based on the geophysical anomalies detected. AOC-42 was again surveyed with EM and GPR instruments in December 1999 to better define the southern extent of the AOC-42 anomalies. The new EM and GPR data confirmed the existence of buried waste and defined the general extent of waste on the northern, southern, and western sides.
The linear trend of the EM anomalies and the proximity of these AOCs leads to the conclusion that material disposed at AOC-42 may be related to disposed material at AOC-52, AOC-62, and AOC-58. Figure AOC42-13 shows the proximity of the AOCs and the linear trending EM anomalies. Mercury and copper were detected in surface soils above CSSA background levels at AOC-42, and buried waste was encountered in one of the borings. AOC-58 had a reported concentration of mercury in one surface sample that exceeded the background value. SWMU B-4, which is located northeast of AOC-42, had nine soil samples with concentrations of mercury exceeding the CSSA background levels.
In March 2001, nine surface soil samples were collected from AOC-42 in locations directly above the anomalies detected in the EM and GPR surveys. Each surface soil sample was analyzed for metals, VOCs, and explosives. Of the metals detected in surface soils, mercury was detected at concentrations exceeding the CSSA background concentrations in four of the nine samples and copper was detected in one sample above the background level. No explosives or VOCs were detected in surface soil samples collected at the site.
Eighteen subsurface soil samples were collected from nine borings performed at AOC-42 in March 2001 based on the results of the geophysical surveys. All samples were tested from VOCs, explosives, and metals. No VOCs or explosives were detected above the laboratory reporting limits. Although barium, nickel, cadmium, and zinc were detected at concentrations slightly exceeding CSSA background levels, all concentrations of barium and zinc, with one exception, are well below the Texas–specific median background concentrations and therefore are not considered a threat to human health or the environment. The nickel concentration reported only exceeds the Texas–specific median background concentration by 2.01 mg/kg.
3.2 - Evaluation of Data Quality Objectives Attainment
Overall DQOs for the investigations at CSSA are provided in Volume 1-1 behind the RFI Addendum tab (Section 11 of the Work Plan Overview). A detailed list of DQOs for AOC-42, along with an evaluation of whether each DQO has been attained, is provided in Appendix D. As described in Section 1, the main objectives of the AOC-42 investigation are to determine if the site meets TCEQ requirements for RRS1 closure and to meet requirements of the 3008(h) Administrative Consent Order.
All data generated during the AOC-42 investigation were reviewed to confirm conformance with the AFCEE QAPP. The data verification report is included in Appendix C. Some results are flagged with an “F.” These results are usable as the analyte was positively identified, but the associated concentration numerical value is below the RL. Some results are flagged with an “M,” indicating a matrix effect was present. These results are considered usable because the matrix interference is minimal and does not significantly affect the sample results. Some results are flagged with a “J.” These results are usable as the estimation of quantitation does not significantly affect the sample results. All data are considered usable for site characterization and closure purposes.
Buried waste was encountered during drilling operations; therefore, the site does not meet the TCEQ RRS1 requirement for removal of all waste. Furthermore, geophysical surveys indicate likely disposal trenches. Several of the 3008(h) RFI requirements, such as identification of waste characteristics and determination of vertical extent of contamination, have also not yet been met.
Further investigation and removal actions are recommended at AOC-42 to meet TCEQ RRS1 requirements based on the geophysical survey results and the analytical results.
Additional investigation at AOC-42 should include the excavation of areas where geophysical anomalies were detected, and the collection of confirmation soil samples below and adjacent to any waste material excavated. The volume of trenched material is estimated to be 6,000 cubic yards. This estimate is based on the area showing geophysical anomalies and a depth of 6 feet bgs.
Because of the mercury detected in surface soils near AOC-42, additional surface soil samples should be collected and tested for metals to determine the extent of the metals contamination. Based on the proximity of AOC-42 to AOC-58 and other sites that may require further investigation and possibly removal actions, additional investigation and characterization activities at AOC-42 could be performed under a removal action for other nearby sites.