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Final AOC-67 RCRA Facility Investigation Report
Section 3 - Conclusions and Recommendations
AOC-67 consists of a drainage ditch and an associated concrete pad. An AST, which reportedly stored floor cleaning rinsate and possibly contained PCE and TCE from Building 90, was located on the concrete pad until 1997, when it was removed. AOC-67 is located north of AOC-68, the wheelabrator site that contains visible metals debris on the soils, and adjacent to Building 90, where PCE and TCE plumes under and around Building 90 were identified during the soil gas survey performed in January 2001. The soil gas PCE plume extends to within 35 feet east of AOC-67. Finally, two composite samples were collected north of the concrete pad in January 2002. These samples were analyzed for metals.
In April 2001, three borings were advanced at AOC-67 to investigate the existence of potential COCs at this site. PCE and TCE were not detected in the seven soil and bedrock samples, including a field duplicate, collected from AOC-67. This suggests that the PCE and TCE source areas associated with AOC-65 are not affecting AOC-67 and are not related to the previous activities at AOC-67.
VOCs and SVOCs, including naphthalene, were reported, but at concentrations below sample SQLs. These compounds are not naturally occurring, and should require further action. The only VOC identified above RLs in the soil/rock samples was methylene chloride. Methylene chloride was only found in one sample (SB3, 30 to 30.5 feet bgs), and it may be related to laboratory contamination, instead of site activities, for several reasons. Methylene chloride is not a transformation product of any COC at the site and there are no known historical activities associated with methylene chloride use at AOC-67. Because the concentration reported in the sample is low (0.0093 mg/kg, SQL, 0.005 mg/kg), the appearance of methylene chloride is limited to one sample, and because methylene chloride is a contaminant commonly associated with laboratory contamination, the detectable levels of methylene chloride are considered a laboratory artifact. Thus, the single occurrence of methylene chloride is not associated with any release from the site and does not require further action.
Three out of four near surface soil samples associated with the soil borings exceeded metal background concentrations for the CSSA soils. Notably, lead and chromium background concentrations are exceeded in SB1, SB3, and SB3 FD1 samples from within the top 1 foot of soil, with reported lead and chromium concentrations of 252.35 and 50.0 mg/L for SB1, 342.56 and 47.0 mg/L for SB3, and 324.5 and 52.9 mg/kg for SB3 FD . These levels exceed the respective background concentrations of 84.5 and 40.2 mg/kg for lead and chromium. The metals concentrations detected in soil samples from SB3 also exceed background levels for zinc and copper. SB3 FD1 exceeds background for barium, with a reported barium concentration of 218.26 mg/kg, and for zinc, with a reported zinc concentration of 148.08 mg/kg. Background concentrations for the CSSA soils are 186 mg/kg for barium and 72.3 for zinc. The composite sample (AOC67-1) collected near AOC67-2 contained lead and chromium in excess of background. Together, these results suggest that metals contamination exists under the concrete pad as well as north and south of it.
The limestone samples collected from below the soils exceeded metals background concentrations slightly or, in most cases, not at all. The sample collected from SB3 (30 to 30.5 feet bgs) had a nickel concentration of 11.28 mg/kg, which exceeds background (6.8 mg/kg). The concentration of nickel (9.86 mg/kg) in the SB3 surface sample, which did not exceed its soil background concentration, is similar to the concentration at depth (11.28 mg/kg), suggesting that the levels detected are likely to be naturally occurring.
Because the reported soil exceedances are for metals in shallow soil, the probable source of AOC-67 contamination is likely from the AST at AOC-67. Further, it is argued that because the metal concentrations in the soils significantly exceed background and the bedrock metal concentrations do not, or only slightly, exceed background, further investigations or closure actions at AOC-67 should be confined to the top 3 feet of soil.
The lateral extent of near surface metals contamination should be established by collecting additional soil samples at AOC-67. Soil samples should be collected from the surface down to the top of bedrock, which is 0 to 3 feet bgs in the area of AOC-67. Once the extent of contamination is known, the impacted soil should be excavated from AOC-67.
3.2 - Evaluation of Data Quality Objectives Attainment
All data generated during the AOC-67 investigation were reviewed to confirm conformance with the AFCEE QAPP; the data verification report is included in Appendix D. All data are considered usable for site characterization purposes. Although several results are flagged with an �M,� these results are considered usable because the matrix interference is minimal and does not significantly affect the sample results. Review of MS and MDS samples associated with the data marked with �M� flags indicates no pattern that would result in masking of the true compound concentrations.
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-67, along with an evaluation of whether each DQO was attained, is provided in Appendix E. As described in Section 1, the main objectives of the AOC-67 investigation are to determine if the site meets TNRCC requirements for RRS1 closure and requirements of the 3008(h) Administrative Consent Order.
AOC-67 consists of a drainage ditch and a concrete pad that once supported an AST that stored floor cleaning rinsate from Building 90-1, where bluing operations were performed, and may have stored PCE and TCE. Further work is required before site closure under RRS1 can be recommended for AOC-67.
The lateral extent of metals contamination in the soils at AOC-67 must be clarified by soil sampling prior to implementing any removal action to define the actual extent of contamination at the site. Once the extent of contamination is determined, the soils can be removed from the site. Further, several compounds which could be associated with potential use of the AST for oils or ordnance maintenance, including toluene and naphthalene, were detected in the soils at AOC-67 in concentrations below the associated SQLs. Confirmation sampling should be performed after excavation to ensure that these compounds do not exceed SQLs. Closure under RRS1 may be reconsidered for AOC-67 following the remedial action.