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Text Box: The 600 area has been the subject of investigation for a number of years. Recent comments (September 8, 2009) by the USEPA on the 600 Area Data Report and Feasibility Study (February 2009 ) indicate that more work may be slated for the site..

The two general comments made by the USEPA sum up the problems identified with the subject report as follows:

‘1.  The FS Report presents the results of several historical investigations of the 600 Area. However, only limited information regarding the characterization of Area of Concern (AOC) 1, the Former Munitions Waste Pit, has been presented. As summarized in the Executive Summary and elsewhere in the FS Report, “The exact nature and volume of the size of the TCE source material is not known, but is presumed to be a thin layer of impacted soils that is buried under blasted rock debris from the Bldg. 660 site.  Further investigation of the potential source soils is not feasible due to the 10 to 30 feet thick overlying rock debris, and potentially buried MEC in the former munitions testing area” (Executive Summary, Page xiii, fifth paragraph). The uncertainty resulting from this lack of information is briefly discussed in the FS Report, though it does not appear that it has been adequately addressed.

To develop a “gross cleanup timeframe,” limited available information was incorporated into groundwater and leaching models. As described in Section 4, Nature and Extent of Contamination, the two primary contaminants of concern are trichloroethylene (TCE) and cyclonite (RDX). Based on available groundwater monitoring data, the TCE plume appears to be stable (Figure 4-3, AWDF and Monitoring Well TCE Time-Concentration Data), though recent data from one

Extensive EPA Commentary on 600 Area

in-plume well may suggest a recent potential reduction in the source area. Natural attenuation (NA) indicators suggest that NA has not currently nor is expected to significantly impact degradation of observed contamination. Modeling results indicate that approximately 60 years will be required for TCE source area soil concentrations to decline to levels below EPA groundwater criteria.

 

While it is understood that the debris recently placed in AOC 1 presents an impediment to further characterization, additional information appears necessary. Notwithstanding extensive groundwater investigations, it does not appear that the AOC 1 source area has been adequately characterized. In addition, the extent of contamination in the saturated zone has not been fully characterized. For example, Figure 4-4 shows the vertical extent of contamination has not been determined in the vicinity of monitoring wells 13MW-1 and 13MW-2. In addition, the “gross cleanup timeframe” of 60 years for attenuation of TCE in soil to concentrations below leaching criteria appears uncertain and unreasonable.  These factors in conjunction with the observed contaminant plume stability suggest that sufficient information may not be available to facilitate an adequate evaluation of potential remedial alternatives.

 

To facilitate a more comprehensive understanding of the AOC 1 source and enable a defensible evaluation of potential remedial alternatives, revise the FS Report to include an expanded discussion of

 

 

Image of Grass Field including Side View of Undergound Layers of Soil

the characterization of the source area and the uncertainty associated with remedial timeframe projections.  In addition, the FS Report should be revised to propose more routine and thorough evaluations of groundwater contaminant trends in the early years of implementation to ensure that potentially emerging trends in contaminant concentration reduction are confirmed. Further, contingent remedies should be further evaluated and discussed to aid in reducing the level of uncertainty.  Finally, to be consistent with EPA policy and guidance, and the National Contingency Plan (NCP), a source control remedy (e.g., soil removal) capable of reducing contaminant toxicity, mobility and volume in groundwater and surface water should be evaluated. The FS Report should also evaluate the need for protracted and uncertain monitoring timeframes and institutional controls/land use restrictions (ICs/LUCs).

 

2.  As discussed in the Executive Summary (fourth complete paragraph) and elsewhere in the FS Report, “a MNA cleanup timeframe cannot be calculated.”  Although the FS Report discusses a “gross cleanup timeframe” of 60 years for attenuation of TCE in soil to concentrations below leaching criteria, it is unclear when groundwater will ultimately reach the more stringent of the state or federal groundwater standards. Based on the lack of sufficient information for the calculation of a groundwater cleanup timeframe for the dissolved phase contamination in the saturated zone as well as the level of uncertainty associated with source characterization, it is not apparent that a long-term monitoring remedy is appropriate.  Revise the FS Report to address these concerns’.

 

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