DEL PROCEDIMIENTO DE CONTRATACIÓN

Parameters That Affect the Fate of This Contaminant in the Vadose Zone

4.1.1 Contaminant Distribution in Soils at the Former Facility

In the Phase II vertical-profile soil sampling at SB23, SB24, and SB34 (Figure 2.2) at the former CCC/USDA facility, maximum contaminant concentrations in the vadose zone soils were 23 µg/kg at SB23 and 14 µg/kg at SB34. Carbon tetrachloride detected in all three borings at the top of the saturated zone was interpreted to reflect contamination of the groundwater at these greater depths. A maximum carbon tetrachloride concentration of 66 µg/kg was identified for a soil sample from the saturated zone at SB23.

To further assess the contaminant distribution in the vadose zone soils, vertical-profile soil sampling was performed in the Phase III targeted investigation at four additional locations (SB73–SB76) at the former facility (Figure 2.2). The results of VOCs analyses (Appendix B Table B.2) are summarized in Figure 4.1.

Carbon tetrachloride was not found in the soil samples from borings SB73 and SB74 (locations in Figure 2.2). In boring SB74, relatively low levels of chloroform (14 µg/kg and 7 µg/kg) were identified without associated carbon tetrachloride in soil from two depths (approximately 23 ft and 38 ft BGL, respectively) in the lower portion of the vadose zone. Boring SB74 was targeted to evaluate the former site of four grain bins along the southwestern margin of the former facility. Boring SB73 tested the soils at a location (SB11) where elevated levels of carbon tetrachloride were found in groundwater in Phase I. After monitoring well MW1 was installed at this location, elevated levels of carbon tetrachloride were found in groundwater (see Section 4.1.2).

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Carbon tetrachloride concentrations of approximately 9–57 µg/kg were identified in the lower portions of the vadose zone at both SB75 and SB76. Chloroform was also found at these locations, at concentrations of approximately 6–11 µg/kg. These borings lie along the approximate axis of two former rows of grain bins near the center of the former facility. The highest concentrations at each location, found at the bases of the sampled intervals (33–38 ft BGL), ranged from 47 µg/kg to 57 µg/kg. These concentrations also represent the highest contaminant levels detected in unsaturated soils at the former CCC/USDA facility.

The chloroform concentrations observed at SB74, SB75, and SB76 are low in absolute value but are relatively high in comparison to the associated carbon tetrachloride levels (Figure 4.1). Chloroform is present at roughly 20–80% of the carbon tetrachloride concentration (on a molar basis) at SB75 and SB76, and it occurs alone at SB74. Chloroform can be formed in the subsurface by the biological degradation (reductive dechlorination) of carbon tetrachloride under anaerobic conditions. The observed concentration relationships suggest that natural biodegradation has assisted and continues to assist, to a limited extent, in reducing carbon tetrachloride concentrations in the vadose zone soils beneath the former CCC/USDA facility.

4.1.2 Contaminant Distribution in Groundwater at the Former Facility

Groundwater samples were collected for VOCs analyses at the locations of borings SB73–SB76 as a direct indicator of potential continuing source effects associated with the contaminated soils at the former CCC/USDA facility. The results of these analyses (Appendix C, Table C.2) are illustrated in Figures 4.1 and 4.2.

Neither carbon tetrachloride nor chloroform was detected in the groundwater at SB76, and low levels of carbon tetrachloride (< 3 µg/L) were identified at SB74 and SB75. Carbon tetrachloride at 28 µg/L and chloroform at 2.8 µg/L were identified in the groundwater sample collected in November 2003 from well MW1 (screened at 41–51 ft BGL), installed at the location of soil boring SB73 during the Phase III targeted investigation. In a verification sample collected from well MW1 in June 2004, carbon tetrachloride was detected at 14 µg/L, and chloroform was detected at 1.8 µg/L. In comparison, higher carbon tetrachloride concentrations of 100 µg/L (at 48.5–52.5 ft BGL) and 727 µg/L (at 43.5–47.5 ft BGL) were identified in groundwater samples collected with the ECPT at this location (originally SB11) during the Phase I investigation (Argonne 2001).

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4.1.3 Human Health Risks Associated with Exposure to Soils at the Former Facility

Results from sampling and analysis of vadose zone soils were compared against the Tier 2 risk-based standards for carbon tetrachloride and chloroform presented in Risk Based Standards for Kansas (RSK Manual; KDHE 2003). In deriving these standards based on risk to human health, the KDHE evaluated exposure pathways including incidental ingestion of soil, inhalation of airborne particulates, inhalation of chemical volatilizing from soil, and direct dermal contact. Risk-based concentrations are provided for two land use settings: residential and nonresidential. The risk-based standards for carbon tetrachloride in the Tier 2 Risk-Based Summary Table (Appendix A in RSK Manual; KDHE 2003) are 2,500 µg/kg for a residential setting and 7,000 µg/kg for a nonresidential setting. The corresponding risk-based standards for chloroform are 3,900 µg/kg and 6,000 µg/kg.

As discussed in Section 4.1.3 of the Everest Phase II report (Argonne 2003), a comparison of the results from the analysis of near-surface soils demonstrated that no unacceptable health risks are associated with potential human exposure to the near-surface soils at the former Everest CCC/USDA facility. Similarly, comparison of the analytical results for subsurface soil samples indicates that no unacceptable health risks are associated with potential human exposure to the subsurface soils at the former Everest CCC/USDA facility.

Potential health risks associated with the soil-to-groundwater contamination pathway are also addressed in the RSK Manual (KDHE 2003). For this pathway the risk-based standards for carbon tetrachloride and chloroform are 200 µg/kg and 1,200 µg/kg, respectively. Comparison with the maximum values found for carbon tetrachloride (57 µg/kg) and chloroform (11 µg/kg) indicates that the contaminant levels detected in soils at the former CCC/USDA facility are well below these target action levels and that these soils do not pose an unacceptable risk to groundwater.

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4.2 Confirm the Interpreted Patterns of Groundwater Flow and the Potential