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S E Mr.Binayak Acharya Project 22074-001.001 February 13, 1996 h Page 2 if : RESPONSE TO AGENCY COMMENTS I Response No. 1. EMCON believes that the carbon contentlkof i percent is conservative. ' It is at the lower end of the range cited by the PHS/EHD, and it appropriately represents typical carbon contents as referenced in Standard Guide for Risk-Based Corrective Action Applied at Petroleum Release Sites (ASTM E 1739-95). However, for the purposes of the residual fate and transport modeling, EMCON has considered a carbon content of 0.06 percent the most conservative value within the range cited by PHS/EHD. =s F Response No. 2. EMCON believes the soil data collected in November 1994 are representative of current soil conditions at the site. The field notes provided by PHS/EHD do not indicate that the soil represented by samples S1 and S2 was left in the excavation. The soil samples collected in November 1994 were collected immediately adjacent to or within the former excavation. The physical dimensions of the excavation and November 1994 sampling locations means that these recent samples were�collected no more than 5 to 8 feet from where the S 1 and S2 samples were collected. The levels of contamination detected in samples S 1 and S2 were not confirmed by the recent November 1994 samples. Therefore, is not likely that the compounds and concentrations detected in samples S 1 and S2 remain in the excavation. However, for the purposes of the residual fate and transport modeling, EMCON has considered the results from samples S 1 and S2. Because of the location of sample S3 (along the roadside), it is not considered representative of site conditions and was not considered in the fate and transport modeling. 4 REVISED CALCULATIONS EMCON revised the fate and transport calculations as presented in Revised Remedial Action Plan. The revisions included using a carbon content of 0.06 percent and the highest concentrations of any compounds detected in both the November 1994 samples and } samples S 1 and S2. The modeling approach and remaining model input parameters are identical to those described in Revised Remedial Action Plan. The attached Table 1 presents the soil concentrations and the revised calculated travel tunes for these compounds to reach groundwater. As shown in Table 1, the minimum time for any compound to reach groundwater at concentrations exceeding any drinking-water standard is almost 800 years. However, it should be noted that this is,the estimated time it would take before a drinking-water standard was exceeded in a drop of water immediately above the groundwater table. This estimate does not represent the time for groundwater at the site to exceed a drinking-water standard. �4 f I:1P321MAS 009 i 8.DQC-961jI c:3 f�