SITE INFORMATION AND CORRESPONDENCE_2005-2014

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Review of the Response to Comments on the Draft OU2 Human Health Risk Assessment DDJC Sharpe Site Lathrop, California August 2006 RESPONSE TO GENERAL COMMENTS Response to General Comment 1: The response to this comment appears to be adequate. The response indicates that the requested information, including figures and analytical results associated with previous characterization and/or confirmation studies have been provided in the revised Figures 3 through 5. Integration of such historical data is consonant with U.S. EPA's RAGs Part A in addition to U.S. EPA's Remedial Investigation and Feasibility Study (RI/FS) Guidance(EPA, 1988). However, the revised figures were not provided with the responses, thus, an assessment of areas where remediation and sampling associated with previous characterization and/or confirmation studies could not be conducted. The figures and revised text referred to in the response to the comment will need to be reviewed in the next version of the sampling and analysis plan(SAP) to confirm the adequacy of the response. Response to General Comment 2: The response to this comment appears to be adequate,but the supporting figure and text referenced in the response were not provided for review, and thus a complete evaluation of the response will need to be conducted by reviewing the revised SAP. The response also indicates that the volatile organic compound (VOC) groundwater plume and its associated risks are being addressed within the scope of the Operable Unit 1 (OUl)remedial action and that inclusion of this medium in the OU2 investigation may generate redundancy. It appears that separation of soil and groundwater exposure media was agreed upon by all parties and thus, the exclusion of this pathway from the OU2 risk assessment is acceptable. However, as part of the final risk assessment report,please address the uncertainties associated with the OU2 risk assessment and specifically address the additional risks that may result from indirect exposure to groundwater until remediation under the OU1 plan has been completed. Response to General Comment 3: The response to this comment is partially adequate. The response indicates that soil gas values will be used to convert to bulk soil concentrations through the employment of a volume averaging approach to account for potential exposures via incidental ingestion of VOCs in shallow and subsurface soil. However, the methodology that will be used to convert soil gas data to bulk soil concentrations was not provided. There are several concerns with the use of soil gas data in this manner. As noted in Minnich,.et.al, 1997, there are numerous difficulties in measuring soil VOCs. "These difficulties are exacerbated by the VOCs' multiphasic nature and their affinity for the vapor phase. The multiphase problem has led to debates over the use of soil vapor measurements vs. soil extraction techniques for the quantification of VOCs. Soil gas measurements offer an economical and rapid screening method at some sites, but soil gas concentrations cannot be used to predict the total concentration of soil VOCs (vapor, sorbed, dissolved, and nonaqueous-phase liquids). In contrast, soil samples,if properly sealed and preserved, can be used to measure VOCs in all phases. However, 1