SITE INFORMATION AND CORRESPONDENCE

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Mr.M.Scott Mansholt—CEMC Soil Sampling Work Plan August 1,2006 Surland Homes Property Page 3 of 8 monitoring for the two on-site groundwater monitoring wells. On May 5,2003 the RWQCB issued a letter approving the work,and a report detailing the sampling results was submitted in October 2003. INITIAL RESPONSE TO COMMENTS The DTSC letter dated December 19,2003 contained comments and questions regarding the HRSE. The comments were divided into two categories: site characterization and HRSE comments. Preliminary responses have been developed below for certain of the comments. Presented below is a discussion regarding the site conceptual model(SCM),which as noted in the DTSC letter,has not previously been developed for the site. Following the SCM discussion are responses to specific items in the DTSC letter. For reference,the DTSC letter is included as Attachment A. SCM A SCM evaluates sources,transport mechanisms,exposure media,and exposure pathways. Potential source media include surface and subsurface soil,groundwater and surface water. Transport mechanisms include dust emissions,volatilization,and leaching to groundwater. Potential exposure media include soil, outdoor and indoor air,groundwater, and surface water. Potential exposure pathways include ingestion of and/or dermal contact with, soil,groundwater, or surface water,inhalation of volatiles,and consumption of aquatic species from surface water. The HRSE performed by Geomatrix included inhalation of volatile organic chemicals from soil and groundwater, inhalation of airborne dust from soil, ingestion of soil and groundwater, and dermal contact with soil and groundwater. Figure 3 presents a SCM for the site taking into account current site conditions. Ingestion, inhalation of volatiles from, and dermal contact with groundwater have been eliminated as exposure pathways as the residential development is supplied with water from the City of Tracy (the City). The City has enacted an ordinance prohibiting installation of private water supply wells;therefore, exposure to groundwater is not a complete exposure pathway. Based on the absence of surface water in the vicinity of the site,no complete exposure pathways exist for this exposure medium. As noted in the DTSC letter,site data do not suggest the need to quantitatively evaluate risk from indoor air exposure, and thus inhalation of outdoor air has also been eliminated. Therefore,ingestion and dermal contact with soil remain as the exposure pathways,and will be evaluated. As seen on Figure 3,the status of the ecological receptors has not been established. As requested in the DTSC letter,an ecological risk evaluation will be performed. The following section contains responses to the DTSC comments(the comments are paraphrased below in italics and are followed by SAIC's response). RESPONSE TO SITE CHARACTERIZATION COMMENTS 1. Pipelines. Locations and depths of pipelines, if pipeline removed, and source of backjill need to be indicated. To the extent the information can be collected, this information will be provided in the revised health risk assessment. 2. West Side Irrigation Ditch Feature. A description of this feature, uses, and potential contamination is requested, as well as when and how potential contamination in this area will be addressed.