SITE INFORMATION AND CORRESPONDENCE

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r � � page 2, 1360 Escalon Ave. SJC/EHD commented that the results of a RBCA model in the context of a site conceptual model (SCM)are one factor utilized while evaluating a site for closure. The SCM should discuss the contaminant source(s), masses released(if possible), potential contaminant migration routes, and residual contaminant mass left in place. SJC/EHD had a geological/hydrological/contaminant fate and transport SCM in mind, not the conceptual model utilized for the RBCA model. Potential migration pathways as described in an SCM refer to the subsurface lithological characteristics and the preferential pathways for contaminant transport or migration through the subsurface that may exist. They are not the same as potential contaminant exposure pathways or receptors as described in a RBCA model and as referred to in the MJK letter response.The intent is to help demonstrate the presence or absence of a contaminant mass under the building that would source the vapors of concern in the RBCA model. This is just basic geological, hydrological and contaminant plume modeling. The risk assessment report did include cross sectional diagrams of the site, but a relationship between the known location of the contamination in the subsurface near the west dispenser island area and the prediction of the RBCA model that there may be a threat to the indoor air quality of a building located approximately 60 feet away from that location was not made. This is an important factor relating the SCM to the RBCA model used by MJK in the report. Illustrated examples provided by ASTM in their RBCA guidance document show contaminants directly underlying the buildings on the sites; there has been no demonstration that contaminants directly underlie the buildings at the 1360 Escalon Ave. site. SJC/EHD also requested presentation of the contaminant mass calculation. In their response correspondence, MJK did not do this. Data used for the calculation, plume segmentation and averaging techniques should be presented in the same document to be checked for reasonableness of methods and assumptions. MJK gave a narrative description of how they estimated the listed impacted area, but still failed to provide the calculations and diagrams used to determine this. MJK calculated the contaminant mass utilizing a method unfamiliar to SJC/EHD that involved specific gravities and specific gravity rations. Please submit to SJC/EHD the method in the form of a mathematical expression with some sample calculations. In their conclusion MJK states that additional corrective action should not be undertaken and requests that a"No Further Action"Letter be issued to the site. This is in contradiction to their conclusions in the previous report, based on the RBCA model,which were to conduct an indoor air survey of the building and a soil vapor survey. With a RBCA model showing a risk and hazard to workers in the building, MJK's general statement"Based on current usage of the facility the potential for indoor vapor exposures in excess of existing conditions is unlikely. Based on the above discussion, additional corrective action should not be undertaken and a 'No Further Action Letter' issued for the site using the risk assessment report and this discussion in place of a Tri- Regional Guidelines,Appendix B review"cannot be accepted. Please prepare a hydrogeological/contaminant distribution and transportation SCM, and a revised RBCA model, if appropriate, and submit it to SJC/EHD by November 22,2002. If you have any questions please call Lori Duncan at(209)468-0337. Donna Heran, REHS, Director Environmental Health Department Lori Duncan, Senior REHS Nuel C. Hii Jr., R.G. LOP/Site Mitigation Unit IV LOP/SitegUnit IV cc: Marty Hartzell, CVRWQCB Mike Kloberdanz, MJK