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2.1 Site Conceptual Exposure Model A Site Conceptual Exposure Model depicting the source of contamination and methods of transport is depicted on Figure 9. The only two potentially complete pathways to receptors are impacted groundwater to nearby water wells and inhalation of vapors to onsite and future onsite workers from the impacted subsurface soil and dissolved groundwater plume. Based on previous investigations for r adjacent facilities, two municipal water wells are located within the site vicinity. One well is located approximately 1,100 feet southwest (crossgradient) of the Site, and one well is located approximately r 575 feet northwest (upgradient) of the Site. Based on the distance of these wells and their locations relative to the Site, this pathway is considered generally incomplete. The inhalation pathway is further considered generally incomplete as the concrete slab floor of the gas house (the building located nearest to the groundwater plume) would minimize the vertical migration of vapors into the building, the building is occupied only over brief intervals, and any vapors migrating outside of the building would be expected to dissipate prior to reaching the workers' breathing zone. 3.0 ADDITIONAL INVESTIGATION r Based on the results of the additional site investigation and Third and Fourth Quarter—2008 groundwater monitoring results it appears that monitoring data for the deep-screened wells at the Site may not be r representative of current site conditions. We therefore recommend destruction of six of the nine deep- screened wells at the Site and installation of three downgradient and one upgradient shallow-screened i" wells. We propose to keep deep-screened monitoring wells MW-2, MW-10 and MW-11 in the event that groundwater levels drop below the screened intervals in the new wells; however, wells MW-2, MW-10 and MW-11 will not be part of the quarterly groundwater monitoring program. The proposed monitoring well locations are depicted on Figure 10. The following are general procedures to complete our recommend scope of services. r 3.1 Pre-field Activities • Utilize the existing project-specific health and safety plan (HSP) to provide guidelines on the use of personal protective equipment and the health and safety procedures to be implemented during the proposed field activities. y • Mark the proposed soil boring locations with white paint as required by law and provide a minimum of 48-hours notice to the local public utilities via Underground Service Alert(USA). We will also subcontract with a private utility locating service to further attempt to delineate y subsurface public utilities and conduits in proximity to the proposed boring locations. • Obtain permits and pay requisite fees to the SJCEHD for the hollow-stem auger (HSA) well installation and well destruction activities. • Retain the services of Gregg Drilling, a Caltrans-approved, C57-licensed drilling company to perform the well installation and well destruction activities. r r Project No.Stockton Yard,Task Order No.45 -4- Caltrans Contract 03A1368,EA No. 10-ON8300 Project No.S9300-06-45 January 28,2009 r