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Extended Site Characterization Report: 7500 West Eleventh Street, Tracy, Cil. Page 56 Ethyl tertiary-butyl ether EPA Method 8260FAB tertiary-Amyl methyl ether EPA Method 8260FAB 1,2 Dichloroethane EPA Method 8260FAB 10.2 Modification of Groundwater-quality Monitoring Schedule it is recommended that three additional rounds of groundwater sampling and analysis be performed on a quarterly schedule using the complete array of 15 monitoring wells currently installed for the 7500 West Eleventh Street site (i.e., Wells MW-1, MW-2, r ! MW-3, MW-3A, MW-3B, MW-4, MW-5, MW-6, MW-7, MW-8, MW-9, MW-10, MW- 11, MW-12 and MW-12A). _j Those sampling rounds should be conducted at approximate three-month intervals so that any seasonal changes in the hydrogeological regime that might affect the distribution of hydrocarbons in the subsurface, particularly to the north of West Eleventh Street, can be evaluated and to permit continued observation of any changes in analyte concentrations that occur with time. However, it is clear from the very slow migration rates of components of fuel hydrocarbons in the subsurface beneath the site, as demonstrated in Section 7.3.2.3, that unless the results from the proposed additional quarterly rounds reveal unexpected changes in the characteristics of the plume of affected groundwater, :.<y little new information of significance to the present understanding of the plume will be gained by continuing to monitor groundwater quality at that frequency. Accordingly, to provide for a cost-effective program, consideration should be given to reducing to the monitoring to a semi-annual frequency when the three additional rounds of quarterly t" monitoring have been completed. 10.3 Closure of Wells Penetrating to Deep Aquifers Each of the deep water-quality monitoring wells installed in March 2002 as part of the extended site characterization program (i.e., MW-3A, MW-3B and MW-12A) penetrates . .i through the protective clay aquitard that is present beneath the 7500 West Street property at depth of approximately 22 ft. That aquitard was clearly identified by SJC using 2000 as part of the first phase of site characterization. Groundwater-quality data obtained from Monitoring Wells MW-3A, MW-313 and MW- 12A did serve the purpose of verifying the maximum depth to which the subsurface beneath the 7500 West Eleventh Street site was affected, although that was already well established based on data gathered from the exploratory borings and shallow monitoring wells installed in April 2000 and analysis of the hydrostratigraphy developed therefrom. (See Figures 14 and 15.) The deep wells do, however, pose a significant risk to groundwater quality beneath the near-surface aquitard that prevents movement of affected groundwater at shallow depth from reaching the unaffected groundwater in the sic 1