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, 1 Mr. Philip S. Isorenao October 29, 1999 Page 2 (McDonald and Harbaugh 1988). The existing three GWETS wells plus three additional downgradient wells were applied to the subsurface hydrogeologic model that was used in the site VOC fate and transport modeling study (Parsons 1999b). The additional downgradient wells (GW-4, GW-5, and GW-6) were placed at various trial locations. Extraction wells GW-4 and GW-6, pumping at 3 gpm, were screened in the shallow groundwater layer. GW-5, pumping at 1.5 gpm, was screened in the intermediate groundwater layer. The analysis indicates that the proposed arrangement of new wells will capture flow sufficiently in the two impacted groundwater zones in the downgradient portion of the plume (Figures 2 and 3). Figure 2 depicts the modeled piezometric surface in summer in the shallow groundwater zone (model layer 2) and Figure 3 depicts the intermediate zone (model layer 3). At least one observation well screened in the shallow groundwater zone (e.g. OB-3) will be needed to confirm the extent of the capture area. Currently, groundwater treatment is being accomplished with a series of three 200-pound drums of granular activated carbon(GAC). The design flow rate of the drums is 10 gpm. Sampling is performed weekly at the intermediate points to monitor for contaminant breakthrough between drums. The first drum in the treatment stream is removed for regeneration approximately every other week. This is done in order to assure compliance with the City of Stockton Municipal Utilities Department discharge requirement of ND (< 0.5 gg/L). The addition of three new wells will increase the total GWETS flow rate to between 15 and 20 gpm. The GAC treatment system will be increased to two 1,000-pound canisters to handle the increased flow and to reduce the frequency of carbon exchanges. Sampling of water at the canister intermediate sampling ports will be done after the first week of service and biweekly thereafter. Project WQO's The WQO's for constituents of concern listed in Table 1 of the above reference letter will be adopted. Per BP Amoco's letter to the RWQCB (Sicvol 1999), it is planned to operate the additional extraction wells and expanded treatment system until such time as groundwater contaminant concentrations have decreased to background, to below the project WQO concentrations, or until asymptotic mass removal rates are being achieved. Then it is planned to evaluate the economic feasibility of continued operation of the three additional wells (GW-4, GW-5, and GW-6) based on the factors presented in the RWQCB letter dated 13 October 1998. The analysis will support a proposal for discontinuing operation of wells and a follow-up monitoring period. Analytical Reporting Limits Until now results of all groundwater analyses for chlorinated VOCs by U.S. EPA Method SW8021B (formerly SW8010) have been reported with a practical quantitation limit (PQL) of 1.0 pg/L. The project analytical laboratory, SPL of Houston, Texas, verified that lower reporting limits can be achieved by modifying the laboratory MS/MSD sample AM END3 WP.DOC