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feasibility Study—Enhancement of Existing Remediation Activities 8200 N.Highway 99, Stockton, CA Page 5 of 10 2.3 Evaluation of Remedial Alternatives Each of the alternatives presented above was evaluated according to the following criteria: 1. Implementation Issues 2. Effectiveness 3. Cost 4. Regulatory and Community Acceptance 2.3.1 Alternative l - Groundwater Extraction and Treatment X Criterion 1 —Implementation Issues: This alternative is feasible from a technical perspective. Access to the proposed extraction well location is not restricted. Subsurface and equipment infrastructure is already in place and a new extraction well can easily be tied into the existing system. Treated groundwater is currently discharged to the ground surface pursuant to Waste Discharge Requirements (WDR) General Order No. R5-2003-004412 and the groundwater extracted from the proposed extraction well will be treated by the existing treatment system. Monitoring and reporting activities will follow the existing WDR. X Criterion 2 -Effectiveness: Extraction from EW2 has influenced the depth-to-water in MW 12 located approximately 15-feet upgradient of EW2. GZA proposes the installation of a new extraction well (EW3) approximately 15 and 20-feet upgradient of MW12 (Figure 3). The location would provide a small overlap of influence with EW2 as well as capturing contaminant migrating downgradient from the source area. Existing groundwater extraction and treatment activities appear to have been effective. The following summarizes the reductions of TPHg, benzene, and MTBE measured in EW 1 (Discussed in detail in GZAs Quarterly Groundwater Monitoring and Treatment System Operational Report—April through June 2007 dated August 3, 2007): • October 29, 2003 — One week prior to extraction activities: TPHg (96,000 pg/L), benzene (9,700 pg/L), and MtBE(11,000 pg/L) • March 6, 2007 — Last sample event prior to air stripper malfunction: TPHg (5,600 pg/L), benzene(480 pg/L), and MtBE (520 pg/L.) o The results suggest a decrease in TPHg, benzene, and MtBE concentrations by one to two orders of magnitude. • June 27, 2007 — Most recent sampling event: TPHg (20,000 �tg/L), benzene (2,500 pg/L,), and MtBE (1,400 µg/L) o Chemicals of concern appear to have rebounded due to the absence of treatment for approximately 90 days. o Even with contaminant rebound, the results suggest a decrease in TPHg benzene, and MtBE constituents by 74%to 83% since groundwater treatment began. G:IGROUNDZEITULEBURG\Reports10709 F$rem enhancement.doc