SITE INFORMATION AND CORRESPONDENCE_FILE 2

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r Chapin Brothers Investment, Inc. -2 - 16 February 200 1766 Monte Diablo, Stockton, San Joaquin County of the SVE and AS remedial systems, which were shut down in June 1999 when groundwater reached 10 feet bgs. Three addition first water monitoring wells(MW-5, MW-6, and MW-7,Figure 2) screened 6 to 25, 16 to 30, and 5 to 25 feet bgs,respectfully,were installed in November 1999 to assess the lateral extent of contamination. A prior investigation to 35 feet bgs into a clay aquitard in May 1999 reported no soil or groundwater contamination below 25 feet bgs. A pump test was performed in March 2001,and a Groundwater Pump and Treatment System (GWP&TS)began in April 2002 with discharge to City of Stockton sanitarysewer from four wells(EW-1 to EW-3 and VW-6). A GWP&TS rebound test was conducted during November 2003 and resulted in groundwater contaminant concentrations doubling by December 2003 for TPH-g in EW-1 (910 µg/L to 1,800 gg/L),with similar responses in other extraction wells by January 2004,when groundwater extraction and treatment was restarted. The GWET is currently operating. In January 2004,groundwater modeling began, to support a closure request. In December 1993,the maximum groundwater contamination was reported as: Total Petroleum Hydrocarbons as gasoline(TPH-g, 140,000 gg/L)and benzene(45,000 gg/L). As of June 2004(last quarterly report),maximum groundwater contamination reported in extraction well EW-1 was TPH-g,(8,300 µg/L)and benzene(260 gg/L). The Closure Report states 5.89 pounds of benzene and 146.7 pounds of gasoline had been removed. The Closure Report estimates TPH-g remaining in soil as 5,733 pounds and 2.59 pounds remaining in groundwater. Comments: 1)The Petitioner has not demonstrated that the site is ready for closure: The Report recommends and the Petition requests closure with no further action. Post remediation monitoring after shutdown of the pump and treatment system is not proposed prior to closure, which is a departure from accepted practices. Site conditions(hydraulic control, declining contaminant concentrations)are currently the result of active pumping from the groundwater pump and treatment system operations. The above-mentioned 2003 rebound test showed a significant rise in contaminant concentrations after one month,which required restarting of the groundwater pump and treatment system. As such,there is a great deal of uncertainty whether the groundwater plume is stable and concentrations will continue to decline at a reasonable rate to achieve water quality goals. Post-remediation groundwater monitoring is necessary prior to closure, and the data must show plume stability and a declining concentration trend before I would agree with the Petitioner that the site is ready for closure with no further action. 2)The groundwater model used to predict fate and transport for closure in the Report should be revised per the following: • The groundwater model used by the Petitioner does not use all available soil data, specifically soil core data collected in November 2003 under the building,which results in a low estimate for the contaminant mass remaining in soil that is available to leach into groundwater. That data, incorporated into the groundwater model, would increase the time needed for site conditions to reach water quality objectives. As such the groundwater model should use all available data or provide the rationale for rejecting data. • The groundwater model apparently was not sensitivity tested to show what variations for specific input values affect the output results. Sensitivity testing is necessary to show the validity of a groundwater model, since minor changes to certain inputs can cause significant changes in output. Whether the use of California Environmental Protection Agency �a Recycled Paper