SITE INFORMATION AND CORRESPONDENCE

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Mr. Ken Price - 2 - • 14 June 2004 , As illustrated in the Annual 2003 Monitoring Report and First Semiannual 2004 Monitoring Report, constituent concentrations in groundwater downgradient of the soil excavation area declined by about an order of magnitude between 2002 and 2004. For example, in MW-27, downgradient of the excavation, 1,2-DCP decreased from about 150 ug/l in 2002 to 7 ug/1 in 2004, 1,2,3-TCP decreased from about 3 to 0.2 ug/l, atrazine decreased from about 4 to less than 0.5 ug/l, chlorobenzene decreased from about 4 to less than 0.5 ug/l, and nitrate decreased from about 200 mg/l to 50 mg/l in 2004. Regional Board staff believe that these dramatic declines in groundwater constituents are a direct result of MSRM's extensive soil removal and treatment activities. Lateral and Vertical Extent of Groundwater Constituents The intermediate water-bearing zone contains chlorobenzene at a concentration of about 1.3 ug/l in one monitoring well (MW-18), which underlies the fornler wash pad. Other constituents of concern identified in the intermediate zone prior to 2000 are no longer detected. In the shallow water-bearing zone, the data show that nitrate (as nitrogen) concentrations in the furthest downgradient monitoring wells (MW-23, MW-25, and MW-29)range from about 20 to 40 mg/1 and concentrations are stable, given the seasonal variation exhibited throughout the monitoring history. While chloride is present in these downgradient monitoring wells, the data show a potential for wide variation in background chloride concentrations, and it is not apparent whether the chloride detected is related to site activities. Organic constituents of concern are not detected in these downgradient monitoring wells. Constituents in the shallow monitoring wells between the facility and the furthest downgradient wells (MW-26, MW-27, MW-28) are declining in concentration, most notably since the soil excavation and treatment. Therefore, Regional Board staff concur that the plume is stable and defined. Prior to 2003, the center of mass of the groundwater pollution straddled the downgradient boundary of the facility with the adjacent residential subdivision. Since the soil removal and treatment activities in late 2001, concentrations of constituents of concern in groundwater have drastically decreased beneath the subdivision, and as a result, the center of mass of pollutants is currently beneath the facility. These reductions in groundwater pollutants were not yet apparent when Geomatrix prepared the FS/RO in mid- 2003. Feasibility Study/Remedial Options Evaluation Report In the FS/RO, MSRM evaluates four possible alternatives for remediation of groundwater, as listed below: • On-site phytoremediation with monitored natural attenuation for off-site constituents • Enhanced in situ bioremediation with monitored natural attenuation for chloride • In situ chemical oxidation with monitored natural attenuation for nitrate and chloride • Groundwater extraction, treatment with activated carbon and ion exchange, and discharge to sanitary sewer. Geomatrix Consultants identifies on-site phytoremediation as the preferred alternative. For the monitored natural attenuation component of this alternative, Geomatrix proposes to develop a