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GEOMATRIX <br /> Ms. Mary Meays <br /> San Joaquin County Public Health Services <br /> 29 September 1993 <br /> Page 2 <br /> - Throughout the document Alton states that no "liquid-phase hydrocarbons" were <br /> detected beneath the site. Assuming that Alton is referring to separate-phase <br /> hydrocarbons by this phrase, this statement also is not accurate. Groundwater data <br /> from Boring B-7, drilled after the soil excavation took place and within the limits of <br /> the area that was excavated, indicate gasoline concentrations of 110,000 micrograms <br /> per liter (ppb) and diesel concentrations of 60,000 ppb. Both of these concentrations <br /> are above the respective solubility limits for gasoline and diesel, indicating that <br /> separate-phase hydrocarbons are present beneath the site. Also, soil data from borings <br /> for recently installed wells, where up to 3700 milligrams per kilogram (ppm) of diesel <br /> and 590 ppm gasoline were detected, indicate that separate-phase hydrocarbons remain <br /> in soil beneath the site. <br /> - No samples of the sidewalls of the excavation were collected, therefore, the lateral <br /> extent of petroleum hydrocarbons due to spills at the former Mobil site has not been <br /> established. <br /> - Because the severity of contamination left beneath the site and adjacent to the site has <br /> not been adequately characterized, the risk evaluation presented in the report is flawed. <br /> The assessment should consider that separate-phase hydrocarbons are present 10 feet <br /> beneath the site and that high concentrations of hydrocarbons are likely present in <br /> shallow soil adjacent to the site. <br /> - Alton's recommended remedial plan, which involves only future monitoring of <br /> groundwater, is not appropriate for this site considering that separate-phase - <br /> hydrocarbons are still present, and the lateral extent of contamination to the north, <br /> east, and west has not been established. <br /> - It is clear that active remediation of the separate-phase hydrocarbons at this site and <br /> additional investigation of the lateral extent of contamination are necessary. <br /> - As you know from your own records, since 1986 many of the reports that Mobil has <br /> submitted for this site have ended with the conclusion that no further work is <br /> necessary, even though you have subsequently requested additional work. It is time for <br /> Mobil to stop delaying and complete the remediation it began in 1990. <br />