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GEDMATRIX <br /> Ms. Mary Ways <br /> San Joaquin County Public Health Services <br /> 29 September 1993 <br /> Page $ <br /> - The statement that "the concentrationof benzen' jn on-site groundwater monitoring <br /> wells has historically been low" is'misleading. The:only groundwater,sample collected <br /> near the water table (at B-7) contained over 500 ppb benzene. <br /> Sections 3.0 through 6:0 , <br /> No comments have been generated by Geomatrix at this time. <br /> Section 7.0, Pages 7-1 through'7-3 <br /> Again AIton states that the lateral and vertical.limits of soil contamination were defined <br /> and that "the excavation removed as-much of the-impacted soil as was accessible or <br /> known to exist. This statement is not accurate. The limits of contamination were not <br /> determined and the soil below 10 feet, which still contains very high concentrations of <br /> petroleum hydrocarbons, wa's.accessible and.was known to exist at the time of the <br /> excavation. . <br /> - As discussed above, the risk assessment.did not evaluate all appropriate future uses of <br /> the property. A' <br /> - Alton states that the remediation plan is consistent with the LUFT;manual and that <br /> "remedial activities conducted to date have remediated soils to the°maximum extent <br /> possible..." The plan is not,consistent with the LUFT manual in that petroleum <br /> hydrocarbons at concentrations .greater than 3000 gpm have been left in soil,that is in <br /> close contact with groundwater. These soils can and.:should be remediated to ensure <br /> safe future use of the property. <br /> In summary, Mobil has.inadequately characterized and remediated petroleum hydrocarbons <br /> at this site. In this report, Alton has inaccurately assessed the significance -of the remaining <br /> contamination at the site and its affect on future site use. The following additional <br /> investigations should be conducted at-the site: <br /> - Determination of lateral and 3'vertical exterit.of'contamination; . <br /> - Evaluation of cross-contamination due to deep screens-in on-site monitoring wells; <br /> - Evaluation of health risks for potential residential, commercial, and industrial use, <br /> considering the presence of separate-phase hydrocarbons beneath the site and the <br />