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Mr. Paul Supple :i <br /> Arco Station No. 5469 <br /> 130 S. Wilson}Way, Stockton <br /> Page 2 of 3 <br /> i <br /> consistent with the hydrogeological framework in the SCM and account for the <br /> observed contaminant mass distribution and migration history. The model should <br /> 1= b <br /> adequately demonstrate all your consultant's interpretations and opinions. <br /> Review of the 130 site data reveals some unexpected relationships. None of the <br /> soil samples collected from the UST area piping run contained significant sorbed <br /> MTBE, while several soil samples from lithe dispenser area piping runs did <br /> contain significant MTBE concentrations, ilup 'to 11.1 milligrams per kilograms <br /> (mg/kg). Yet the high concentrations of;;dissolved MTBE appear to migrate <br /> northward and northeastward from the UST area rather than from the dispenser <br /> area. Initial groundwater elevation maps for the site indicate a southeastward <br /> flow direction that swung northerly to nortl4easterly aver time. EHD is concerned <br /> °E that the plume may extent southward of the dispenser island. Please submit to <br /> EHD by 01 '}November 2005 a work plan to install'three first-water monitoring <br /> wells, one southeast and one southwest of the dispenser island and one.'between <br /> the dispenser island and MW-5; the latter to demonstrate whether or not the. <br /> dispenser island is the source of the dissolved MTBE plume. <br /> p <br /> EHD notes a that a previously approved' work plan for installing additional` <br /> groundwater monitoring wells-'east of your;1building has been held up for lack of <br /> an access ;agreement from the adjacent property owner. EHD views the <br /> proposed wells east of the building to be necessary for this site characterization <br /> and evaluating the risks posed by your pluilne. Roek Brothers II, P.T.P. no longer <br /> owns the property, the current owner is Mohammad°'Asghar, 601 Wade'Avenue, <br /> Modesto, CA 95351. Please direct your request for an access agreement to Mr. <br /> Asghar. If you are unsuccessful, EHD will request the new owner ;to grant <br /> ri <br /> access. <br /> :E. <br /> ii <br /> f9 <br /> The ozone sparge system on your site has:been in operation since 26 December <br /> w' 2002, and much data has been collected on contaminant concentrations and <br /> various natural attenuation parameters. Include in your SCM an evaluation of the <br /> effectiveness of the ozone system for contaminant mass reduction : and an <br /> evaluation of the natural attenuation parameter data. Questions that should;be <br /> addressed include- <br /> N <br /> • Is the ozone system more effective than natural attenuation or other <br /> engineered remediation systems? <br /> • Can the ozone system be made more effective and, if so, how? <br /> • Should another groundwater remediation system be proposed? <br /> a Is the :natural attenuation data demonstrating the likely occurrence of <br /> _g natural attenuation or are there significant impediments to the 'effective <br /> use of natural attenuation? <br /> ' ii <br />