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Tosco's technical opinion is that the most prudent approach to determine the presence of TBA and ether compounds is <br /> an initial screening process in the source area. As stated in the workplan,groundwater monitoring well MW-2 will be <br /> sampled in the fourth quarter 1997 for ether based oxygenates and TBA by EPA Method 8260. If oxygenates are <br /> detected(other than MtBE)in this sample,Tosco will propose a program to assess the monitoring well network for <br /> their presence. However,if oxygenates are not detected during this initial screening process,Tosco will discontinue <br /> these analysis from MW-2 and will not analyze other water samples from the wells at the site by EPA Method 8260. <br /> Tosco believes that with the installation of the monitoring well proposed in the attached GSI workplan,will delineate <br /> the dissolved hydrocarbon plume. The effectiveness of the treatment system in removing residual hydrocarbons from <br /> the unsaturated zone will be evaluated by the proposed boring. Samples collected from MW-2 during the third quarter <br /> 1998 will assess the effectiveness of ORC in enhancing the natural attenuation of the dissolved hydrocarbons. Should <br /> data indicate a continued decline in hydrocarbon concentration beneath the site,Tosco will propose consideration of <br /> this site for low risk closure. <br /> If you have any questions,please feel free to contact me at(510)277-2311 <br /> Sincerely, <br /> — n <br /> Edward C.Ralston <br /> Project Manager <br /> cc: Elizabeth Thayer, CVRWQCB <br /> Greg Gurss,GSI <br />