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gasoline constituents detected in soil and groundwater at the former Chevron site are a result of <br /> discharges from the Chevron facility. The weight of evidence indicates that the contamination <br /> originates from the Opal Cliffs site. <br /> The letter order under petition was issued pursuant to the Regional Board's <br /> authority under Water Code section 13267. That section broadly authorizes the regional water <br /> quality control boards to require persons who discharge, have discharged, or are"suspected of <br /> having discharged or discharging" waste that could affect water quality to furnish technical or <br /> monitoring program reports' When acting under this broad authority, regional boards must <br /> identify the evidence that supports requiring that person to provide the reports.' The evidence <br /> can be any relevant evidence on which responsible persons are accustomed to rely in the conduct <br /> of serious affairs.' <br /> The Regional Board's two-plume theory depends upon a finding that the former <br /> Chevron site is not downgradient of the Opal Cliffs release. The Regional Board maintains that <br /> the groundwater flow direction has been consistently to the southwest and views this as evidence <br /> that the release from the Opal Cliffs site did not migrate onto the site of the former Chevron <br /> station. Such a southwesterly flow of the Opal Cliffs plume would completely bypass the former <br /> Chevron facility to the northwest. Hence,the Regional Board concludes that a separate plume <br /> must originate from the location of the former Chevron USTs in order to account for detections <br /> downgradient from that site. <br /> State Board technical analysis of the evidence in the record indicates that the <br /> former Chevron facility is directly downgradient from the Opal Cliffs site. A substantial mass of <br /> gasoline is present in soil and groundwater in the eastern portion of the Opal Cliffs site, and the <br /> data show groundwater flowing to the southwest toward the former Chevron site. The Regional <br /> Board in its interpretation of this evidence appears to have considered only the direction of <br /> groundwater flow near the western portion of the Opal Cliffs site. <br /> The distribution of contaminants in soil and groundwater also supports Chevron's <br /> contention that no appreciable releases occurred from the former Chevron USTs. Monitoring <br /> 2 Water Code§ 13267(b)(1). <br /> ' /d. <br /> Water Code§ 13267(e). <br /> 4. <br />