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responsibility, it is imperative that regional boards pursue all available avenues for gathering the necessary information to proceed to cleanup. This clearly includes requiring that all parties with potential responsibility participate in investigating the sources and extent of pollution. In this case, however,evidence gathered pursuant to the earlier investigations does not support continuing requirements that Chevron investigate and remediate petroleum hydrocarbon contamination at its former site. The Chevron station may have had minor surface releases during its operations. Low detections of hydrocarbons and trace concentrations of toluene and xylene in shallow soil may be remnants of such releases. However,there is sufficient evidence available in the record to determine that the magnitude of any possible releases prior to 1972 were minor in nature,did not materially contribute to the plume, and would not warrant further investigation or remediation. Under these circumstances,we are unable to conclude that the Regional Board appropriately named Chevron as a party responsible for the ongoing investigation and remediation of a plume originating off-site. The State Board has previously found that"it is appropriate and responsible for a Regional Board to name all parties for which there is reasonable evidence of responsibility, even in cases of disputed responsibility" in a cleanup order." However, the Regional Board must show substantial evidence to support naming a party in a cleanup order,and this conclusion may be applied to later phases of investigation orders where a substantial body of evidence has been produced." Chevron has complied with earlier stages of an investigation that has produced evidence showing another party's responsibility for discharges affecting groundwater. This evidence outweighs the evidence relied upon by the Regional Board to name Chevron in the letter order. Under these facts, Chevron should not be required to participate in the continuing investigation and remediation of those discharges." The conclusions of this Order are based upon the unique facts presented and are not intended to have precedential effect on investigation or cleanup activities at other sites. " State Board Order No.WQ 85-7,at p. 11. 12 See,State Board Order No.WOO 2003-0020,at p.8. " If Opal Cliffs'continuing investigation and cleanup produces substantial new evidence that the former Chevron station or any other source has discharged or may have discharged waste that contributes to the need for cleanup at the site,the Regional Board should require any such entities to participate in appropriate phases of the investigation and remediation. 7.