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current property owner, Walter Eller, recollects no indication of soil contamination when he personally excavated and constructed the basement and dewatering sump to a depth of 10 to 13 feet bgs in 1978.' The Regional Board correctly responds that testimony of personal observations does not provide conclusive evidence that a release did not occur.' However,the very low odor threshold for gasoline constituents, combined with evidence of high contaminant levels in 1998, is credible evidence that a release of the magnitude detected in 1998 was not present in 1973 or 1978' The Regional Board relied in part on evidence regarding benzene detections downgradient of the former Chevron site coupled with data regarding the typical characteristics of benzene plumes. While the Regional Board noted that monitoring well data showed benzene concentrations downgradient of the Chevron site higher than would be expected if originating from Opal Cliffs, a State Board study on the subject of benzene does not clearly support this conclusion.10 Therefore,the benzene data cited by the Regional Board are insufficient to reject the body of evidence indicating that the likely source of contamination is Opal Cliffs. We note that the Regional Board's authority to require investigation under Water Code section 13267 is extremely broad. The Regional Board need only cite evidence sufficient to suspect that a discharge has occurred. In most cases, evidence of petroleum hydrocarbon contamination in the vicinity of a former service station would easily meet this standard, and the Regional Board in the initial phases appropriately ordered Chevron to conduct an investigation on its former site. For sites where insufficient data are available to make a determination as to ' Deposition of Walt Eller,December 18,2002. ' Regional Board Response to Petition,September 11,2003,at p.3. e The odor threshold for gasoline is 10 ppb for a strong odor. In addition,a sump pump operated periodically between 1978 and 1996,discharging water to a paved swale where it flowed about 200 feet to a storm drain. The current property owner has testified that he did not observe or smell any petroleum hydrocarbons in the discharged water. 10 The Regional Board's September 12,2000,letter,which set forth findings confirmed in the directive of November 9,2000,cited an American Petroleum Institute(API)publication showing that most BTEX groundwater plumes are less than 200 feet in length. The Regional Board in its response to the petition cites unstable benzene concentrations in a downgradient monitoring well located more than 400 feet from the Opal Cliffs site. The API report is a compilation of data and findings published by state agencies that manage UST sites,and the report includes findings and conclusions from a State Board study titled California Leaking Underground Fuel Tank Historical Case Analysis(1995). In that study,the length of a benzene plume was measured not from the leaking UST,but from the plume's"center of mass",or the monitoring well showing the highest concentration of benzene. 6.