The Utah Division of Water Quality said twice-daily tests taken since the late Dec. 1 leak showed contamination, but only in the initial sample.
Subsequent monitoring at several locations near the spill have revealed no evidence of petroleum in the creek, which is about 50 feet from where emergency crews built earthen berms to stop oil from flowing into the waterway.
Officials say a faulty valve apparently leaked 500 barrels (21,000 gallons) from Chevron's pipeline, which runs about 260 miles from Rangely, Colo., to the company's refinery in northern Salt Lake City.
"We think the petroleum compounds found in the creek actually resulted from the petroleum contaminants becoming airborne and then being deposited into the stream," said Walt Baker, who oversees Utah's water-quality programs. "The good news is that, at the levels seen, they pose no risk to aquatic life or the public."
Environmental officials continue to test soil and water in the spill area, which is about 100 yards from a June 11-12 spill that poured 800 barrels (33,600 gallons) into the stream. That disaster saw crude flowing down the creek for miles, through upscale east-side neighborhoods to Liberty Park's pond and stretches of the west side's Jordan River.
Chevron spokesman Mickey Driver said the second spill's cleanup, already declared 95-percent complete, continues. Discussions about how to tackle the long-term remediation have begun. An incident command center has been established at the University Marriott in Research Park, not far from the spill site.
"We're working very closely with the University of Utah and the Red Butte Garden," Driver said.
Utah Department of Environmental Quality scientists said Tuesday they do not know how the oil contaminants reached Red Butte Creek. And Renee Zollinger, Salt Lake City's oil spill czar, said it's possible no one ever will be able to explain it. She said it could have been runoff, vapor, the past spill or some other factor.
"There are a lot of possibilities," she said, "and it may not matter if these concentrations are gone."
Since that first test Thursday morning, the state has found no traces of petroleum contamination in the stream.
"From the city's perspective," Zollinger said, "we want to see these [petroleum contamination] levels return to background levels."
Peter Hayes, a Yalecrest neighborhood activist and high school biology teacher, found it plausible that the crude oil vapors tainted the stream that first night, given that the vapor is heavier than air.
He also noted that the environmental testing this time has been more focused and precise than during the June spill.
"That's a good step," he said, "something the Citizens Response Committee [formed after the June spill] wanted to happen."
Hayes and Zollinger agreed more work needs to be done to ensure there is no Red Butte Spill III. To that end, Mayor Ralph Becker has urged federal regulators to keep the pipeline shut down indefinitely until third-party experts can signify its safety.