U.S. Environmental Protection Agency engineers say they think they understand where black goo leaking into Sand Creek north of downtown Denver is coming from, but they haven’t stopped the flow.

“The material appears to be coming from Suncor property, migrating under the Metro Wastewater property and daylighting in Sand Creek,” said EPA emergency response manager Curtis Kimbel.

State health department managers today told the Associated Press that Suncor Energy reported a break in a spur of an underground pipe that runs between a storage tank and refinery about a half mile from where the oily ooze is leaking into the creek.

Hazardous Waste Corrective Action Unit supervisor Walter Avramenko said more tests are needed to confirm the break is the source.

Workers contracted by Suncor are still using vacuum trucks to suck up water fouled with the black liquid. They also are using absorbent material to keep more of the the still unidentified liquid from reaching the South Platte River.

“Our goal still is to completely stop it from going into the creek,” Kimbel said.

At 3 p.m., workers will begin digging a trench to try to stanch the flow of the material, which was first reported leaking into Sand Creek and the South Platte by a fisherman Sunday morning.

Federal environmental officials have been managing the spill since Monday afternoon.

Workers overnight dammed off a catchment area and banded it with absorbent material hoping slow the spread into the river.

The EPA also has launched comprehensive water and soil sampling along Sand Creek and the South Platte. The first lab results from earlier tests are expected this afternoon, and EPA contractor said.

First responders are still using vapor monitors that indicate an ongoing need for respirators.

The area is part of the Sand Creek Regional Greenway, billed as “wilderness in the city.”

Colorado Department of Public Health and Environment officials have known about hazardous leakages in the area for at least a month, documents show. And for a week, toxic vapors at the nearby Metro Wastewater Reclamation District facility have forced workers to wear respirators.

But nobody checked the rivers or tried to stop the seepage. Damage remains unassessed.

Lab tests of water and soil samples taken late Monday and early Tuesday have not been completed, and the source remained a mystery in an industrial area where refineries have existed since 1938. Kimbel today said there is a long history of problems at the site.

“We might be able to find people who have some historical knowledge of those” problems, he said.

At the adjacent Metro Wastewater plant, operations director Steve Rogowski said petroleum odors worsened significantly over the past week, forcing a partial closure of a technical-services building.

“There have been levels of benzene, there have been levels of VOCs (volatile organic compounds),” he said. “And so, as a precaution, we provided respirators.”

Fisherman found spill

A fisherman wading in the river Sunday morning detected the spill and tried to notify authorities, scratching out phone numbers on the sand until he finally reached a state response coordinator who asked if he could call him back in 20 minutes.

“I said, ‘No! Is this how Colorado responds to oil spills?’ He said, ‘You are right’ and took down the info,” said angler Trevor Tanner, 37, an aerospace engineer. “What I pictured was, when I make a call that there was an oil spill, people come in with sirens blazing.”

Reports of spills often are referred to local agencies. After receiving the report from the state health department, Tri-County Health Department investigators tried to locate the spill but could not.

Meanwhile, Tanner wrote about what had happened on his fishing blog, and a reader in Boise, Idaho, later called The Denver Post.

EPA duty officer Craig Myers dispatched Kimbel shortly before noon Monday.

Kimbel stood on a bike-path overpass and looked down at the South Platte. He could not see the sheen but smelled the odor. He then hiked along Sand Creek from the confluence with the South Platte to where the black goo was thickest — about a mile northwest of the refinery and launched an EPA response.

Suncor officials later reported a spill.

The state health department was well aware of problems at the site.

An Oct. 26 letter from the health department to Suncor’s senior remediation officer said “recent releases of hazardous waste and hazardous constituents onsite are now migrating offsite in excess of applicable standards.”

Tuesday evening, EPA and Suncor officials met at the refinery and decided to try digging the trench. Health department officials did not attend because agency experts were needed to respond to media calls, health department spokesman Warren Smith said.

It is unclear how long the oily material has been leaking into the creek and South Platte.

Health department officials doubt the muck has been leaking into the creek for as long as a month, Avramenko said.

“Somebody would have noticed it,” Avramenko said. “My guess is what we are talking about at the present moment is something that happened only recently.”

Health risks unknown

He said health department officials “only learned of the vapor-intrusion issue at the Metro Wastewater facility” on Monday.

Potential public-health risks have not been assessed. Lab test results “will help us evaluate what risk may exist, if any,” Avramenko said.

A Colorado Division of Parks and Wildlife employee checked out Sand Creek on Tuesday morning and did not notice dead fish, suggesting less than acute toxicity in the water.

State health officials said they notified downriver municipal and agricultural water providers about the spill. At least one agency shut off its intake from the South Platte and was drawing water instead from Standley Lake on the west side of the metro area.

Suncor has been monitoring contaminated groundwater under the refinery site it inherited from Conoco, including a hydrocarbon plume that has spread onto adjacent property owned by Metro Wastewater.

But “we really don’t have a sense of where this material is coming from,” Suncor vice president for refining John Gallagher said. “It is coming out of the ground. It is not coming out in a rapid manner. But it is coming out.”

Suncor’s water testing focuses on water running off its property, and company employees “do not monitor water that is downstream of our location,” Gallagher said.

The health department and other state agencies also did not test river water this past month.

Tanner said he will discuss this incident with Trout Unlimited, the fisheries conservation advocacy group.

“I could see the currents in the water. They kind of glowed funny. I noticed the gas smell but kind of brushed it off as being Commerce City. But I really started to smell it once I got across the Platte. I could see a weird brown sludge at the confluence — a light white film across the top.

“I did put my hand in it. Hopefully it is not heavily carcinogenic,” he said. “You could smell it on my flies. I really love the river. I love the fish. So this is quite distressing.”

Bruce Finley: 303-954-1700

or bfinley@denverpost.com