DENVER (AP) — Colorado state health officials fear that a leaky underground pipe that leads to a tank at a Suncor Energy refinery might have leaked a "sizable" amount of petroleum near the South Platte River, a major source of water for Colorado and Nebraska.

The leak reported by the refinery this summer is about a half-mile away from where an oily substance began seeping into Sand Creek on Monday, raising concerns about how much petroleum would need to leak for it to migrate underground from the source to the creek, said the health department's Hazardous Waste Corrective Action Unit supervisor, Walter Avramenko.

"We were very surprised," Avramenko said Wednesday of the oily substance seeping into the creek from the refinery, which for decades has been dealing with contamination. "It's a fairly sizable quantity of oil."

More tests are needed to confirm the leak is the source, he said.

Avramenko said Suncor reported the leak in a capped section of pipe that comes off a pipeline that runs between a storage tank and the refinery.

Suncor Energy Inc.'s vice president of refining, John Gallagher, said a crude oil pipeline to the refinery from Wyoming has been ruled out as a source. He said the company is responsible for the substance leaking into the creek but said it's "dealing in facts, not speculation" about where it's coming from.

"We don't know," Gallagher said, adding that he believes the spill has been contained. "We're committed to protecting the environment. That's really where we're at."

Crews on Wednesday built three small dams around a 90- to 100-foot section of the Sand Creek bank that's about one-tenth of a mile from the South Platte River, said Environmental Protection Agency spokeswoman Karen Edson. The dam seemed to contain most of the mucky oily substance seeping out of the creek bank that was then vacuumed out by crews on the scene.

Edson said between 20 and 50 gallons of an oily substance had been recovered by crews, but it was unclear how much may have seeped into the creek or made it into the river. EPA officials on the scene said they're focusing on clean up and haven't pinpointed the source.

Crews were to dig a trench Wednesday as part of long-term containment efforts, Edson said.

Gallagher said the refinery produces jet fuel, gasoline, diesel fuel, and asphalt mostly from oil from Colorado and Wyoming. About 10 to 15 percent of the oil refined there comes from oil sands from Canada, adding that the refinery has been there since 1938 and was designed to handle local crude. The Calgary, Canada-based company has three refineries in Canada and in Commerce City.

State health officials have long known about pools of oil in the ground from the 1980s and 1990s when it was owned by other companies. Avramenko said those former pools of oil had become more tar-like, stable and less likely to move off the refinery grounds. Monitoring wells showed that the groundwater quality had even improved over the years as the refinery pumped groundwater to rid it of contaminates and took other measures as part of a state health department corrective order over contamination.

Last year, Avramenko said monitoring wells detected elevated levels of petroleum contaminates between the creek and an underground barrier wall, called a curtain, that is meant to contain the contamination on refinery grounds. Suncor repaired a corroded outlet pipe on that barrier that was suspected of allowing contaminates to leave the refinery.

But then Suncor reported other anomalies, including an oily sheen on a pond and in a ditch on refinery grounds, as well as an oily sheen in the creek this summer that went away.

"These appearances began to cause us to reconsider what was going on," Avramenko said.

On Oct. 26, the state ordered Suncor to step up its efforts to contain the contamination, including shoring up the barrier wall near the current seepage, pumping more groundwater and treating it, and providing the state with a model for how the contamination is moving underground, as well as a year to determine the source. Ancient creek beds, sand channels, clay layers and gravel beds all could influence how petroleum moves underground.

The "notice of additional work requirements" followed Suncor's informal report of the leaky pipe over the summer.

"We're stepping back and taking a more formal approach," Avramenko said. "We thought we had our hands around it."