Hill Air Force Base officers want to begin soon to clean up many thousands of gallons of hazardous solvents that were allowed to sink into the groundwater for more than a decade.

The cleanup site has been designated Operable Unit 2, which is only one of seven contaminated areas that have been discovered at Hill. It is located near the base's eastern boundary."The sources of the solvents could be from various Air Force base operations, which would include airplane maintenance, cleaning of machinery, that sort of thing," said Robert Stites, Denver, the Environmental Protection Agency's remedial project manager assigned to cover Hill projects.

None of the unit's contamination is believed to have migrated off base, he said. "The migration rate is slow. We do not perceive any immediate threat to human health and the environment."

Between 1967 and 1979, great quantities of solvents used for degreasing warplanes and stripping paint were dumped in a chemical disposal pit. "A liner was not required by regulations at that time," Stites said.

After it was poured out, the material worked its way through the ground. Because the material was generally denser than water, it sank below the groundwater layer. Then it stopped, trapped by a clay layer.

"This material tends to be pretty viscous and doesn't flow as much as the water," Stites said. Small portions of it are soluble in the underground water, however.

Among the chemicals are trichloroethylene, toluene, tetrachloroethylene and other hazardous material. The pollution that accumulated underground at the site is at least 10,000 gallons and may be as much as 100,000 gallons.

"We're working to refine that," Stites said of the estimate. It has a large uncertainty factor "because records weren't required to be kept back when this was going on."

A preliminary estimate of cleanup costs that was made last year, before the magnitude of the contamination was known, was $2 million. "It could go higher," Stites warned.

A consulting company, RADIAN Corp. of Austin, Texas, recommended to Hill that the contaminated liquid should be recovered and treated on site.

Groundwater tested from two wells within Operable Unit 2 had two layers of material: dense, oil-contaminated solvents and a watery layer saturated with the material. The material comes out mixed as an emulsion, but when left undisturbed it settles into the two layers.

Groundwater from other wells within the unit contained only dissolved organic material and low levels of heavy metals. Organic solvents are the main contaminants.

Although the selection of the final alternative for cleanup is 14 months away, and cleanup is more than three years away, Hill Air Force Base wants to begin some of the work soon, said Capt. Edward Heyse, manager of the base's Installation Restoration Program.

"It doesn't make any sense to wait more than three years to get on with the cleanup when contaminants remain in the ground." He added that the EPA encourages expeditious actions in such case.

Before any cleanup starts, a 30-day public comment period will be held, said Hill's Debbie Berry-Smith.

"They aren't going to start pumping it out until after we have that record of decision, or decisions document," Stites said. "The design for the construction will have to undergo a review."

The review will start as soon as possible after a decision document is signed. "We can't start designing right now because that would be circumventing the public comment process," he said.