The Air Force has spent nearly $100 million over the past two decades finding and cleaning up hazardous chemical sites at Hill Air Force Base.

And it will take many more millions of dollars and more decades to finish the job, according to Bob Elliott, chief of environmental restoration at the base.Elliott reviewed the cleanup program Tuesday for the Davis County Health Board.

There are 77 identified sites that need or have been cleaned up, Elliott said, ranging from underground storage tanks that leaked jet fuel to landfills and chemical dumping pits.

Of those sites, 47 are underground tanks and only four of those remain to be cleaned up, he said.

The most serious problem sites are the old waste pits where solvents, degreasers, and other toxic chemicals were dumped, Elliott said. Several of them are on the base's perimeter and are leaking chemicals into groundwater.

Elliott said he does not believe the sites present a health hazard to nearby residents. The groundwater leaching through the sites and picking up the chemical residue is not used for drinking water and the amount found in the water is barely above the detectable level, he told the board.

That conclusion was also reached in an independent risk assessment done by the federal disease center in Atlanta, Elliott said, but is not universally held.

Some residents near the base blame various health problems on the contamination, he said.

"We try to assess each claim and deal with it with understanding," Elliott said, adding team members are available to test wells, springs, and other water that residents fear is contaminated.

Engineers have found six sites, or plumes, where the waste has spread off the base, Elliott said, but in most instances the plumes are in low population or undevel-oped areas.

The only exposure residents may have is groundwater seeping out of hillsides or into a basement and being pumped out with a sump pump, Elliott said.

They recently surveyed 800 homes in Layton on the base's perimeter and found only 10 with water in their basements, he said. Air monitors are installed in the homes to determine if the chemicals are evaporating into the air.

"But it's difficult to put a monitor in a basement, sometimes right next to a woodshop with paints and solvents in it, and try to determine the source of what you turn up," Elliott said.

The geology of the land underneath the base is both helping and hindering efforts to contain the leaks, Elliott said.

There is a layer of impermeable clay under the base that acts like a pan to catch and hold some of the liquid. At one site, according to Elliott, engineers have pumped 35,000 gallons of solvent out of the ground since 1993, trapped there by the clay.

But the groundwater above the clay moves through the sites at a rate of 10 to 100 feet per year, picking up the chemicals as it drains off the plateau the base sits on.

The plumes of contaminants don't appear to be spreading, Elliott said, theorizing the chemicals picked up by the groundwater are degrading or breaking down at about the same rate they're being carried off.

While cleaning up some sites was as easy as trucking away some contaminated soil, others are tougher, Elliott said.

It takes between 11 and 13 years after a site is identified to begin the actual cleanup, he said, with all the hearings, reviews, and oversight procedures required by federal regulations.

One site on the north perimeter of the base may take decades to clean up, he said, estimating the final bill for the work at $256 million.

Hill is "one of the worst contamination sites the Air Force has" and was put on the federal Superfund list of high priority areas in 1987, a move Elliott said is "a dubious honor for your county" but noted the listing also gives the base leverage in getting funding.

And it has attracted study teams from various universities and research centers, yielding millions of dollars worth of information, Elliott said.