In a May 23 letter to Selma Sierra, director of the Utah office of the federal Bureau of Land Management, the EPA said BLM will have to write a supplemental environmental study that would have to go to the public for comment and then back to EPA before the project could proceed.

The requirement appears certain to slow or even halt Denver-based Bill Barrett Corp.'s plan to drill 807 natural gas wells on the West Tavaputs Plateau.

Critics of the project have said dust and chemicals kicked up by hundreds of big rigs traveling to and from the drilling site are corroding peerless Ancient Puebloan ruins and rock art.

The EPA said the draft environmental impact study's finding that the project would have very little impact on the area's ground-level ozone "is not technically defensible."

Bill Barrett estimates the full-field development project would yield 1 trillion cubic feet of natural gas during more than 30 years of drilling. The yield is equivalent to about 17 days of supply at today's national consumption level.

BLM spokeswoman Lola Bird said she couldn't comment on the EPA letter because she hadn't seen it.