STATE COLLEGE, PA (AP) The $700 million Interstate 99 construction project has hit a serious snag, with crews unearthing vast quantities of a "wicked" pollutant that threatens a waterway, state officials said.

Gary Byron, assistant regional director of the state Department of Environmental Protection, said he’s never before seen such potential for stream damage as the acid rock taken from a sandstone layer at Skytop Mountain during construction.

"It’s the worst I’ve seen ... and I’ve been here 27 years," he said. "The bottom line is, PennDOT’s environmental-impact work for I-99 missed this geologic formation."

It is the biggest construction setback for the state Department of Transportation since it began building this stretch of highway five years ago, and might delay this year’s scheduled opening of I-99 from Grays Woods to Port Matilda.

That would prolong commuting delays for thousands of people who live in the Philipsburg and Altoona areas and work in State College. Those motorists have already endured traffic jams and detours on U.S. Routes 322 and 220 for the last two years.

Workers dug up between 500,000 and 1 million cu. yds. (382,277 and 764,554 cu m) of acid rock that had been safely sealed in geologic formations. The rock is now exposed to rainwater that drains into Buffalo Run, a high-quality tributary of Spring Creek.

The state Fish and Boat Commission said recent data showed a decrease in the number of aquatic insects and other wildlife in stream, although officials said more testing would be needed to tell whether the reduced counts are the result of acid-rock drainage or natural variations.

The Skytop acid rock "is rare, but it is very wicked in chemical composition," Byron said.

The discharge is being temporarily neutralized with 10,000 lbs. (4,536 kg) of soda-ash briquettes as it washes into the stream. PennDOT has not yet decided how to permanently repair the problem, but hopes to come up with a solution by May.

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