In 2002 and 2003, PennDOT was constructing Interstate 99. Moving some earth exposed pyrite, which caused several problems.
Pyrite in the ground is harmless. But when it is exposed to air and water, it forms sulfuric acid and ferric hydroxide. Hydrogenn is released which lowers the pH, making it more acidic.
The results added much time, expense, and mitigation efforts. The pyrite had to be re-covered, to stop the toxic reaction.
Now you can see large expanses of black plastic covered with limestone gravel that have been placed over the pyrite. There are several of these areas along I 99 in the Skytop area, around mile marker 66.
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The Skytop section of I-99 is four lanes and two and a half miles long. PennDot accepted an initial construction bid from a company named HRI, Inc. for around $39 million.
For the completed project, road and bridge building costs fell in close to that, at $44 million, according to Marlaine Fannin, a PennDot community relations coordinator. Efforts to fix the problems caused by construction soared to more than twice the price of building the road. They are now in the $100 million range.
People have known that there was pyrite in Bald Eagle Ridge for decades, Penn State geologist Duff Gold said. They just didn't know how much.
A Pittsburgh engineering firm, American Geotechnical and Evironmental Services, took the core samples for the project in 1998. They took samples of the rock by drilling straight down and then horizontally into the mountain. Gold said that the firm used the wrong sampling strategy.
Because of the way the mountain formed millions of years ago, the layers of pyritic rock in Bald Eagle Ridge were vertical, not horizontal. A few steps to one side or the other and the core sample taken from above could miss the bulk of the pyrite. That's what happened on Skytop.
Skytop's pyrite has been described numerous times by people involved in the project as a particularly "wicked" type.
It's a "totally unique formation ... found nowhere else in the world," PennDot's retired district executive Khoury said.
When contractors started digging into the hill, they had no idea the amount of pyrite underneath. Two percent sulfur is the highest level of sulfur in the core samples. Gold says the amount of pyrite they found in Skytop is closer to five percent.
A five percent ore body is a high enough percentage to mine the area, if anyone wanted to mine pyrite for its sulfur.
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