"On a cool November morning, Charles Cravotta was highlighting a smelly environmental success story — and one that might have a surprising side effect.

A combination of ponds, limestone and compost were treating a portion of the acidic chemical cocktail draining from a long-abandoned coal mine and heading toward Shamokin Creek in central Pennsylvania.

Over many decades, dissolved iron in the acidic runoff had stained the streambed orange from rust-like deposits. The iron and other metals had rendered the stream fishless — a common occurrence in Pennsylvania’s coal mining regions.

“When this treatment system was built, it was obviously working, because the air smelled really bad,” said Charles Cravotta, a retired U.S. Geological Survey scientist.

That’s because the compost material changed the sulfur in the water to sulfide, which reduces its acidity, but produces the pungent “rotten egg smell” in the process. “Your nose is more sensitive than a lot of instruments that might be used to measure it,” he noted.

The odor was still present, showing that the system continues to reduce contamination more than 20 years after it was built.

Decades of work, which is accelerating in recent years, is helping decrease acidic runoff from abandoned mines and bring streams once devoid of fish and aquatic insects back to life.

But Cravotta now suspects those improvements might be unexpectedly contributing to another problem. . ."