It was polluted due to the construction of a hydroelectric dam in 1940 and acid drainage from the Tamagawa Hot Spring (onsen).

These kinds of release are much less obvious and may take the form of acid drainage or dry tailings dust being blown away from the storage area.

Limestone, which is used to neutralize the acid drainage, can actually exacerbate the expansion problem by promoting sulfate-sulfate reactions that form the minerals thaumasite and ettringite, which have even higher molar volumes.

Intense rains contribute to the seepage of waste deposits into the groundwater supply, generating and carrying the acid drainage from abandoned mines into the nearby rivers and streams.

But environmentalists say the acid drainage from the project could flow into the park through Soda Butte Creek, and ultimately threaten the survival of the endangered Yellowstone Cutthroat fish.

As a legacy of past mines, sections of local streams still run rust red from acid drainage.

Long-term acid drainage, a staple of earlier generations of mines, is highly unlikely, he said, because of new, clean-as-you-go technologies.

The vast majority of those mines have no surviving owners to bear financial responsibility, or else the known owners aren't rich enough to reclaim the mine and treat its acid drainage in perpetuity.

Further environmental problems caused by hardrock mining involve water pollution by metals themselves, processing chemicals, acid drainage, and sediment.

A bulldozer had dug through 12 feet of buried bony to sculpture a chain of six trapezoid-shape treatment ponds for breaking down the acid drainage that still flows out of these hills.