Acid mine drainage is a form of water pollution from various types of mineral extraction. I am most familiar with its effects in relation to Pennsylvania Anthracite coal mining, but it occurs almost anywhere that big holes are dug in rocks.
Rocks tend to contain a fair amount of pyrite, (iron sulfide) which is a highly metallic mineral. When tunnels are driven into bedrock, they expose these iron bearing rocks to air and water. Most mining takes place below the water table, mines have giant pumps to dewater them while the mine is being used. When the mine is abandoned and the pumps are turned off, it fills with water which begins to dissolve the pyrite. With the help of bacteria and other enzymes and catalysts, this iron bearing water becomes highly acidic and bright orange colored.
A buildup of water in abandoned mine cavities can cause problems. like raising the local water table to the point where it floods basements. It can also cause sinkholes and subsidence. This becomes a big problem when the abandoned mine cavity is a sprawling network of tunnels on multiple levels which interconnect below an entire town.
In the 1960’s, the city of Scranton PA was severely threatened by rising water levels in its underground mine pool. Deep tunnel mining in the area primarily ceased after the Knox Mine Disaster in 1959, when the Susquehanna River breached a mine tunnel and flooded the entire system, killing 12 miners. The hundreds of miles of tunnels extending thousands of feet below the city were flooded, and a vast underground lake formed. Newspapers would publish the daily water level, and people were constantly fearing flooded basements.
The city’s solution was the Old Forge Borehole. It’s a giant drainpipe drilled at the lowest point in the valley which drains into the Lackawanna River. It was drilled by the Army Corps of Engineers in 1961. It drains over 100 million gallons of iron-saturated and acidic water each day. It is thought to be the largest single point source of pollution to the Chesapeake Bay, where the water eventually reaches.
Acid mine discharge can be treated by installing processing plants to remove the iron and other pollutants from water which exits the boreholes. The recovered iron can be processed and sold for industrial and commercial applications, including the manufacture of pigments. The Eastern Pennsylvania Coalition for Abandoned Mine Reclamation has done good work cleaning up various AMD sites, and doing test projects to recover iron from polluted water and using it to make glazes for ceramics.