Although Mr. Thompson has written an excellent description of mine drainage, When our river turned orange, High Country News, August 9, 2015, and the complexity of problems related to the Gold King release he falls short of adequately describing the processes of “acid mine drainage”. Iron pyrite is a common sulfide mineral in many geologic environments. It slowly oxidizes in the presence of water and oxygen leading to the release of sulfuric acid and ferric sulfate. Although many ecosystems contain enough buffering compounds to neutralize the acids, the high altitude environment within the San Juan Mountain’s numerous volcanic calderas have limited buffering capacity. The geochemically altered calderas contain widespread finely disseminated iron pyrite, ideal for acid production and the accumulation of ferric sulfate which is oxidized to ferric iron, in turn, responsible for dissolving the heavy metals from metal sulfides (cadmium, copper, lead, etc.). Compounding this is that when a pH of 4.8, or so, is reached a “bio-catalytic” reaction begins where the sulfidic metals are further oxidized by masses of extremophilic bacteria (e.g. Acidithiobacillus ferrodoxidans) capable of releasing magnitudes of more acid that dissolves even more metal sulfides. This accelerated reaction can lead to an environment with a pH as low as negative three (e.g. Iron Mtn., CA).
The reason I mention this is that it is important to understand the entire process so one can intervene. One way is to eliminate water and/or oxygen. The installation of hydrological controls to keep water from entering a mine or mine wastes is both logical and highly successful
in those locations where it is practical. Another way is to install concrete bulkheads, hypothetically, to turn a mine into an anaerobic (lacking oxygen) pool of water inhibiting further dissolution of metals. The bulkheads in the Sunnyside Mine were, to some degree, intended to
do this, however oxygen has not been completely excluded). If anaerobic conditions are created it is even possible that other bacteria known as “sulfur reducing bacteria” (SRB) can reverse the AMD process to create insoluble metal sulfides. The ARSG has insisted that piping be installed
by the EPA in the Red and Bonita bulkhead so we could feed these little critters (e.g. energy source such as alcohol)
By Bill Simon,
Restoration Ecologist and ARSG Co-coordinator