The mining industry has allowed the geologic term Acid Rock Drainage "ARD" to be modified by detractors of mining to "Acid Mine Drainage" which is now even used by regulatory bodies. This potentially misleading term suggests that the natural, and acid producing process of weathering sulphide minerals (natural minerals comprised of metal and sulphur) only occurs if mining extracts and exposes these sulphide minerals. Folks who buy this story don't get around much geologically. Large volumes of sulphides are exposed and cause ARD all over the earth. For example pyrite (iron and sulphur; also called fool's gold) has the highest acid producing potential of all sulphides but is an uneconomic mineral. Pyrite is also the world's most abundant sulphide mineral. It occurs over vast areas of the planet, exposed at surface where it weathers naturally and acidifies streams. And guess what? Miners just aren't interested in it!
For example the Colorado Geological Survey may have been a bit stunned to discover and report that large areas of naturally exposed pyrite and other sulphides, unmodified by humans with no historic mining disturbance, are producing significant ARD with high levels of dissolved metals, NATURALLY. High metal contents are common in acidic waters because low pH (acidification) also makes metals more water soluble in water. This is why ARD commonly results in rusty iron and metal rich seeps like what the Colorado Geological Survey studied. They summarise that 76% of the samples taken from areas with no human disturbance showed pH levels and metal contents that are higher than the limits set by the State of Colorado for drinking quality! This brings up some difficult questions for regulators. For example should a mine be found and proposed in these areas of natural ARD, what standard for downstream water quality should they be held to? Where ARD occurs downstream from historic mines how much is really caused by human disturbance? Was there significant ARD before such historic mining? Since the earth is weathering and eroding rapidly (in geologic terms) could modern mining that prevents and monitors for ARD actually be sequestering sulphides so that they don't cause natural ARD? In this way could mining actually "improve nature" preventing natural ARD that will eventually occur?
MiningFocus reader, consider this blog post the parting shot of an active intellectual and scientific engagement we plan for you. For too long geologic realities have not been part of the debate. We are going to change that. Look for updates here!