A crash course on hazardous mine drainage
Throughout the Arkansas River Valley sagging mine shafts offer a glimpse into a bygone era of mining, but these sites are far from dormant. Minerals unearthed over a century ago continue to react with water and oxygen, creating a toxic legacy: acid mine drainage.
By exposing large surface areas of sulfide-bearing minerals like gold, silver and copper to the elements, mining activity jump-starts acid mine drainage. Exposed sulfides react with oxygen and water flowing through surface mine waste and mine voids to form sulfuric acid. As sulfuric acid comes into contact with rocks, it dissolves heavy metals such as cadmium, copper, mercury, manganese, lead and arsenic from surrounding rock and into the runoff. This chain of chemical reactions perpetuates itself, breaking down more rock to release additional sulfides and toxic metals. Left unchecked, this cycle can last for centuries.
Acid mine drainage can contaminate drinking water, stunt the growth and reproduction of aquatic species and corrode infrastructure such as culverts and bridges.
We’ve seen this happen: In the 1970s the Arkansas River was devoid of aquatic life from Leadville to Balltown due to acid mine drainage. The Bureau of Reclamation’s Leadville Mine Drainage Tunnel Treatment Plant (LMDT) was critical in bringing life back to that section of the river after its establishment in 1992, and it continues to mitigate many acid mine drainage inputs into the upper watershed today.
The LMDT treats raw acid mine drainage collected via underground pipelines from mine sites throughout Leadville. At the treatment plant, toxic drainage flows through a system of pipes, pumps and hulking metal tanks all the while undergoing a complex series of chemical dosings, precipitations, flocculations and filtrations. The goal is to separate clean water and sludge. Sludge, or hazardous waste including the heavy metals and solids from treated acid mine drainage, is chemically neutralized and disposed of in a landfill. Clean water is released into the Arkansas River.
The LMDT significantly decreased the harms of acid mine drainage locally, to the extent that local environmental scientist and geologist Devin Castendyk isn’t very worried about acid mine drainage impacting us in BV and Salida. He is more concerned about another type of hazardous runoff closely related to acid mine drainage that occurs in Chalk Creek: neutral mine drainage.
Neutral mine drainage is formed by the same chemical reactions as acid mine drainage. It differs, however, in that it occurs in environments containing large amounts of carbonate – a compound found in limestone that neutralizes the acid generated by sulfur minerals. The resulting runoff loses the acidity characteristic of acid mine drainage. Carbonate does nothing to intercept harmful heavy metals in runoff, however, so while acidity isn’t a problem with neutral mine drainage, heavy metals are.
Written by Rose Nadelhoffer - A Blue Elements Watershed Steward serving with the Greater Arkansas River Nature Association.
