Just what exactly constitutes acid mine drainage?

By Ryan J. McHale
Posted 4/5/10

Editor’s note: This is part two of a three-part series. There are a lot of preconceived notions and misinformation out there regarding mining. It is understandable because we only have to look …

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Just what exactly constitutes acid mine drainage?


Editor’s note: This is part two of a three-part series.

There are a lot of preconceived notions and misinformation out there regarding mining. It is understandable because we only have to look around at these hillsides to see the pock-marks and scars left behind by unregulated mining of the past.

This week I want to focus on the specific issue of acid mine runoff from mining tailings and dump piles. You have seen them: the ugly, barren yellow piles that are prevalent in the old mining districts. When it rains, the water runs off like a gross orange-looking acid milkshake you could only imagine in a nightmare. What’s going on here?

In the early days, the hard rock miner would be digging away following narrow veins of precious metal containing ore. He would fill his cart with the high-grade ore and any necessary surrounding rock. Once outside the mine, he would hand-sort the contents.  The high-grade would be set safely aside and the waste rock would be dumped on the ground. The high-grade would later be sent directly to a smelter.  

It was not economically feasible to ship everything to a mill for processing. Flotation milling technology didn’t become mainstream until the 1930s. An elaborate transportation infrastructure did not exist then. Many of the roads were toll roads. I once saw an old toll schedule that charged $1.50 for one-way passage of a two-mule drawn cart. In the late 1800s, the price of gold was about $20 per troy ounce. You had better have some high-grade in tow!  

That’s how many of those piles ended up all over the place, more properly defined as waste rock piles. So where does the acid runoff come from?  

Much of the underground ore within our mining districts is composed of metallic sulfide minerals. These minerals contain varying proportions of iron, copper, lead, zinc, precious metals, other trace heavy metals and sulfur. For instance, iron sulfide is very prevalent. We commonly know its name as pyrite or “fool’s gold.” These minerals were injected into faults and fractures of the surrounding rock by high temperature, high pressure hydrothermal vents from deep within the earth.

Natural weathering of rainwater draining into the ground and the reaction with air break down these metallic sulfides. As these minerals weather, the sulfur component reacts with the water and air, creating dilute acid. In an acidic solution, many of the heavy metals are liberated and become suspended. The combination of this acid and heavy metal solution is what can wreak havoc in our area’s surface and groundwaters.  

But wait one minute, I have not yet mentioned a miner’s activity in the foregoing two paragraphs. This natural weathering has gone on for tens of thousands of years . . . way before George Jackson’s “discovery” of 1859. This acid solution has leached into the waters and affected the environment through this natural phenomenon for a very long time.

It was the unregulated mining industry of the late 19th and early 20th centuries and lack of technology that greatly exacerbated this weathering cycle. By constructing tunnels, adits and shafts, the waters were more readily exposed to this highly mineralized ore and conveyed out into the surface waters. Waste rock piles on the surface were exposed to more rapid natural weathering, air and rain. There were no provisions then to control and treat drainage waters from the mines or stormwater runoff.

As a society, now we are better educated and have learned from past mistakes. The Colorado Division of Reclamation, Mining and Safety is responsible for regulating mining. It incorporates a vast framework of protective regulations designed to safeguard our waters, air and lands.

As an engineer applying for a permit, I have found the process arduous and never-ending at times. But it is understood that it is of utmost importance to ensure that proper safeguards are in place to prevent repeating the problems of the past. Performance bonds are now required so that money is available for proper cleanup and reclamation.

Next week we will look at how these old waste rock piles can be cleaned up and bring jobs and business back to Clear Creek County.

Ryan J. McHale is an engineer and Clear Creek County resident.


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