What is acid mine drainage?Acid Mine Drainage is one of the main environmental problems found at the

What is acid mine drainage?Acid Mine Drainage is one of the main environmental problems found at the Earth’s surface. It is triggered by exposed rocks from old, inactive mines, and billions of dollars are spent yearly managing it. One example is seen here, in this photo from an old mine (and now Mushroom farm) in Ohio, but what is it that has turned this water red? Our story begins, like many on Earth, 4.56 billion years ago. When the Earth formed, its atmosphere was chemically in equilibrium with the rocks in the mantle. Oxygen is the most abundant element in the mantle, but all that oxygen is taken up bonding with elements like silicon, magnesium, and iron. There was so much of these elements that there was no oxygen left in the atmosphere – the planet’s atmosphere started off reducing, filled with gases like methane that would burn in today’s atmosphere.2.5 billion years ago, the atmosphere changed. Oxygen built up in the atmosphere after life developed photosynthesis, but this oxygen didn’t reach the mantle. Today, the inside of the planet is out of equilibrium with the atmosphere; rocks from the mantle brought to the surface will chemically react with the atmosphere, giving off energy.An atmosphere and a planet out of equilibrium with each other is what drives acid mine drainage. Rocks that contain ores like gold and silver are formed out of the mantle, distilled and concentrated by magmatic processes. However, they’re still in the ground, still in equilibrium with the mantle.When those rocks are exposed to the surface, they begin to chemically react. One of the most reactive elements in mantle rocks is iron. It’s extremely common in the mantle and as anyone who has seen a piece of rusted metal can tell you, when it is exposed at the surface it iron will chemically react with the atmosphere to form rust.In rocks such as those here in Eastern Ohio much of the iron is contained within the mineral pyrite, which is commonly known as fools gold. The chemical formula of Pyrite is FeS2, so it has iron in the structure. That mineral is extremely common on Earth; it’s found in all sorts of ore bodies, coal layers, and other rocks that are mined. Pyrite is stable when surrounded by other rocks in the crust, but when it is exposed to oxygen and water, it starts to react.In other words, when we bring pyrite up to the surface, we cause it to react. It’s happy when its buried, but pyrite plus oxygen causes a chemical reaction.This reaction uses oxygen and releases sulfuric acid. In the process, it oxidizes the iron – basically the iron rusts. Rusted iron does not dissolve easily in water; instead it precipitates to form tiny grains of solid minerals. These mineral grains can be carried downstream by the water in suspension or settle out onto the bottom of rivers, coating rocks and plants in the process. These minerals are commonly termed “Yellow boy” and they are the source of the mustard color seen in this spot.Although the iron is the most easily recognized product of acid mine drainage due to its distinct color, it’s not the worst part of this chemical brew. Iron can kill life in streams by coating it and eliminating plants or food sources, but it doesn’t easily poison life, and you’re not likely to drink a glass of yellow colored water. Instead, it’s the other parts of acid mine drainage that are the real worry, at least for humansPyrite reacts more easily with acidic water, so the acid generated by pyrite weathering makes the pyrite weather even faster. Acid mine drainage is a runaway process that can make water extremely acidic; pH values as low as -3.6 (that’s a pH of negative 3) have been measured in a California mine (when the pH gets too acidic, the activity coefficient of that ion explodes, making pH’s drop to an extreme level).Extremely acidic water can dissolve all sorts of other nasty compounds. Some compounds like arsenic can be contained within the pyrite, others can be contained within the surrounding rocks, but when they touch highly acidic water they dissolve easily. These other compounds are extremely worrisome as they can harm humans if they are concentrated enough.The exact chemistry of the water depends on the chemistry of the exposed rocks, but huge amounts of metal can dissolve in waters this acidic. Arsenic, copper, zinc, cadmium, nickel, lead, uranium, and so on can be found in these waters. Obviously people won’t directly drink mustard-colored water, but the dissolved compounds can leak into groundwater and contaminate large areas where the color isn’t obvious.Acid mine drainage is a huge problem worldwide. Pyrite is extremely common, found in igneous, metamorphic, and sedimentary settings, and it is directly associated with many rock types that humans want to use. There are tens of thousands of mines in the U.S. alone that expose pyrite at the surface in the mine waste. They are a legacy of centuries of mining worldwide.There’s a very good chance that right now there are mines leaking acidic water into the streams that feed whatever streams flow near you. Most water treatment systems are designed to take those pollutants out of the water and those compounds are diluted by water from fresher streams before they reach you, but it is a serious problem worldwide, it costs a lot of money to manage and treat, and often in low income or disadvantaged communities, the waste isn’t even treated at all.-JBBImage credit: https://commons.wikimedia.org/wiki/File:Ohio_Valley_Mushroom_Farm,_Acid-Mine_Drainage_(AMD)_(13671209554).jpgReferenceshttp://water.usgs.gov/edu/mining-waterquality.htmlhttps://www.usgs.gov/mission-areas/water-resources/science/mine-drainagehttp://digitalcommons.unl.edu/usgsstaffpub/479/ -- source link
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