What is Acid Rock Drainage?
Acid rock drainage makes streams and soils acidic - fish can not live in the water and plants can not grow in the soil. Acid rock drainage occurs naturally as it does in Red Dog Creek in the western Brooks Range. But acidic soils and water can also form from improperly disposed waste rock and tailings from metal mining operations. The acidic orange soils and red creek in the photograph are the likely the result of acid rock drainage from historic mining operations dating back to the Roman times near the Rio Tinto river in Spain.
Acid rock drainage can form wherever there are sulfide minerals - whether or not there is mining. Sulfide minerals are combinations of metals with sulfur. The dark but sparkly minerals in the rock on the left are sulfide minerals.
The most common sulfide mineral in the world is pyrite. Pyrite occurs in many rocks and it commonly accompanies other sulfide minerals in metal deposits. Pyrite combines iron and sulfur. This photograph is of pyrite crystals. The cubic shapes and brassy yellow color are distinctive. Where pyrite occurs in small pieces as in sand along a gravel bar, the brassy yellow color makes some people think it is gold – hence the nickname "Fools Gold". The problem with pyrite is that it is the main cause of acid rock drainage.
How does Acid Rock Drainage Occur?
When pyrite comes in contact with oxygen in air or water it changes or "oxidizes". The iron turns into rusty material and the sulfur turns into sulfuric acid. If the sulfuric acid mixes with water in the ground or in a nearby stream, acid rock drainage can begin to occur.
How do we prevent Acid Rock Drainage?
Preventing oxygen from contacting pyrite is very important to preventing or controlling acid rock drainage.
It is common for waste rocks and tailings to contain pyrite - so properly disposing of them involves techniques to isolate the pyrite in the waste material from oxygen in air or water. This example is from some reclaimed mine lands in Utah. Specially designed and constructed soil layers can inhibit oxygen from contacting underlying pyrite-bearing rocks. The layers can include barriers or compacted material that inhibits infiltration of water. By adding material like limestone to the layer above the pyrite, the acid can be neutralized. In addition to the covering layers, the land surface can be shaped so that water runs off easily and does not soak in.
If the pyrite-bearing material is in a low area, another way to prevent reaction with oxygen is to flood the material with water. These photographs shows a tailings area before and after flooding. There's only a few places this can be done but it works very well - and creates excellent wildlife habitat in the process.
Images Courtesy United States Geological Survey; Image source: Earth Science World Image Bank