Visual & Sensory Characteristics of Acid Mine Drainage
Acid mine drainage forms from the oxidation of pyrite. Pyrite is an iron sulfide, FeS 2, commonly called fool’s gold. Pyrite goes through a series of reactions when it is exposed to the environment, which can occur when land is disturbed. When exposed to oxygen and water, pyrite forms yellow and white salts of iron sulfate and sulfuric acid. As these rocks are flushed by rainfall or snowmelt, the salts and acids are released in the water and enter the streams. In the streams, the iron and other metals may remain dissolved in the water or they may fall out (precipitate) as solids in the stream, causing the common yellow-orange staining color and the sterile look, since few organisms can survive in these streams.
The most obvious visual sign of AMD is the yellow-orange color of the rocks and sediments in the stream. These colors mean that iron, aluminum, and other metals are precipitating in the stream and coating the rocks and streambed with “yellowboy,” a common term for iron hydroxide. This material clogs the spaces between rocks where macroinvertebrates live, making it hard or impossible for them to survive.
Another way to tell if a stream is impacted by AMD is if the water has a rotten egg smell. If someone has had occasion to taste the water (which is not recommended), the water may have a bitter taste, and may perhaps produce a slight stinging sensation, which is due to the low pH conditions of the water. In all AMD, a metallic taste is very common, which comes from the iron and other metals in the water. These senses (smell and taste) should only be used as secondary signs.
Another visual sign is the amount of biological life in the stream. Many AMD-impacted streams have some life, but those that are more severely impacted (especially those with clear water) will have no sign of life or only green algae growing in them. Humans must be especially careful about those AMD-affected streams that are clear. If they do not have the common visual signs with yellowboy staining on the rocks, then water pH may be very low. These streams are clear because the iron and other metals remain completely dissolved in the acid water.
Household sewage is also a common pollutant of streams. Sewage in a stream may not be as easy to distinguish as AMD, and the pipes or other sources of sewage to a stream may not be obvious. Visual symptoms of sewage-impacted streams are dark-colored rocks on the streambed (organic matter), abundant algal growth, a sewage smell, laundry-type suds, and certain types of trash in the stream. People working in streams where sewage may be present should wear rubber gloves (much preferred and highly recommended) or wash hands thoroughly at the first opportunity after contact with the water. If rubber gloves are not worn, contact between hands and face must be avoided. Rubber gloves must be worn by those with open cuts on hands or arms.