Definition: Abandoned mine drainage (AMD) is caused by the flow of water through abandoned coal mine environments such as surface mines, deep mines, or coal refuse piles. When surface water or ground water comes into contact with an iron-sulfide mineral such as pyrite (or fool's gold) that is found in the unreclaimed waste rock or leftover coal strata, a chemical reaction occurs in the presence of oxygen to create sulfuric acid and iron hydroxide. Sulfuric acid causes the low pH values that are typical of AMD impacted streams, particularly in streams that do not have a good acid neutralizing capacity or natural alkaline buffering system. These acidic waters act as a leaching agent that dissolves additional metals such as aluminum and manganese, which are commonly found in clays and soils. Iron hydroxide is the cause for the yellow-orange to red staining that is often observed in AMD-impacted streams. Aluminum precipitate will stain the streambed white and black staining is indicative of manganese precipitate.
Actual Ecosystem Stress: AMD impacts the aquatic ecosystem in a number of ways. Metal precipitates (i.e. aluminum, iron) may clog the gills of macroinvertebrates and fish and create a slippery surface on the stream bottom, which prevents the macroinvertebrates from attaching to the substrate. Low pH can be directly lethal to aquatic organisms because it may result in respiratory or osmoregulatory failure and can create ionic imbalances. Low pH also adversely affects the growth rates and reproduction of fish. Although brook trout are tolerant of pH as low as 5.0, their tolerance is rapidly decreased when a low pH is present in combination with a heavy metal such as aluminum. Of the three main heavy metals typically found in AMD - aluminum, iron, and manganese - aluminum is the most toxic to aquatic life.
Measures to Mitigate AMD: Successfully addressing AMD pollution requires a thorough assessment to answer questions such as, but not limited to, 1) What is the source of the AMD - is it a deep mine, unreclaimed or poorly reclaimed surface mine, refuse pile, or a combination of the above? 2) What is the average flow of the discharge and what type of chemistry does it have? 3) What are the impacts to the receiving stream(s)? In most instances, the AMD should be monitored on at least a monthly basis for a minimum of one year to establish a solid baseline for flow and chemistry. Simply put, good monitoring data results in a much better chance for a successful remediation strategy.
There is no one cookbook method to mitigate AMD because each individual source of AMD is characterized by a complex combination of influences from geology, hydrology, chemistry, hydrogeology, biology, and the method of mining used to extract the coal in the first place. Some of the more common approaches used are collectively referred to as passive treatment methods or active treatment methods. Passive treatment typically incorporates a combination of components such as wetlands, limestone-filled channels or ponds, and ponds containing both limestone and organic compost. Active treatment methods require the addition of a chemical or other acid-neutralizing material to the AMD-impacted water. Reclamation, which is sometimes done in combination with remining of the remaining coal reserves, is another mitigation alternative.