Runoff from abandoned mines has a unique set of problems associated with it. For example, coal mine drainage has high concentrations of metals, elevated sulfate levels, low pH, and excessive suspended solids. The following BMPs are associated with acid runoff from abandoned mines. There are many abandoned coal mines in the Appalachian region, including Eastern Tennessee. This information is adapted from the EPA publication, “A Citizen’s Handbook to Address Contaminated Coal Mine Drainage” (EPA 903-K-97-003) and addresses the problems of coal mine drainage (CMD).
There are two types of treatment systems, active and passive. Active treatment systems use strong alkaline chemicals such as lime, caustic soda, ammonia and calcium oxide to neutralize acid so that metals can be precipitated and removed. There are six chemical reagents that may be used to treat contaminated mine drainage in active treatment systems. Which chemical is used will depend upon technical and economic factors. They are:
Passive treatment systems remove or neutralize contaminants in CMD by exposing them to air, limestone vegetation in ponds, neutralizing ditches, buried channels, and wetlands. Exposing CMD to air helps to precipitate metals through oxidation processes, limestone contact neutralizes acid by adding alkalinity, vegetation such as cattails filters contaminants and aids in oxidation and metals removal, and organic wetlands remove metals and provide habitat for bacteria that remove sulfates.
Since passive systems are designed to make use of gravity flow through ponds, buried channels, ditches, and wetlands, they can treat drainage without the continual addition of chemicals or neutralizing agents. In general, larger ponds are used to collect mine drainage, settle out larger particles of sediment, oxidize metals, and can reduce acidity when underlain with organic material and limestone. Ditches convey limestone rock for acid reduction. Wetlands serve a variety of treatment functions, including filtration of smaller sediment particles, uptake of water and some contaminants, oxidation/adsorption of metals, and removal of sulfates through bacterial action.
Aerobic Wetlands are used for low-acid CMD to collect flows, settle out sediments and provide residence time so that metals in the water can precipitate. Cattails and other wetland vegetation are planted in an aerobic wetlands substrate to promote the uptake of water and small quantities of metals and other contaminants.
Anaerobic Wetlands are used to treat CMD that is higher in acidity or sulfate concentrations. Anaerobic wetlands are similar to aerobic wetlands in appearance however, they are underlain with organic muck (substrate) and a layer of limestone.
Anoxic Limestone Drains (ALDs) are buried trenches or channels containing crushed limestone into which acidic CMD is channeled. As the CMD flows through, the limestone is dissolved, alkalinity is added and pH is increased. The channels are covered to reduce or eliminate the presence of oxygen, the elimination of oxygen prevents the development of iron oxide coating (armor) on the limestone.
Alkalinity Producing Systems and Successive Alkalinity Producing Systems combine the use of ALDs and anaerobic wetlands. Elevated dissolved oxygen concentrations are often a design limitation for ALDs. If dissolved oxygen concentrations are above 1 or 2 mg/L, the water can be collected in a pond underlain with drainage pipes, which are covered by limestone and capped off with organic material.
Limestone Ponds are a new passive treatment approach in which a pond is constructed on the upwelling of an acid mine drain seep or underground water discharge point. Limestone is placed in the bottom of the pond and the contaminated water flows upward through the neutralizing limestone.
Reverse Alkalinity Producing Systems are similar to limestone ponds in design and installation, but are more efficient in removing metals.
Open Limestone Channels can be used to increase alkalinity and reduce acid, but armoring reactions promoted by contact with the air will reduce their effectiveness somewhat.