Acid Mine Drainage - AMD

The products of AMD formation, acidity and iron, can devastate water resources by lowering the pH and coating stream bottoms with iron hydroxide, forming the familiar orange colored "yellow boy" common in areas with abandoned mine drainage.

Many areas also contain naturally occurring limestone (CaCO3) deposits which neutralizes acidity. To determine whether or not a mine will create acidic drainage, coal companies must analyze how much pyrite and neutralizers are in the rocks which will be disturbed by mining. Then DEP can determine whether or not a site can be mined without harming the environment.

By law, DEP cannot issue a permit for new coal mining where it is determined mining will cause acid mine drainage.

As acidity increases, fewer and fewer living things can tolerate the harsh conditions. And the corrosive acid also attacks culverts and bridge abutments, resulting in a shorter than normal life span for exposed infrastructure.

Small amounts of AMD can harm the life in streams because the metals, sulfates and/or other suspended solids drop out of the water and coat the rocks and gravel on the stream bottom. When this happens, the insects that live on and under the rocks literally are smothered because they cannot get oxygen out of the water. And if the aquatic insects die, the fish have little or no food.

Most of the pollution, by far, comes from old mining that took place before the 1977 law, the Surface Mining Control and Reclamation Act or SMCRA. Occasionally, current mine operators have a problem that results in a fish kill or an orange stream, but SMCRA holds them accountable -- they pay fines and must quickly remedy such situations today.

When water comes into contact with pyrite in coal and the rock surrounding it (the overburden), chemical reactions take place which cause the water to gain acidity and to pick up in solution iron, aluminum, manganese and other minerals. Precipitation of these minerals gives the water its tell-tale color.

In deep mines, the metals are dissolved in the water and stay in solution beneath the earth due to the lack of oxygen. When water emerges from the mine through a shaft, bore hole or natural cracks in the earth, it reacts with the oxygen once it hits the air. When that happens, chemical reactions take place that bring the minerals out of solution and they precipitate in the stream, leaving deposits of iron, manganese and aluminum on rocks and the stream bed.

SCRIP studies identified 467 separate sources of AMD in the Stonycreek and Upper Conemaugh watershed. These sources range from as little as 1 gallon per minute (gpm) to as much as 4500 gpm. Many different concentrations and combinations of contaminants are present and depend on the characteristics of the coal and the overburden through which the water flows.

The affect of AMD on local streams varies with the size of the stream and the total pollution load put on the stream. The pollution load is the product of the concentration of contaminants and the stream's flow. AMD has eliminated fish completely from the major portions of the Conemaugh and Little Conemaugh Rivers. The upper reaches of the Stonycreek River are considered to be one of the best reclaimed trout fisheries in America, and that fishery has recently been extended approximately seven miles downstream because of AMD treatment methods, which have been used in Oven Run and other SCRIP project.

Recent research has shown that mine drainage also has many other metals in lower concentrations, including some relatively high priced metals such as strontium and magnesium. Resource Recovery efforts are under way to recover and sell iron and aluminum from mine drainage because of their higher concentrations, but the idea of recovering other metals has been suggested.

One important way that people can help our local effort is to join The Stream Team, which monitors the pollution levels on a quarterly basis.

PASSIVE TREATMENT: Since the early 1990s, passive treatment systems have been developed that to treat AMD with only periodic maintenance, which greatly reduces long term costs. However, these systems have some drawbacks. In particular, they use a lot more land. Often there is not enough land available to treat large discharges or discharges that come out on stream banks or in communities. That's why we've pursued new approaches to big, killer discharges, such as Resource Recovery, trying to recover value from the pollution to offset treatment costs.
But where sufficient land is available and the landowners are cooperative, passive systems have generally worked well if the appropriate system is designed based on the chemistry and flow of the individual dishcarge.

Some discharges are alkaline or near alkaline, in which case the treatment system only needs to remove metals. With many discharges, two things need to be done: acidity needs to be neutralized and metals need to precipitate. Passive systems typically have separate components for each function.

A brief description of the passive systems most commonly used in the Bituminous coal fields of Pennsylvania follows, along with links to sites with more information.
 

Systems to Add Alkalinity

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• Open Limestone Channels or Limestone Drains: These are simply open ditches filled with limestone. As the AMD flows over the limestone, the stone dissolves, which produces alkalinity to increase pH. These systems generally only work with low flows with a long distance to the nearest receiving stream because, as iron and aluminum precipitate from the AMD, the stone get coated or armored by the metals and then dissolve at a slower rate.

• Anoxic Limestone Drains (ALD): Limestone is buried in trenches. As the AMD flows through, the limestone dissolves, alkalinity is added and pH is increased. To prevent the limestone from becoming coated or armored with precipitated iron or other metals, the AMD must have very little or no oxygen. Deep mine discharges often have no oxygen, so the water can be channeled directly into the drain, which is covered with clay and/or plastic liners so no oxygen gets in. If the AMD already has oxygen, or if a series of ALD is used (see SAPS below), then the water must be put through an anaerobic wetland in which organic material removes the oxygen, after which the water is channeled into the ALD.

• Diversion Wells:the stream or a portion of the stream is diverted into a well or tall container filled with crushed limestone. The fast flow churns the limestone, preventing iron or other metals from coating or armoring the limestone. The limestone needs to be replaced more frequently than with drains, and the AMD contaminants drop out in the stream, not in settling ponds or wetlands.

• Limestone Sand: Fine particles of limestone are placed on stream banks or in the stream itself. This low-tech approach has its limits: the limestone sand washes away and needs to be replaced often, usually at least once a year; this works only with low metals concentrations, and the metals still drop out in the stream, although they tend to drop out much faster and impact a smaller part of the stream.

Systems to Remove Metals (For more information on this topic, please visit: Removing Metals )

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• Aerobic Wetlands: These typical wetlands with 6 to 18 inches of water, cattails and other wetland vegetation remove metals through oxidation reactions that form oxides or hydroxides. However, they work best if the in-flowing AMD has plenty of oxygen and the pH is at least 5.5 (fairly close no neutral). The cattails and microbes in the water also take in some metals, and the slow rate at which water moves through the wetlands allows time for metals and sediments to drop out.

• Compost Wetlands or Anaerobic Wetlands: These wetlands look like regular wetlands with cattails and other typical vegetation atop the water, but they have a layer or organic material along with some limestone underneath. The AMD slowly flows down through the organic material, which can be mushroom compost, peat moss, wood chips, sawdust or even hay. Microbes in the organic layer reduce sulfates, and the substrate promotes chemical and microbe actions that generate alkalinity and increase pH. In the SAPS system described below, compost wetlands are used to remove oxygen before mine drainage flows into ALDs.

• SAPS or Successive Alkalinity Producing System: also called Vertical Flow Wetlands or Vertical Flow Reactors (VFR): These systems combine a compost wetland with an anoxic limestone drain. As the water flows through the wetland's compost, oxygen is removed so the metals do not precipitate in the drain, which would armor or coat the limestone.

• Aluminator (C): This is basically a limestone drain "designed to fail." It works only with high-aluminum discharges. The idea is to encourage aluminum to precipitate in the drain, but design the drain so that it can easily be backflushed to remove the aluminum. The drains need to be flushing as often as twice a month, but this system is showing promise as a method of Resource Recovery with the goal of recovering and selling the aluminum to offset treatment costs.

• Pyrolusite (R) Process: This process uses microbes that are grown in a laboratory for the specific discharge. The microbes remove iron, manganese and aluminum through oxydation, and at the same time etch away limestone to generate alkalinity. The process is patented, but has shown promising results.

• Ponds: As their name suggests, these are rather simple but can be very important in serving three functions. In the front of a treatment system, they slow down the discharge flow, allowing dirt and organic material -- twigs, leaves, typical forest debris -- to settle out before it gets into other system components; typically, some metals will begin to precipitate in the settling ponds. In many systems, oxydation ponds are constructed after limestone drains to keep the water moving very slowly, allowing time for metals to oxydize and precipitate out. And "polishing" ponds often used at the end of treatment systems to allow extra time for metals to drop out, "polishing" the water before it is discharged to the streams.

• Resource Recovery:Our efforts to recover value from AMD to offset treatments costs, is making progress in not only removing metals but selling them to offset treatment costs.

Science of Acid Mine Drainage and Passive Treatment, an excellent although fairly technical overview by the state Department of Environmental Protection.

Abandoned Mine Reclamation Clearinghouse of the Western Pennsylvania Coalition for Abandoned Mine Reclamation.

Some Passive and Semi-Passive Treatment Methods for Acide Mine Drainage, the site of Carl S. Kirby, Bucknell University Associate Professor of Geology.

Resource Recovery is the concept of recovering value from acid mine drainage (AMD).

Pennsylvania needs an estimated $5 billion to address its AMD problems, but all funding sources combined provide only about $5 million a year. Resource recovery can help to shift the financial balance.

And just as important, many discharges come out on stream banks, on steep slopes or in communities, where there is not enough flat land available for passive treatment. Resource Recovery may enable us to afford active treatment methods that use far less land.
assive methods have not been demonstrated to work effectively with very large flows -- the so-called "killer discharges" that do the most damage to our streams and rivers.

The pressure is growing, too, as several recent bankruptcies of coal companies and steel companies that had coal subsidiaries have resulted in bond forfeitures, leaving the Commonwealth of Pennsylvania to try to figure out how to continue treatment or else let the AMD damage our waterways.

• Offset treatment costs by bringing in revenues.
• Treat discharges that otherwise cannot be treated.

• Clean up watersheds that otherwise will not be cleaned up.

Value? What Value?

• We can recover and sell the metals or minerals. For instance, iron oxide from mine drainage is being sold to a paint company as pigment.

 
• The clean water has an ever increasing value. There are proposals to use AMD as the source of water for a pump storage electric generation facility and as coolant for a co-generation plant that would produce electricity from old coal refuse or bony piles. Likewise, in Tennessee, there is a proposal to treat a mine discharge and build a lake that potentially could become a state park. And the AMD & Art project used clean water from the end of its Vintondale treatment system to make additional wetlands, which were sold to PennDOT as mitigation for wetlands taken in nearby highway projects.

 
• AMD sludge has potential uses as agricultural micro nutrients and in certain environmental processes because heavier metals adsorb to charged iron particles in the AMD.

• There are proposals to make bricks or other building materials from AMD sludge.

Unfortunately, the biggest "value" of all is one for which we cannot derive a revenue stream: the "value" of the restored waterways that now support fishing, boating, wildlife, fish and quality of life in nearby communities.