Stephanie Nebel

Geology 206

March 4, 2005

 

Acid mine drainage: causes, effects, and mitigation techniques

Abstract

            Acid mine drainage, formed from the breaking down of the mineral pyrite, has serious repercussions for both ecosystems and watersheds.  Pyrite occurs naturally and is often found around coal seams.  The mining of coal, a major energy resource, exposes this pyrite to both oxygen and water.  Such exposure causes pyrite form sulfuric acid and yellowboy, a rust colored precipitate that is detrimental to stream ecosystems.  Pennsylvania and West Virginia are particularly susceptible to acid mine drainage because of recent coal mining activity.  Therefore, several techniques have been developed and applied in order to restore watersheds and ecosystems.

Introduction

            Acid mine drainage is found seeping from the outlets of abandoned coal mines and from coal waste.  This is particularly the case in Pennsylvania and West Virginia, which has a high concentration of both anthracite and bituminous coal.  Coal mining activity in the mid Atlantic region had been high, both presently and in the past. 

AMD poses a serious environmental threat, particularly to the watersheds nearby an abandoned or active coal mine.  The pH in streams and rivers affected by AMD is often significantly lower than normal.  This in effect causes plants and animals that live in and around a stream to die.  Furthermore, dissolved metals such as iron and aluminum seep into streams and can sometimes contaminate the groundwater.  These dissolved metals, if found in high concentrations, are highly toxic.  Dissolved iron in combination with hydroxide ions (which forms from water) produces an orange rust colored precipitate (Figure 1)

Figure 1: A stream affected by the iron hydroxide precipitate yellowboy. From: http://www.dep.state.pa.us/dep/deputate/minres/bamr/amd/science_of_amd.htm

 

that forms at the bottom of stream channels.  This precipitate, termed “yellowboy” is unsightly and degrades the natural quality of an area. 

AMD Chemistry

            AMD occurs when pyrite (FeS2) breaks down.  Pyrite is commonly found in and surrounding coal seams.  Pyrite poses no risk to the environment if it is not exposed to air and water.  The mining of a coal seam, however, allows pyrite to become uncovered.  After coal is mined, the leftover pyrite is left untreated in large piles, therefore allowing for the breaking down of this material.  The breaking down of pyrite has serious environmental implications, particularly for streams and rivers surrounding both active and inactive coal mines. 

            The breaking down of pyrite happens almost spontaneously when it is exposed at the surface.  The rate of this reaction occurs quickly, oftentimes several orders of magnitude greater than that of natural geological weathering (which can take several hundreds of thousands of years to occur).  The oxidation of Pyrite, which then becomes AMD, occurs in four steps as follows:

2 FeS2 (s) + 7 O2 (g) + 2 H2O (l) à 2 Fe­2+ (aq) + 4 SO42- (aq) + 4 H+ (aq)                     (1)

Pyrite + Oxygen + Water à Ferrous Iron (dissolved) + Sulfate (dissolved) + Acidity

            In this step of the reaction, the dissolved iron and dissolved hydrogen ions are released into the water, oftentimes causing the pH of the water to decrease.  The aqueous form of both the ferrous iron and the sulfate are colorless, thereby causing water to appear clear. 

4 Fe2+ (aq) + O2 (g) + 4 H+ (aq) à 4 Fe3+ (aq) + 2 H2O (l)                                              (2)

Ferrous Iron (dissolved) + Oxygen + Acidity à Ferric Iron (dissolved) + Water

            In this step of the reaction, ferrous iron is converted to ferric iron in the presence of oxygen.  Little quantities of oxygen are present underground, therefore this part of the reaction primarily occurs after pyrite has been brought to the surface.  H+ ions responsible for acidic waters are consumed in this process, thereby decreasing acidity slightly.  Certain forms of bacteria speed up this reaction.  Acidic (pH of 2-3) waters are incapable of sustaining such bacteria.  This acts to help to slow this part of the reaction.  The reaction occurs at rates that are several orders of magnitude faster in water with higher pH because bacteria thrive in such conditions (~5). 

4 Fe3+ (aq) + 12 H2O (l) à 4 Fe(OH)3 (s) + 12 H+ (aq)                                                   (3)

Ferric Iron (dissolved) + water à Ferric Hydroxide (termed “yellowboy”) + Acidity

            In this step of the reaction, ferric iron hydrolyzes, or splits water molecule in order to form yellowboy.  The yellowboy is an orange-yellow solid precipitate which forms on the bottom or stream beds, killing organisms which dwell in these areas.  This precipitate forms in streams where water ph is 3.5 or greater.  Acidity is further increased during this step of the reaction. 

FeS2 (s) + 14 Fe3+ (aq) + 8 H2O (l) à 15 Fe2+ (aq) + 2 SO42- (aq) + 16 H+ (aq)              (4)

Pyrite + Ferric Iron (dissolved) + Water à Ferrous Iron (dissolved) + Sulfate (dissolved)         + Acidity

            In this step of the reaction, pyrite is further oxidized by the dissolved ferric iron, forming more ferrous iron.  Iron becomes the oxidizing agent, causing the reaction to cycle until either the ferric iron or pyrite is completely used up.  This reaction occurs very rapidly.

Overall, this series or reactions can be summarized as follows:

4 FeS2 + 15 O2 + 14 H2O à 4 Fe(OH)­3 + 8 H2SO4

Pyrite (s) + Oxygen (g) + Water (l) à Yellowboy (s) + Sulfuric Acid (aq)

Environmental Effects

            Presently, AMD is the main source of water contamination in the mid-Atlantic region (www.epa.gov).  Furthermore, more than 4,500 miles of stream are contaminated in the mid-Atlantic (figure 2) (www.epa.gov).  Mines built after the 1800’s used a technique termed gravity drainage.  Such a system allowed water to effectively be drained from the mines by preventing water buildup.  However, as a result, much of the AMD contaminated water are free to flow into the waterways. 

           

           

Figure 2: Map depicting the streams and rivers affected by AMD (in red) in the Mid-Atlantic Region.  From: www.epa.gov

 

There are several major problems which stem from AMD.  First, waters with a low pH (high acidity) form when waste from mines comes in contact with water.  Acid concentrations in water affected by AMD can be 10,000 times higher than that of waters not affected by AMD.  This acidic water has the potential to leach toxic metals (such as aluminum, manganese, iron and other heavy metals) from the rocks around a mine site.  Acidic water, after entering the watershed, has the potential to corrode metal structures (bridges, pipes, ect.), decompose concrete, and devastate the surrounding ecology.  Metal ions that wash into streams act as suspended sediments and are toxic to fish and plants. Furthermore, the yellowboy precipitate obstructs fish gills, killing fish populations. 

            Soil erosion is prevalent at abandoned mine sites.  Therefore, soils and small sediments such as silt can be easily washed into stream beds after heavy storms.  This also has a significant effect on organisms living in streams.  Such sediments act to obstruct gills and cover fish eggs that are found at the bottom of a stream bed.  Such high sedimentation increases the cost of water treatment.  In certain regions, limestone (CaCO3) is present in the surficial geology.  When acidic water produced by AMD comes in contact with this limestone, it is neutralized, and in some cases, the water becomes alkaline.  The acidity in the water is neutralized due to this alkalinity.

Treatment options

            AMD can be combated through several chemical, physical and biological methods.  These various methods are employed in order to isolate, neutralize stabilize and remove AMD contamination.  AMD can be targeted by active of passive method. 

            Active methods involve the addition of materials which are alkaline in nature.  Such materials include lime, soda ash and ammonia.  This added alkalinity causes a decrease in waters acidity which further aides in the removal of dissolved metals. 

            Passive techniques require the building of a structure, which include retention ponds, ditches in which acidity is neutralized and buried channels.  They further attempt to expose the polluted water in air in order to oxidize the dissolved metals.  Gravity flow is a major component of passive systems.  Such a process is achieved through the building of ponds, wetlands, ditches and buried channels and required little use of water neutralizing chemicals.

            Neutralizing Acid

            The addition of certain chemicals such as crushed limestone efficiently increases pH thereby neutralizing acidic waters.  Consultants must oftentimes be called upon to determine how much of a particular neutralizing chemical should be added to an effected area.  Crushed limestone is most widely used because it is the safest and most inexpensive way to neutralize the acid. 

            Anoxic Limestone Drains

            AMD can be diverted through a buried culvert or trench which contains crushed limestone.  This is another effective way to heighten pH of AMD waters.  In this case, it important that the water not be exposed to oxygen during the treatment process.  If exposed to oxygen, iron oxide will precipitate from solution and encrust the limestone, thereby decreasing the limestones’ effectiveness on polluted waters.  An anoxic limestone drain is pictured in figure 3.

Figure 3:  Pictoral representation of an anoxic limestone drain. 

From: http://www.dep.state.pa.us/dep/deputate/minres/bamr/amd/science_of_amd.htm

           

 

Diversion Wells

            A diversion well is another technique utilized in order to increase pH.  In such a system, AMD effected waters are forced through a vertical pipe from upstream to downstream.  Downstream, the water is forced through a well which contains crushed limestone.  The drop in elevation created by the pipe causes the limestone to further crush itself, thereby further increasing acid neutralization.  Water then flows back into the stream.  The stream bed becomes lined with large limestone chunks which further aides in the neutralization process.  On disadvantage to such a system is that limestone needs to be continuously replenished (every two weeks).  Such a system is most effective in areas where there is a low ph and low dissolved metal content. 

            Aerobic Wetlands

            Aerobic wetlands (figure 4) are constructed so that suspended metals have the

Figure 4: Cross section of an Aerobic Wetland

From: http://www.dep.state.pa.us/dep/deputate/minres/bamr/amd/science_of_amd.htm

 

opportunity to settle out of solutions.  Such a technique is only an option in waters with high pH.  Peak effectiveness occurs in water about 6-18 inched deep.  Vegetation is planted in and around such areas.  Aerobic wetlands further have a large surface area, meaning that there is high dissolved oxygen content.  Ideally, water should be oxygenated through some other method before entering the pond.  Oxidation reactions form oxides and hydroxides.  Planed vegetation consumes water and small concentrations of metals.  Microbes, both water and soil borne, are also used in order to decrease dissolved metal content. 

            Anaerobic Wetlands

            Anaerobic wetlands (Figure 5) are utilized in areas with low pH and high sulfate

Figure 5: Cross section of an Anaerobic Wetland.  The Limestone layer, which underlies the organic material is not shown.

From: http://www.dep.state.pa.us/dep/deputate/minres/bamr/amd/science_of_amd.htm

 

concentrations.  Anaerobic wetlands are constructed with a layer of limestone and organic material lining the bottom.  As the name suggests, dissolved oxygen content in these wetlands is not very high.

            Successive Alkalinity Producing Systems

            In successive alkalinity producing systems, two AMD remediation techniques are utilized, anoxic limestone drains and anaerobic wetlands.  Dissolved oxygen concentrations must be above 1-2 mg/L in order to use this technique.  If such concentrations of oxygen are found in an area which acidic waters flow, a wetland can be constructed.  Limestone lines the bottom of such a pond.  The top of the pond is covered with organic materials.  Before water enters this wetland, it is first diverted through an anoxic limestone drain in order to elevate pH levels.

            Vertical Flow Reactors

            Use of vertical flow reactors (figure 6) is a fairly new technology.  In such a

Figure 6:  Cross-sectional diagram of a vertical flow reactor. 

From: http://www.dep.state.pa.us/dep/deputate/minres/bamr/amd/science_of_amd.htm

 

system, a channel is constructed, which is, at the bottom, covered in limestone.  It is then sealed on top with a layer of organic material.  Water first flows through the top organic layer and then through the bottom limestone layer.  It is then forced through pipes that underlay the limestone layer.  The pH is lowered by the limestone.  Water is then collected in and anaerobic wetland where metals can precipitate out of solution.  They then settle to the bottom of the collections.   Vertical flow reactors can be constructed in series to treat highly acidic waters. 

            Limestone Channels

            The construction of a limestone channel requires a large amount of limestone, however, it is a more simple technique that is often utilized.  Limestone channels can be constructed in two ways.  The first method involved dropping limestone directly into the affected stream, while the second method mandates that a parallel channel, through which acidic waters can flow, be built.  Limestone often becomes encrusted in an iron oxide precipitate thereby decreasing the effectiveness of the limestone.   

            Sulfate reducing bioreactors

            Microorganisms are used to convert pollutants to compounds which are less harmful.  These microorganisms reduce the suspended materials and target metal sulfides through precipitation.  This sulfate reduction treats heavy metal pollution and also raises pH levels. 

            Limestone Sands

            Small sand sized particles of limestone are added to small streams. 

Much of this limestone is dissolved by acidity in the water.  Occasional flooding is advantageous because it further aides in limestone movement.  The technique requires the continual replenishment of limestone sand and is most effective in waters with low ph and low levels of dissolved metals.

            Metal Reduction

            The most effective way to passively reduce metal concentrations is through constructions of both ponds and anaerobic/aerobic wetlands.  In such situations metals are oxidized and precipitate out of solution.

            Settling Ponds

            Settling ponds are similar to anaerobic/aerobic wetlands in that they promote the precipitations of suspended solids.  Settling ponds offer a place of clam where this can happen.  Solids such as the yellowboy precipitate collect at the bottom of such ponds.  Such ponds much be dredged occasionally.  However, water that exits such a pond is noticeably cleaner than when it entered.  Furthermore, oxygen is present in such pond environments because of a large surface area.  Ferrous (Fe2+) ions tend not to precipate out of solutions.  Ferric (Fe3+) ions readily combine with H2O to form Yellowboy (Fe(OH)3).  Oxygen, on the other hand, will convert Fe2+ to Fe3+ so that this to can settle out of solutions. 

Summary and Conclusions

1.      Acid mine drainage poses a serious threat to ecosystems that surround both active and abandoned coal mines.  Furthermore, AMD greatly threatens nearby watersheds and had a severe impact on the natural and aesthetic quality of streams and rivers. 

2.      Acid mine drainage occurs as a result of the breaking down of pyrite (FeS2), a common mineral found in and around coal seams.  Pyrite poses a serious threat when exposed to both oxygen and water.  When broken down, the iron combines with hydroxide to form a rust colored precipitate termed yellowboy while sulfur, in the presence of water, forms sulfuric acid.

3.      Several techniques now exist which attempt to combat and mitigate the devastating effects of acid mine drainage.  These techniques attempt to restore ecosystems and the natural water quality to areas affected by AMD.

4.      Given the large amounts of coal refuse still present, acid mine drainage will continue for some time.  Therefore, treatment of areas affected must continue. 

 

 

 

 

 

 

 

 

 

 

 

 

 

References:

 

The Science of Acid Mine Drainage and Passive Treatment: http://www.dep.state.pa.us/dep/deputate/minres/bamr/amd/science_of_amd.htm, March 1, 2005

 

United States Geological Survey: www.usgs.gov, March 1, 2005.

 

The Office of Surface Mining:

http://www.osmre.gov/amdint.htm, February 28, 2005

 

Abandoned Mine Reclamation Clearinghouse:

http://www.amrclearinghouse.org, March 2, 2005

 

The Environmental Protection Agency:  www.epa.gov, March 2, 2005