INTRODUCTION Imagine going fishing on a cool Autumn day, the trees are all different shades of orange, brown and red and the birds are singing their beautiful songs, but their is a serious problem because when you arrive at the river all plant and animal life are gone. This is by no means a recent phenomenon. This is due to the effects of acid rock drainage (ARD). This is a problem that has been occurring since ancient times, but it was not until the 1800’s when fast growing industrialization and heavy mining that it caught alot of attention. Acid rock drainage is the term used to describe leachate, seepage, or drainage that has been affected by the natural oxidation of sulfur minerals contained in rock which is exposed to air and water. The major components of ARD formation are reactive sulfide minerals, oxygen, and water. Biological activity and reactions is what is responsible for the production of ARD. These reactions make low pH water that has the ability to mobilize heavy metals contained in geological materials with which it comes in contact. “ARD causes a devastating impact on the quality of the ground or surface water it discharges to. (Ellison & Hutchison)”
ACID MINE DRAINAGE
Within the mining process there are several sources that cause ARD. No matter what activities occur, ARD usually occurs when certain conditions are met. These conditions are the factors that limit or accelerate the release of ARD. The initial release of ARD can occur anywhere from a few months to many decades after the sulfide containing material is disturbed or deposited. ARD has been associated with mines since mining began. When ARD occurs due to the effects of mining it is called acid mine drainage, or AMD. The coal mining industry here in the eastern United States has been associated with a major source of AMD for decades. When water comes in contact with pyrite in coal and the rock surrounding it, chemical reactions take place which cause the water to gain acidity and to pick up iron, manganese and aluminum. Water that comes into contact with coal has a orange-red yellow and sometimes white color. The metals stay in the solution beneath the earth due to the lack of oxygen.
When the water comes out of the mine or the borehole it reacts with the oxygen in the air or some that may be deposited in the stream and deposits the iron, manganese and aluminum and deposits it on the rocks and the stream bed. Each of the chemicals in acid mine drainage is toxic to fish and aquatic insects in moderate concentrations. At real high concentrations all plant life is killed. “Underground mines that are likely to result in ARD are those where mining is located above the water table. (Kelly 1988)” Most of the mines are also located in mountainous terrain. “Underground workings usually result in a ground water table that has been lowered significantly and permanently. (Kelly 1988)” Mining also helps in the breaking of rock exposing more surface area to oxidation.
OTHER SOURCES OF ARD ARD is not necessarily confined to these mining activities. “Any process, natural or anthropogenic, that exposes sulfide- bearing rock to air and water will cause it to occur. (Ellison & Hutchison)” There are examples of natural ARD where springs produce acidic water. These are found near outcrops of sulfide-bearing rock, but not all exposing sulfide rock will result in ARD formation. “Acid drainage will not occur if sulfide minerals are nonreactive, the rock contains sufficient alkaline material to neutralize any acid produced, or the climate is arid and there is not adequate rainfall infiltration to cause leakage. (Ellison & Hutchison 1992)” CHEMISTRY “The most important factor in determining the extent of the acid mine drainage is not the pH, but the total acidity. (Ellison &Hutchison 1992)” Total acidity is a measure of the excess amount of H+ ions over other ions in the solution. A high acidity is accompanied by a low pH in AMD. This is what separates AMD from acid rain, which has a low pH and a low acidity. These differences are due to the sources of acid in different ecosystems.
A buffer, as we learned in class, “is a compound that tends to maintain the pH of a solution over a narrow range as small amounts of acid or base are added.(Rhankin 1996)” This is also a substance that can also be either an acid or a base. A low pH has a lot of bad effects on the “bicarbonate buffering system.”(Kelly 1988) At low pH solutions carbonate and bicarbonate are changed over to carbonic acid and then on to water and carbon dioxide. Because of this water looses its ability to buffer the pH of the water and plants in and around the water that use the bicarbonate in the process of photosynthesis. Another effect of low pH is the increase in the rate of the decomposition of clay minerals and carbonates, releasing toxic metals such as aluminum and silica. Ironically however, Aluminum silicates can aid in the “buffering” of pH. HEAVY METALS The presence of high concentrations of heavy metals from acid mine drainage is just as much a threat to the environment as acidity is. When sulfide is oxidized, heavy metal ions are released into the water. “The key concept in this case is the specialization of the metal distinguishes between filterable’ and particulate’ fraction of a metal.(Kelly 1988)”
Filterable means that particles can be trapped by a filter. The particulate fraction of the metal includes solid minerals, crystals, and metals that set up into organisms. The presence of heavy metals in the aquatic environment can have a serious effect on the plants and animals in an ecosystem. Plants uptake the metals and because plants are at the bottom of the food chain, these metals are passed on to animals. The animals become contaminated with the metals through eating and drinking. There are actually some types of algae that actually thrive in harsh metal environments because they are not affected by the toxicity and therefore they have no competition. These types of species are blue-green algae: Plectonema, and green algae: Mougeoutia, Stigeoclonium, and Holmidium rivular (Kelly 1988). These species are the exception because there are “very few aquatic plants known to be naturally tolerant to heavy metals.(Kelly 1988)”
LAWS AND REGULATIONS
Recently, many laws and regulations have been passed to help treat and control the acid mine drainage. The EPA has helped establish new limits and regulations such as no net acidity of drainage (pH between 6-9), average total iron content of discharge must be less than 3 mg/L, and the average total manganese content less than 2 mg/L. Processes used now to prevent acid discharge are proper filtering equipment and drainage ponds that contain acid rock indefinitely. The most common methods of treating acid mine drainage are through chemical and biological processes.(Klepper 1989) The Appalachian Clean Stream Initiative was established by the Office of Surface Mining (OSM) and is trying to clean up acid drainage by combining the efforts of citizen groups, corporations and government agencies. President of the OSM, Robert Uram said, “Private organizations both grassroots and national have joined, in addition to government programs at the federal, state, and local levels.” “The most effective way to control acid generation is to prevent its initiation.(Siwik 1989)” The biggest part of the reclamation and restoration is to research into the use of peat/wetland treatment for heavy metal removal from acid mine drainage.(Siwik 1989) According to the EPA standards, many of the mines will have to be designed and operated to meet the standard of “zero discharge” from the mines.
Chemical treatment is the most common method used to eliminate acid drainage from abandoned underground mines. There is three major working parts that do just this; complexion, oxidation, and reduction”(Kelly 1989) Neutralization of acid water with lime is a common practice. Chemicals commonly used in neutralization techniques are lime and sodium bicarbonate or “costic soda.” Other examples of substances that have been found to reduce acid mine drainage are bactericides including antibiotics, detergents, heavy metals and food preservatives. Antibiotics and heavy metals are to costly and to dangerous to the surrounding aquatic life. Alconex, an inexpensive detergent, and sodium laurel sulfate both are found to reduce acid in mine drainage.
Some choose to use biological treatment to treat acid mine drainage and these ways can include: Biodegration of a chemical into basic oxidation products such as carbon dioxide, water, and nitrogen. To me, a very interesting way of treating acid mine drainage successfully and also high metal removal. The reason for this is that the plants that are in the wetland are anaerobic and therefore the rates of decomposition and mineralization of organic matter from the plants of the wetlands is slowed, and organic matter tends to accumulate on the surface of sediments. Wetland, therefore can gather and transform organic material and nutrients.(Bastian 1993) Natural and constructed wetland have been used to treat wastewater. The first one that was ever constructed was in 1982. There are over 200 systems in Appalachia alone.(Bastian 1993) Even though this is safer for the ecosystem it is found that at most sites, chemical treatment is still necessary to meet efficient standards, but the costs of chemical treatment is greatly reduced with the initial biological treatment. Most operators find that the costs of the construction of the wetlands are made up within one year due to the money saved on chemicals.
In conclusion, acid rock drainage is a big problem all throughout the world due to alot of industrialization and mining. This is not only a serious problem around the world, it touches home here, especially here in Appalachia, but it seems to be under or getting under control with all the new regulations and standards the EPA is setting. Low pH and a high acidity level is harmful to us our wildlife and our plants. With the help of more education and more research it will not have to be a problem for our future.