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The Monastery Run
Improvement Project: |
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1997 |
2002 |
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Formation of AMD Coal seams were often located near or below the groundwater level. Groundwater is water that is found underground in cracks and spaces between particles of rock and soil or in crevices and cracks in rocks. The area where the cracks and crevices are filled with both air and water, this is called the unsaturated zone. The level below which all the spaces are filled with water is called the water table. The area below the water table is the saturated zone. The water in the saturated zone is called the ground water. The water table may be only a foot below the ground’s surface or it may be hundreds of feet down. Because the mine was near the water table or ground water, the mine would fill up with water if the company did not pump the water out of the mine during the mining operation. In Early mining operations, when the mine was "played out", the company abandoned the mine and stopped pumping the water out of the mine. In the roof of the mine, was pyrite or fool’s gold, The two elements that make up pyrite are Iron and Sulfide. Pyrite is commonly found in the rock layers overlying coal seams. Eventually, the roof would collapse, if not from retreat mining, from stress, and the mine would be filled with the rocks and minerals that were in the roof, things such as pyrite, sandstone, shale and limestone. Just as water dissolving sugar, the groundwater will begin to dissolve the minerals, and the pyrite will dissolve into iron (Fe+2) particles and sulfate (SO4-2) particles. Also produced in the weathering or dissolving of pyrite is the production of acid, specifically H2SO4. (1) 2 FeS2 + 7 O2 + 2 H2O ® 2 Fe2+ + 4 SO42- + 4 H+ (1) Pyrite + Oxygen + Water ® Ferrous Iron + Sulfate + Acidity When the mine water comes in contact with oxygen, the ferrous iron is oxidized to ferric iron. Certain bacteria increase this rate of oxidation from ferrous to ferric iron, but this reaction rate is pH dependant with the reaction proceeding slowly under acidic conditions (pH 2-3) with no bacteria present and several orders of magnitude faster at pH values near 5. This reaction is referred to as the "rate determining step" in the overall acid-generating sequence. (2) 4 Fe2+ + O2 + 4 H+ ® 4 Fe3+ + 2 H2O (2) Ferrous Iron + Oxygen + Acidity ® Ferric Iron + Water The third reaction, which may occur, is the hydrolysis of iron. Hydrolysis is a reaction which splits the water molecule into one hydrogen ion(H+) and one hydroxide ion(OH-). The hydroxide ion reacts with the ferric ion to form ferric hydroxide, a precipitate (solid) which is commonly know as yellow boy. This reaction is also pH dependent. Solids form if the pH is above about 3.5 but below pH 3.5 little or no solids will precipitate. (3) 4 Fe3+ + 12 H2O ® 4 Fe(OH)3 ¯ + 12 H+ (3) Ferric Iron + Water ® Ferric Hydroxide (yellowboy) + AcidityMany areas also contain naturally occurring limestone (CaCO3) deposits which neutralizes acidity. When the water is alkaline or above a pH of 3.5, the formation of yellowboy or the characteristic orange color associated with mine drainage will occur. 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. The strict regulations currently used provide an opportunity to determine if the mine will produce Acidic or Alkaline drainage. To determine this 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. Other laws and regulation require the cleanup of the site prior to closing the mine, to ensure that there will be no mine drainage to harm local waterways.
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