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iron and other metals that co-precipitate with iron. Settling ponds should be <br />designed fora 24-hour or greater retention time wherever possible. <br />Sulfate reducing wetlands are often called bioreactors. These systems treat <br />water through bacterial reduction of heavy metals. Sulfate reducing bacteria <br />(SRB) utilize the oxygen in sulfates for respiration, producing sulfides. The <br />sulfides then combine with heavy metals to form relatively insoluble metal <br />sulfides. The bacteria derive their energy from a carbon source such as cow <br />manure or mushroom compost. There are many other substrates that are an <br />acceptable source of carbon, but most have a low hydraulic conductivity that can <br />result in short circuiting of the system by formation of preferential flow paths. <br />Sulfate reducing bacteria cannot survive in a drainage with pH below 4.5. <br />Highly acidic drainages will require a pH increase before the effluent eaters the <br />bioreactor. <br />Sulfate reducing wetlands should generally not be constructed near population <br />centers. These systems commonly produce excess hydrogen sulfide, which can <br />cause undesirable odors up to three miles from the system. V~'hen initially <br />started; organics in the substrate discolor the treated water for several months, <br />making water quality appear, to the layman, to be worse than that entering the <br />system. <br />Aqueous lime injection is a passive method to introduce neutralizing agents into <br />mine drainage. This system requires a clean water source. Clean water is <br />passed through a pond containing neutralizing agent, then the high pH effluent is <br />mixed with the mine drainage before it enters a settling pond. This system can <br />be cost effective if the alkaline wastes such as kiln dusts or fly ash are available. <br />Although still in the experimental phase, the method holds promise for some <br />mine sites. Neutralizing materials may also be injected into stopes and drifts. <br />Limestone water jets are an aerobic method of accelerating the dissolving of <br />limestone. In situations where mine drainage flows down a steep slope, the <br />discharge can be piped, and the resultant head can produce ahigh-pressure water <br />jet. The high-pressure jet can be either sprayed onto loose crushed limestone, or <br />passed upward through a vessel containing limestone. In both situations, the <br />limestone does not become coated because of abrasion by the water jet, and <br />agitation of the surrounding clasts. The system using a vessel can result in <br />higher alkalinity in the effluent due to greater abrasion. Both system types are <br />in the experimental phase. <br />Oxidation wetlands are what most people think of as `wetlands". They differ <br />from sulfate reducing systems in that metals are precipitated through oxidation, <br />and aquatic plants must be established. This treatment method is applicable <br />where the pH of mine drainage is approximately 6.5 or higher, and where metals <br />concentrations in the drainage are primarily a problem during summer months. <br />Aeration is an important part of the system. The plant materials provide aeration <br />