Simple anaerobic reactors were installed to treat metal-contaminated water in an underground coal mine and at a smelting residues dump in Pennsylvania. The reactors consisted of barrels and tanks filled with spent mushroom compost, within which bacterial sulfate reduction became established. Concentrations of Al, Cd, Fe, Mn, Ni, and Zn were typically lowered by over 95% as contaminated water flowed through the reactors. Cadmium, Fe, Ni, and some Zn were retained as insoluble metal sulfides following their reaction with bacterially generated H(2)S. Aluminum, Mn, and some Zn hydrolyzed and were retained as insoluble hydroxides or carbonates. Reactor effluents were typically circumneutral in pH and contained net alkalinity. The principal sources of alkalinity in the reactors were bacterial sulfate reduction and limestone dissolution. This article examines the chemistry of the reactor systems and the opportunities for enhancing their metal-retaining and alkalinity-generating potential.

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The water treatment performances of two anoxic limestone drains (ALDs) were evaluated. Anoxic limestone drains are buried beds limestone that are intended to add bicarbonate alkalinity to flowthrough acid mine drainage. Both ALDs received mine water contaminated with Fe 2+ (216-279 mg-I) and Mn (41-51 m g L-~). F low through the Howe Bridge ALD increased alkalinity by an average 128 mg L-~ (CaCO3 equivalent) and Ca by 52 mg L-i, while concentrations of Fe, K, Mg, Mn, Na, and SO~-were unchanged. The Morrison ALD increased alkalinity by an average 248 mg L-I and Ca by III mg L-~. Concentrations of K, Mg, Mn, and SO~-all decreased by an average 17%, an effect attributed to dilution with uncontaminated water. Iron, which decreased by 30%, was partially retained within the Morrison ALD. Calcite dissolution was enhanced at both sites by high Pco2. Untreated mine waters at the Howe Bridge and Morrison sites had average calculated Pco~ values of 6.39 kPa (I0-1.z0 atm) and 9.24 kPa (I0-I'°~ atm), respectively. At both sites, concentrations of bicarbonate alkalinity stabilized at undersaturated values (SIc~k~t~ I0-I"~ at Howe Bridge and I0-°'s at Morrison) after flowing through approximately half of the limestone beds. Flow through the second half of each ALD had little additional effect on mine water chemistry. At the current rates of calcite solubilization, 17.9 kg d-i CaCO3 at Howe Bridge and 2.7 kg d-i CaCO3 at Morrison, the ALDs have theoretical effective lifetimes in excess of 20 yr. By significantly increasing alkalinity concentrations in the mine waters, both ALDs increased metal removal in downstream constructed wetlands.

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A derelict "valley dyke" tailings pond at the abandoned Mount Wellington Tin Mine, Cornwall, UK. The mine closed before this pond could be used as intended, and it now serves only as a seasonal source of acidic pollution to the Carnon River. The orange high water mark is controlled by decant through the lower of the two redundant draw-off towers; the burgundy low water mark reflects evaporation and seepage through the dam (which is composed of waste rock). Notice the losing battle of Mother Nature to re-establish a healthy fauna: this book is about how we can help Nature to overcome water pollution at mine sites world-wide. (Photo: P.L. Younger).