Balance of S in a constructed wetland built to treat acid mine drainage, Idaho Springs, Colorado, U.S.A. |
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| Authors: | Steven D Machemer Julia S Reynolds Leslie S Laudon Thomas R Wildeman |
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| Institution: | 1. Simon Fraser University, Department of Earth Sciences, 8888 University Drive, Burnaby, BC V5A 1S6, Canada;2. British Columbia Geological Survey, 1810 Blanshard St., Victoria, BC V8T 4J1, Canada;3. Boise State University, Department of Geosciences, 1910 University Drive, Boise, ID 83725, USA;4. University of Florida, Department of Geological Sciences, PO Box 2120, Gainvesville, FL 32611-2120, USA |
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| Abstract: | The wetland constructed at the Big Five Tunnel in Idaho Springs, Colorado was designed to remove, passively, heavy metals from acid mine drainage. In optimizing the design of such a wetland, an improved understanding of the chemical processes operating there was required, particularly SO42− reduction and sulfide precipitation. For this purpose, field and laboratory data were collected to study the balance of S in the system. Field data collected included water analyses of the mine drainage and wetland effluents and measurements of H2S gas emissions from the wetland. The concentration of sulfide in the wetland effluent ranged from 10−4 to 10−3 mol/l. The average rates of H2S emission from the surface of the substrate were 150 nmol/cm2/d in the summer and 0.17 and 0.35 nmol/cm2/d in the winter. This maximum estimated loss of sulfide was not significant in reducing the amount of sulfide available for precipitation with metals. Sequential extraction experiments for S on wetland substrates showed that acid volatile sulfides (AVS) increased with time in the wetland substrate. A serum bottle experiment was conducted to study the S balance in the Big Five wetland by quantitatively measuring the amount of S in different phases as microbial SO42− reduction progressed. The increase in AVS reasonably balanced the decrease in SO42− concentration in the experiment, suggesting that the decrease in SO42− concentration represented the amount of SO42− reduced and that nearly all of the sulfide produced was precipitated as AVS. Sulfide precipitation was determined to be the primary metal removal process in the wetland system and amorphous FeS is the primary iron sulfide formed in the substrate. |
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