Controls on pH and Pyrite Oxidation Pathways across the Phreatic Surface of a Coal Waste Deposit in Southwest Indiana

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2010-06
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During acid mine drainage (AMD) generation, the factors controlling pH and pyrite oxidation can differ between the saturated and unsaturated zones. These differences, however, may not always be considered during field-scale remediation efforts. At an unreclaimed coal waste deposit in southwest Indiana, preliminary studies have identified a horizontal pH gradient that increases from 2.8 to 6.5 along a 90 meter shallow groundwater flow path. This site provides a unique opportunity to determine pyrite oxidation pathways under varying conditions within both the saturated and unsaturated zones of coal mine refuse being subjected to weathering processes. Vertical profiles of pore-water were obtained using Diffusion-Controlled Dialysis Membrane Samplers installed across the phreatic surface. Samples were collected at 2 cm intervals from the ground surface to a depth of 40 cm with the bottom 10 cm being fully saturated. Samples collected during the summer reveal a linear trend with respect to pH and depth with pH values of approximately 2.5 near the surface to 3.4 at the base of the profile. Concentrations of sulfate (SO42-) and iron (Fe-total) during the summer are highest near the surface and in the case of Fe, decrease by an order of magnitude at the bottom of the profile. Samples collected during the autumn show that within the saturated zone, ferrous iron (Fe2+) concentrations are elevated relative to ferric iron (Fe3+), but Fe3+ increases with decreasing pH. This indicates that atmospheric oxygen is the limiting factor in pyrite oxidation both through direct oxidation of pyrite and through the oxidation of Fe2+ to Fe3+ that can then oxidize pyrite in both saturated and unsaturated conditions at lower pH values. These results imply that an increase in the thickness of the saturated zone can act as a control on acidity generation by preventing positive feedback cycling of Fe3+ to oxidize pyrite.
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This presentation was given at the 2010 Annual Meeting of the American Society of Mining and Reclamation, Pittsburgh, PA, June 5-10, 2010.
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coal, reclamation, Indiana Geological Survey, Indiana, pyrite, AMD, acid mine drainage
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