Pyrite oxidation in the tertiary sands of the London Basin aquifer |
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| Affiliation: | 1. CSIRO Mineral Resources, PO Box 1130, Bentley, WA 6102, Australia;2. The University of Queensland, Brisbane, EAIT, QLD 4072, Australia;3. CSIRO Energy, PO Box 1130, Bentley, WA 6102, Australia;4. CSIRO Land and Water, Waite Road, Urrbrae, SA 5064, Australia;1. University of Wyoming, Wyoming Center for Environmental Hydrology and Geophysics and Department of Ecosystem Science and Management, 1000 E. University Ave., Dep. 3354, Laramie, WY 82072, USA;2. University of Wyoming, Wyoming Center for Environmental Hydrology and Geophysics and Department of Geology and Geophysics, 1000 E. University Ave., Dep. 3006, Laramie, WY 82072, USA;3. Clemson University, Department of Environmental Engineering and Earth Sciences, 342 Computer Court, Anderson, SC 29625, USA;1. CAS Key Laboratory of Forest Ecology and Management, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110164, China;2. Research Institute of Tropical Forestry, Chinese Academy of Forestry, Guangzhou 510520, China;3. School of Geography, University of Leeds, Leeds, UK;4. Center for Ecological Research, Kyoto University, Shiga 520-2113, Japan;5. Department of Biological Sciences, Binghamton University, The State University of New York, Binghamton, NY 13902, USA;6. Qingyuan Forest CERN, Chinese Academy of Sciences, Shenyang 110016, China;7. College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, China |
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| Abstract: | Possible groundwater quality changes related to pyrite oxidation during artificial groundwater recharge and its storage in the Tertiary sands of the London Basin are investigated. Pyrite textures in the Tertiary sands are examined by scanning electron microscopy while an experimental approach is used to study mechanisms of pyrite oxidation and of some associated chemical reactions. In the Tertiary sands of the London Basin aquifer, pyrite occurs as aggregates made of discrete individual crystals 0.5–5 μm in size or, in a cryptocrystalline form, often as pseudomorphs of biogenic debris. It can expose a very large specific surface area to porefluids. Although ferric iron, which can be an oxidising agent of pyrite, is abundant in the solid phase of the Tertiary sands, it does not appear to take a significant part in this case. Pyrite oxidation seems to rely on a supply of oxygen. Leaching experiments using a 0.001 M H2SO4 solution were carried out to examine interactions between mildly acidic groundwater resulting from pyrite oxidation at a moderate rate and the host-sediment. In the presence of CaCO3 in the solid phase, H+ is rapidly buffered by CaCO3 dissolution. Oscillations of this reaction around equilibrium appear to trigger cation-exchange reactions on clay mineral surfaces, resulting in the release of major cations (e.g. K and Mg) into solution. In the absence of CaCO3 in the solid phase, H+ buffering occurs less efficiently solely through exchange of cations for H+ on clay minerals surfaces. If the rate of pyrite oxidation in the Tertiary sands becomes high enough for the buffering capacity of the system to be exceeded, the groundwater pH begins to fall. Interactions between low pH (2) groundwaters and the host sediments were examined by leaching solid material in 0.01 M and 0.1 M H2SO4 solutions. Concentrations of Fe, Mg and K increase in solution throughout the experiment, indicating partial dissolution of clay minerals. The composition of the porefluid thus depends on the geochemical composition and surface area of the different clay minerals present. |
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