Microbial activity and chemical weathering in the Middendorf aquifer,South Carolina |
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| Authors: | Jungho Park Robert A Sanford Craig M Bethke |
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| Institution: | 1. V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia;2. A.N. Zavaritsky Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences, Pochtovyi per. 7, Yekaterinburg, 620151, Russia;3. Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia;1. Aix-Marseille Université-CNRS, Laboratoire Chimie Environnement, FRE 3416 Marseille, France;2. French Environment and Energy Management Agency, 20 avenue de Grésillé-BP 90406 Angers Cedex 01, France;1. BRGM French Geological Survey, 3 Avenue Claude Guillemin, 45060 Orléans Cedex 2, France;2. Institute of Geosciences, University of São Paulo, Rua do Lago, 562 Butantã, 05508-080 Sao Paulo, Brazil;3. OSUR-Géosciences Rennes, Université Rennes 1 — CNRS, 35000 Rennes, France;4. Civil Engineering Department, Federal University of Pernambuco, 50740 Recife, PE, Brazil;5. Géo-Hyd, 101 rue Jacques Charles, 45160 Olivet, France;1. BRGM French Geological Survey, 3 Avenue Claude Guillemin, 45060 Orléans Cedex 2, France;2. Institute of Geosciences, University of São Paulo, Rua do Lago, 562 Butantã, 05508-080 Sao Paulo, Brazil;3. OSUR-Géosciences Rennes, Université Rennes 1 — CNRS, 35000 Rennes, France;4. Civil Engineering Department, Federal University of Pernambuco, 50740 Recife, PE Brazil;5. CPRM, Brazilian Geologic Survey, Avenida Sul 2291, Recife PE, Brazil;6. Géo-Hyd, 101 rue Jacques Charles, 45160 Olivet, France |
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| Abstract: | We use reactive transport modeling to better understand the kinetics of chemical weathering in the Cretaceous Middendorf aquifer of South Carolina, USA, and the relationship of this process to subsurface microbial activity. We constructed a model accounting for the kinetics of mineral dissolution and precipitation, ion exchange, and the CO2 and bicarbonate produced by iron reducing and sulfate reducing bacteria in the aquifer. We then fit the model to observed trends in the chemical composition of groundwater along the aquifer by adjusting the rate constants for the kinetic reactions considered. The modeling portrays weathering in the Middendorf as a slow process by which groundwater gradually reacts toward equilibrium with minerals in the aquifer. The rate constants predicted are 6 to 7 orders of magnitude smaller than measured in laboratory experiments and 3 to 4 orders of magnitude less than those inferred from weathering rates in soils. The rate constants are smaller even than expected by projecting observed trends with the duration of weathering to the geologic age of the Middendorf. Weathering is driven largely by biological activity: about half the acid consumed is CO2 derived from the recharge area, and about half is supplied by iron reducing bacteria in the aquifer; only about 1% of the acid is of atmospheric origin, from CO2 dissolved in rainwater. |
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