Biologically Induced Clay Formation in Subsurface Granitic Environments
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%LRORJLFDOO\,QGXFHG&OD\)RUPDWLRQLQ6XEVXUIDFH*UDQLWLF(QYLURQPHQWV Victoria A. Tuck 1, J. M. West2, K. Bateman2, P. Coombs2, A. E. Milodowski2, R. Edyvean1 Chemical and Process Engineering, University of Sheffield, S1 3JD, UK 2 British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK 1
$%675$&7 Experiments were conducted to identify the rock-water and microbial interactions influencing accelerated smectite-clay formation. Packed columns and stirred batch reactors contained Äspö granodiorite, artificial groundwater mimicking that from Äspö and combinations of three types of subsurface chemolithotrophic bacteria, two of which were indigenous to the Äspö rocks Results showed evidence that, within 5 days under anaerobic reducing conditions, all three of the bacterial types produced copious biofilamentous 'meshes' across porespaces, apparently using the larger grains as anchor points. The biofilaments quickly became encrusted with fine grained material and surrounded with neoformed clay-like deposits. In contrast, the abiotic controls showed little or no evidence of clay formation suggesting that this process is biologically induced or controlled. A second series of abiotic experiments to determine the effects of increased acidity showed evidence of mineral pitting and dissolution along with an increase in concentration of soluble species thought to be important in smectite formation (i.e. Si, Al, Mg, Fe, Ca, Na). However, there was no evidence of clay formation, and the biotic experiments showed no signs of bulk scale pH change, suggesting that either the bacteria are actively concentrating relevant chemical species at a local level or they are acting as templates or nucleation points for clay formation. ,1752'8&7,21 The natural growth of bacteria in porous rocks can have a significant effect on geochemical and physical characteristics of the host rock [1 - 6]. Many microorganisms produce chemically reactive metabolic by-products such as organic acids and element-specific ligands that are able to alter pH and redox conditions, as well as enhance chelation resulting in increased mobilization of trace elements [7 - 10]. Bacteria will often flourish, even in deep subsurface granitic environments associated with low-nutrient, reducing conditions and saline groundwater. Furthermore, because bacteria have an extremely high surface-to-volume ratio and are very reactive, they can dominate the chemical reactivity of the substrate-water interface. This can alter the attenuation capacity of the environment as well as changing its porosity and permeability. Many species of bacteria are also known to passively and actively act as catalysts or nucleation sites for authigenic mineral phases such as metal sulphides and complex silicates [11, 12]. However, the processes involved are not well defined, appearing to range from large-scale perturbations in bulk groundwater chemistry and mineral-water equilibria, to small-scale interactions where attached biota change geochemical and physical conditions within their microenvi
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