Influence of bacteria on rock-water interaction and clay mineral formation in subsurface granitic environments

Abstract

Studies of the subsurface microbiology of the Äspö Hard Rock Laboratory, Sweden have revealed the presence of many different bacteria in the deep groundwaters which appear to maintain reducing conditions. Experiments were conducted to study the rock-water and microbial interactions. These used crushed Äspö diorite, Äspö groundwater and iron- and sulphate-reducing bacteria in flowing systems under anaerobic conditions. In column experiments, there was evidence of loss and mobilization of fine-grained crushed material (<5 μm) which had originally adhered to grain surfaces in the starting material. The mobilized fines were trapped between grains. The degree of mineralogical alteration was greater in the experiments when bacteria were present. In both column and continuously stirred reactor experiments, there is evidence for the formation of a secondary clay. These experiments have shown that microbial activity can influence rock-water interactions even in nutrient-poor conditions.

Mineralogical comparisons of experimental results investigating the biological impacts on rock transport processes

“…crushed 125 to 250 µm Äspö granodiorite material, 30 packed into 2. borosilicate glass columns, and 3. saline Äspö groundwater containing minimal nutrients (C, S, N, P). The first approach by Hama et al 14 introduced a mixture of iron- 35 and sulphate-reducing bacteria, i.e. Shewanella putrefaciens and Desulfovibrio aespoeensis respectively, in Äspö groundwater flowing through either columns or stirred tank reactors (CSTR) packed with crushed Äspö granodiorite.…”

“…The experiments were aimed to run over a period of 100 days under anaerobic conditions and a flow rate of 0.5 ml hr -1 . However, the columns became blocked soon after the introduction of bacteria and the experiments had to be 45 terminated after 2 days 14 . The second approach by Tuck et al 32 applied granodiorite packed column studies that were inoculated with a single or mixed population of chemolithotrophic bacteria under anaerobic and aerobic conditions.…”

“…This mixture of bacteria is commonly used in bioremediation experiments. As with previous work 14 , the column experiments had to be terminated prematurely after 5 55 days due to blockage. Additional biotic and abiotic CSTR experiments investigated the influence of varying pH (ranging from between 1 and 10) on mineral dissolution aiming to free up' elements important for clay mineral formation 32 .…”

“…These granitic rocks have undergone extensive 5 fracturing in parts and have been subjected to hydrothermal alteration adjacent to rock fractures 33 . A detailed account of the geology of the Äspö site is provided by Hama et al 14 . The Äspö granodiorite samples were taken in immediate proximity to the in situ field-scale experiment (REX -redox experiment in detailed 10 scale) which investigated groundwater redox-buffering capacity of the rock mass within the URL 27 .…”

“…Petrographic work by Hama et al 14 revealed an Äspö granodiorite starting material that is hydrothermally altered, partially recrystallised and relatively homogeneous. The rock 110 matrix was shown to comprise plagioclase and orthoclase feldspar, quartz, 'mica' (muscovite and biotite) and accessory epidote, magnetite, pyrite and calcite.…”