BIOGEOCHEMICAL AND MICROBIOLOGICAL SITE-CHARACTERIZATION OF THE DEEP GEOLOGICAL ENVIRONMENT

摘要

Biogeochemical processes within the deep subsurface have potential impacts on radionuclide migration from subsurface to near surface environments. As most of sedimentary basins in the Japanese Island arc system are disseminated with biogenic methane, it is likely that active microorganisms are present deeply beneath the ground surface. In order to evaluate in-situ biogeochemical and microbiological characteristics in deep sedimentary rocks within an inland terrestrial basin, we conducted 350-m deep drilling by using ultrafiltered and deoxygenated drilling fluid to minimize microbial and geochemical disturbance. The main findings of our previous and present works are as follows: (1) From pore-water samples extracted from 200-350-m deep cores contain nitrite and/or nitrate, and based on 16S rRNA gene sequences, nitrate-respiring microbes belonging to the genus Pseudomonas are dominant. (2) The number and activity of microorganisms increased with upwardly migrated brine enriched in methane. (3) Redox states of groundwaters collected from a multi-packer system installed in the borehole (depleted in nitrite, nitrate and sulfate) were clearly different from those extracted from cores from the corresponding depths (containing nitrite, nitrate and sulfate). Similarly, dominant microbial populations shifted from nitrate-reducing to sulfate-reducing and fermenting bacteria in the borehole.