A biogeochemical assessment of the Tono site, Japan

When designing investigations of microbial populations in the subsurface, it is extremely valuable to undertake scoping calculations to estimate the likely microbial abundances and evaluate the effects of contamination during sampling. A biogeochemical assessment of the groundwater and lithologies of the Tono mine, Japan, has been made using the BGS/NAGRA computer code BGSE (Bacterial Growth in Subsurface Environments). This code enables an assessment to be made of the maximum microbial growth rates that may be achieved in ideal circumstances, based on availability of nutrients and energy calculated from mineralogical and groundwater analyses. The effect of drilling fluid/groundwater mixing on biomass was assessed using a hypothetical drilling fluid composition. The results of modelling the mixing between groundwater and drilling fluid shows that the addition of only small concentrations of drilling fluid (<1% (v/v)) to the groundwater gives rise to significant microbial growth rates for the systems studied. Maximum growth rates were observed at ratios of 50:50 (v/v) (groundwater: drilling fluid) for the Akeyo and Toki lower groundwaters, and ratios of 90:10 (v/v) (groundwater:drilling fluid) for the Toki upper and Granite groundwaters. At low ratios of drilling fluid (<1% (v/v)) the limiting factor in each system was the availability of an energy source. This reflects the fact that the system is approaching pristine conditions. However, there was sufficient energy to permit a significant growth rate to be observed.