A biogeochemical assessment of the Tono site, Japan

When designing investigations of microbial populations in the subsurface, i t is extremely valuable to undertake scoping calculations to estimate the l ikely microbial abundances and evaluate the effects of contamination during sampling. A biogeochemical assessment of the groundwater and lithologies o f the Tono mine, Japan, has been made using the BGS/NAGRA computer code BGS E (Bacterial Growth in Subsurface Environments). This code enables an asses sment to be made of the maximum microbial growth rates that may be achieved in ideal circumstances, based on availability of nutrients and energy calc ulated from mineralogical and groundwater analyses. The effect of drilling fluid/groundwater mixing on biomass was assessed using a hypothetical drill ing fluid composition. The results of modelling the mixing between groundwa ter 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 Ake yo and Toki lower groundwaters, and ratios of 90:10 (v/v) (groundwater:dril ling fluid) for the Toki upper and Granite groundwaters. At low ratios of d rilling fluid (< 1% (v/v)) the limiting factor in each system was the avail ability of an energy source. This reflects the fact that the system is appr oaching pristine conditions. However, there was sufficient energy to permit a significant growth rate to be observed. (C) 1998 Published by Elsevier S cience B.V. All rights reserved.