Microbiology of Transitional Groundwater of the Porous Overburden and Underlying Shallow Fractured Bedrock Aquifers in Olkiluoto, Finland

The subsurface biosphere on Earth appears to be far more expansive and metabolically and phylogenetically complex than previously thought. A diverse suite of subsurface environments have been reported to support microbial ecosystems, extending from a few meters below the surface to several hundred meters. The discovery of a deep biosphere will have several important effects on underground repositories for radioactive wastes. The main potential effects of microorganisms in the context of a KBS-3 type repository for spent fuel in the bedrock of Olkiluoto are: • Oxygen reduction and maintenance of anoxic and reduced conditions. • Bio-immobilisation and bio-mobilisation of radionuclides, and the effects from microbial metabolism on radionuclide mobility. • Sulphate reduction to sulphide and the potential for copper sulphide corrosion. The first main objective of this study was to characterize the geochemistry, biomass and microbial diversity of shallow subsurface groundwater at Olkiluoto, from 4.0 m down to 24.5 m. This objective also permitted the determination of whether or not there is any transition in the shallow depths at Olkiluoto to microbial conditions associated with the deep subsurface. The second main objective was to continue the study of biomass and microbial metabolic diversity in deep groundwater of Olkiluoto to a maximal depth of 525 m, using cultivation methods similar to those applied to the shallow groundwater. This was the first investigation that covered both shallow and deep groundwater microbiology. The analysis of microbiology is very important for proper understanding of the evolution of geochemical processes in and around the underground research facility ONKALO being constructed at Olkiluoto by Posiva since autumn 2004, as well as for the planned KBS-3 type spent fuel repository at Olkiluoto. There are several conclusions and hypotheses with respect to the microbiology that are of great importance for ONKALO and for the spent fuel repository. The following present day hypotheses can be drawn. Continued investigations will update and test them. 1. The transient between the shallow and deep biospheres occurs at a very shallow depth, typically within the first 15-25 m. 2. The shallow biosphere is dominated by oxygen consuming microorganisms that will block oxygen migration to deeper groundwater. 3. The groundwater depression caused by construction of ONKALO will most probably move the borderline between the shallow and deep biosphere downwards. 4. As the groundwater depression zone deepens, oxygen will intrude from above and microbial oxidation of ferrous iron and pyrite will occur with a concomitant decrease in pH and the deposition of ferric iron oxides in the aquifers. Later, when the repository is closed and the groundwater level is restored, those oxides will add to the radionuclide retention capacity of the rock. 5. At present, a deep biosphere signature is found at relatively shallow depths in Olkiluoto compared to other sites investigated with the same methods (The SKB sites Forsmark, Oskarshamn and Aspo). MAAKERROKSESSA JA KALLIOPERASSA LIIKKUVAN POHJAVEDEN MIKROBIOLOGISET TUTKIMUKSET OLKILUODOSSA VUONNA 2004