MICROBIOLOGICAL ANALYSIS AT THE POCOS-DE-CALDAS NATURAL ANALOG STUDY SITES

The Pocos de Caldas project is a wide ranging natural analogue study focused on testing models used in performance assessment of the disposal of radioactive waste. Part of the work has involved characterising microbial populations and their influence on the various processes being studied. Core material and groundwaters have been sampled for microbiological content at various depths from boreholes at the Osamu Utsumi open pit uranium mine and Morro do Ferro Th/REE ore body. Microbes were found in all samples, but numbers do not appear to show any clear trend as a function of depth. Analyses of groundwaters gave higher numbers than with solid material, probably because of sampling methods, and demonstrated the presence of sulphur cycle bacteria. These observations have been compared with predictions of a model used in performance assessment to calculate the maximum biomass/microbial activity based on constraints set by available nutrients and energy. The main conclusions of this analysis are: (i) Low microbial activities can be supported by the energy and nutrients supplied by rock alteration processes (weathering) at or around the redox front. The maximum annual production of 0.010. 1 g biomass (dry)/m2 of redox front is in reasonable agreement with observed standing populations. (ii) The presence of high concentrations of sulphate reducing bacteria around the redox front is compatible with other evidence for a complex sulphur geochemistry which may be predominantly microbially catalysed and could explain the nodular form of pitchblende concretions and the presence of secondary pyrite. (iii) There is little evidence of trace element mobilisation by organic byproducts and the main role of microbes in this system seems to be catalysis of specific redox reactions.