The microbiology of subsurface, salt-based nuclear waste repositories: using microbial ecology, bioenergetics, and projected conditions to help predict microbial effects on repository performance

The evaluation of deep geological settings as sites for nuclear waste disposal is extensive and multidisciplinary, and among the many areas of study is the field of microbiology. The microbiology of granite, basalt, tuff, and clay formations in Europe and the US has been under investigation for decades, and much has been learned about the potential influence of microorganisms on repository performance and about deep subsurface microbiology in general. In spite of this, there is still uncertainty surrounding the effects of microorganisms on salt-based repository performance. One of the reasons for this is that negative findings (i.e., no growth) cannot be used as performance model input, so as a result, conditions are often manipulated to generate positive findings (i.e., growth). Given the unique microbiology of hypersaline environments, these negative results are both valid and meaningful and should be analyzed from the perspective of feasibility. The microbial communities present in hypersaline settings are limited in both structural and functional diversity. This is because, in order to survive at high salt concentrations, these organisms must osmotically balance their internal and external environments. This limits their ability to perform certain modes of metabolism, based on the energy required for survival and the energy derived from a given reaction. The field of repository microbiology has assumed that diverse organisms capable of diverse metabolic processes will be present and active in the repository setting; however, this is not likely to be the case at extremely high salt concentrations.
At the highest salt concentrations, extremely halophilic Archaea are dominant members of the microbial population because of their ability to balance osmotic pressure using a low-energy strategy. These organisms are almost all aerobic with limited anaerobic capability, thus their role in repository microbiology may be confined to early oxic periods. Still, they are able to survive tens of thousands of years encased in salt, such that they will be present throughout repository history. Some extremely halophilic Bacteria also exist in hypersaline environments. In general, these organisms will have a much more diverse metabolic repertoire, including aerobic and anaerobic capabilities. However, these capabilities narrow as salt concentration increases, due to the high-energy cost strategy utilized by bacteria to maintain osmotic balance. Bacteria present in repository waste or introduced during mining operations are not likely to be halophilic and may not survive long-term. However, the role of microorganisms within drums may be significant.
This report summarizes the potential role of microorganisms in salt-based nuclear waste repositories using available information on the microbial ecology of hypersaline environments, the bioenergetics of survival under high ionic strength conditions, and “repository microbiology” related studies. In areas where microbial activity is in question, there may be a need to shift the research focus toward feasibility studies rather than studies that generate actual input for performance assessments. In areas where activity is not necessary to affect performance (e.g., biocolloid transport), repository-relevant data should be generated. Both approaches will lend a realistic perspective to a safety case/performance scenario that will most likely underscore the conservative value of that case.