Modeling Studies on Microbial Effects on Groundwater Chemistry
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0985-NN11-15
Modeling Studies on Microbial Effects on Groundwater Chemistry Yoshikatsu Tochigi, Hideki Yoshikawa, and Mikazu Yui Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency, 4-33 Muramatsu, Tokai-mura, 319-1194, Japan
ABSTRACT The overall goal of this project is to develop a model to predict microbial effects on the performance of a high-level radioactive waste (HLW) repository. As a first step, the effects of microbes on groundwater chemistry have been evaluated with the numerical code 'MINT', using data collected from the borehole HDB-6 in the Horonobe underground research laboratory (URL) in Japan. The MINT code models biochemistry and geochemical equilibrium, with consideration of transport of solute and microbial activity. The MINT code simulates the activities of six major groups of microbes, classified by their metabolism as 'aerobic', 'denitrifying', 'manganese reducing', 'iron reducing', 'sulfate reducing' and 'methanogenic'. The specific activity of each of these groups will depend on the redox potential (Eh) of the groundwater. Sensitivity analyses were performed to investigate the consequences of changes in groundwater composition on the effects of microbial activity. This indicates that the activities of Sulfate Reducing Bacteria (SRB) and methanogens are relatively high. The concentration of dissolved methane produced by such microbial activity is seen to be influenced by sulfate concentration. Based on the observed data from Horonobe URL, the concentration in oxygen is relatively high and the activity of denitrifying bacteria is the highest of the major six groups of microbes. This can, however, be attributable to chemical / microbial contamination of the groundwater during sampling. The modeling results indicate that the concentration of dissolved oxygen and nitrate ion should be quickly reduced by microbial metabolism, reducing the redox potential to a level low enough for active methanogenesis to commence. Such assessment can be important to evaluate the reliability of sampling and measurement techniques for sensitive geochemical parameters in general - and microbiology in particular. INTRODUCTION Recently, in-situ sampling and analysis has shown that microbial populations in the order of 10 to 106 cells/mL can be found in deep geological environments [1] – similar to the kind of levels found in near-surface seawater on the shore and soils. Such microbial populations are dependent on chemolithotrophs, which obtain energy for growth by coupling the oxidation of dissolved organic carbon (DOC; as electron donor) to the reduction of a range of different electron acceptors in the groundwater. The kind of electron acceptor used is determined by its particular metabolism and the chemical conditions (particularly redox potential [Eh]) of a specific subsurface niche. Microbes are categorized into metabolic groups, depending on the particular electron acceptors that they can utilize. The concentration of electron acceptors will thus reflect a balance between their
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