We investigated the diversity and distribution of archaeal and bacterial 16S rRNA gene sequences in deep aquifers of mid-to late Miocene hard shale located in the northernmost region of the Japanese archipelago. A major fault in the north-west-south-east (NW-SE) direction runs across the studied area. We collected three groundwater samples from boreholes on the south-west (SW) side of the fault at depths of 296, 374 and 625 m below ground level (m.b.g.l.) and one sample from the north-east (NE) side of the fault at a depth of 458 m.b.g.l. The groundwater samples were observed to be neutral and weakly saline. The total microbial counts after staining with acridine orange were in the order 10 5 − 10 6 cells mL − 1 and 10 3 cells mL − 1 in the aquifers to the SW and to the NE of the fault, respectively. A total of 407 archaeal and bacterial 16S rRNA gene sequences (204 and 203 sequences, respectively) were determined for clone libraries constructed from all groundwater samples. Phylogenetic analyses showed that the libraries constructed from the SW aquifers were generally coherent but considerably different from those constructed from the NE aquifer. All of the archaeal clone libraries from the SW aquifers were predominated by a single sequence closely related to the archaeon Methanoculleus chikugoensis , and the corresponding bacterial libraries were mostly predominated by the sequences related to Bacteroidetes, Firmicutes and δ -Proteobacteria. In contrast, the libraries from the NE aquifer were dominated by uncultured environmental archaeal clones with no methanogen sequences and by β -proteobacterial clones with no sequences related to Bacteroidetes and δ -Proteobacteria. Hence, the possible coexistence of methanogens and sulphate reducers in Horonobe deep borehole (HDB) on the SW side is suggested, particularly in . Moreover, these organisms might play an important geochemical role in the groundwater obtained from the aquifers. permeability (Fukusawa, 1987;Yamamoto et al ., 2004).An underground research laboratory (URL) is being constructed in the unique sediment by the Japan Atomic Energy Agency (JAEA), and the survey boreholes created for the URL construction are used for geophysical, geological, hydrological 204 S. SHIMIZU et al.
There is considerable interest in the use of thick argillaceous geologic formations to contain nuclear waste. Here, we show that diffusion can be the controlling transport process in these formations and diffusional time scales for δ18O and δ2H in water, dissolved He, and Cl transport in shale‐dominated aquitards are typically over 106 years, well exceeding the regulatory requirements for isolation in most countries. Our scientific understanding of diffusive solute transport processes through argillaceous formations would benefit from the application of additional isotopic tracers (e.g., using new 4He sampling technology), multidimensional diffusive‐dispersive modeling of groundwater flow and diffusive‐dispersive solute transport over long geologic time scales, and an improved understanding of spatial heterogeneity as well as time‐dependent changes in the subsurface conditions and properties of argillaceous formations in response to events such as glaciation. Based on our current isotopic and geochemical understanding of transport, we argue that argillaceous formations can provide favorable long‐term conditions for isolating nuclear wastes.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.