Teruki Iwatsuki

Phylogenetic Characterization of 16S rRNA Gene Clones from Deep-Groundwater Microorganisms That Pass through 0.2-Micrometer-Pore-Size Filters

ABSTRACT A total of 247 clones of 16S rRNA genes from microorganisms captured by 0.2- and 0.1-μm-pore-size filters from sedimentary and granite rock aquifers were amplified and yielded 37 operational taxonomic units (OTUs). Fifteen OTUs captured by 0.1-μm-pore-size filters were affiliated with the candidate divisions OD1 and OP11, representing novel lineages. On the other hand, OTUs captured by 0.2-μm-pore-size filters were largely affiliated with Betaproteobacteria.

Isotopic and morphological features of fracture calcite from granitic rocks of the Tono area, Japan: a promising palaeohydrogeological tool

Abstract This study aimed to develop a methodology for assessing the hydrochemical evolution of a groundwater system, using fracture-filling and fracture-lining calcite. Fracture calcite in deep (to ca. 1000 m) granitic rocks of the Tono area, central Japan, was investigated by optical and electron microscopy, and chemical and isotopic analysis. Coupled with geological evidence, these new data imply 3 main origins for the waters that precipitated calcite: (1) relatively high-temperature hydrothermal solutions, precipitating calcite distinguished by δ18OSMOW from −3 to ca. 10‰, and with δ13CPDB from ca. −18 to −7‰; (2) seawater, probably partly of Miocene age, which precipitated calcite distinguished by δ13CPDB of ca. 0‰ and δ18OSMOW > ca. 20‰; (3) fresh water, with a variable δ13CPDB composition, but which precipitated calcite distinguished by δ13CPDB that was significantly ca. 17‰. Data for 14C suggest that at least some of the fresh-water calcite formed within the last 50 ka. The present day hydrogeological regime in the Tono area is also dominated by fresh groundwater. However, the marine calcite of probable Miocene age found at depth has shown no evidence for dissolution and many different calcite crystal forms have been preserved. Studies of other groundwater systems have correlated similar crystallographic variations with variations in the salinity of coexisting groundwaters. When this correlation is applied to the Tono observations, the calcite crystal forms imply a similar range of groundwater salinity to that inferred from the isotopic data. Thus, the present study suggests that even in presently low-salinity groundwater systems, calcite morphological variations may record the changing salinity of coexisting groundwaters. It is suggested that calcite morphological data, coupled with isotopic data, could provide a powerful palaeohydrogeological tool in such circumstances.

Carbon-14 study of groundwater in the sedimentary rocks at the Tono study site, central Japan

Isotopic investigations using 14C of groundwater were carried out to understand the hydraulic conditions in the sedimentary rocks at the Tono study site, central Japan. 14C activities of groundwater observed range from 2 to 32 % Modern Carbon (pMC). Measured 14C activities of groundwater are corrected by the isotopic mass balance model based on 14C activities, δ13C values, concentrations of the dissolved inorganic carbon (DIC) and δ13C values of non-active carbon dissolved into the groundwater from carbonate minerals and organics. Assuming that the groundwater reservoir is comparable to the piston flow situation, the relative 14C ages of groundwaters were calculated from the corrected 14C activity. The relative 14C age suggests that the groundwater infiltration from the upper part of the sedimentary rocks to the lower part takes several thousands of years, or that the groundwater in the lower part of sedimentary rocks is derived from long distance flow from the surface through the unconformity between the sedimentary rocks and basement granite. The flow rate calculated by relative 14C ages shows similar values to those estimated by computer simulation using hydraulic pressure and conductivity data. Hydraulic conditions at the Tono study site inferred from 14C activity agree with those suggested from hydrogeological analyses. Isotopic approaches using 14C activity can be applied as geochemical evaluation for interpretations from the hydraulic study.

Characterization of saline groundwater at Horonobe, Hokkaido, Japan by SEC-UV-ICP-MS: Speciation of uranium and iodine

The saline groundwater collected at a depth of about 500 m in Horonobe, Japan, where an underground research laboratory (URL) has been built, is rich in saline (Na 4900 ppm, Cl 7600 ppm), iodine (42 ppm), and methane gas. We analyzed the colloids and ions of this groundwater mainly by employing a size exclusion chromatography (SEC) coupled on-line to ultraviolet-visible (UV-Vis) detection and inductively coupled plasma mass spectrometry (ICP-MS) technique and focused on the speciation of uranium and iodine, both of which are of particular importance for radioactive waste disposal. For this purpose, the groundwater sample was introduced to SEC columns after being passed through a 0.45 μm filter but without further pretreatment, such as isolation of colloids. The chromatographic profiles obtained with two different SEC columns were compared. This study revealed that uranium present in the groundwater at several tens of ppt was associated with low molecular weight silica species with neutral charge. The silica species were virtually free of metal elements such as Na, K, Mg, Ca, and Al. This study also found that almost all of the iodine in the groundwater was iodide (I(-)). The groundwater contained an unidentified organic colloid that was not a carrier for the radioactive waste-relevant elements Se, Sr, I, Cs, Th, and U.

Chlorite and biotite weathering, Fe2+-rich corrensite formation, and Fe behavior under low PO2 conditions and their implication for Precambrian weathering

Abstract Fresh and weathered granite from drill cores in Tono, Gifu, Japan, was examined to understand weathering products and the mechanisms of chlorite and biotite weathering under low PO₂ conditions. A fresh sample from 365 m depth, a slightly weathered light-green sample from 367 m depth, and a nearly fresh sample from 369 m depth (but with brown stains on fractures), were investigated. The XRD, SEM, EMPA, and TEM analysis of green grains present within chlorite, biotite, and plagioclase grains and in veins was found to be Fe2+-rich corrensite [about 40 wt% FeO with Fe/(Fe + Mg) = 0.94]. The corrensite is interpreted to have formed from chlorite and biotite via a dissolution-precipitation mechanism. The <2 μm fraction of the weathered sample had an Fe2+/ΣFe value of 0.69, which, when combined with the presence of amorphous Fe3+ (hydr)oxides confirmed by TEM, indicates that the Fe2+/ΣFe value of corrensite is >0.69. These results indicate that on dissolution of chlorite and biotite, Fe2+ was transported as Fe2+ and precipitated as Fe2+-rich corrensite and a part of the dissolved Fe2+ was oxidized to amorphous Fe3+ (hydr)oxides under low PO₂ conditions. The formation of Fe2+-rich corrensite and that of Fe2+-rich smectite or vermiculite in the laboratory at 1 atm of PCO₂ and ≤3 × 10-5 atm of PO₂ (Murakami et al. 2004) suggest that a possible Fe2+-bearing product during Precambrian weathering is Fe2+-rich sheet silicates but not siderite.

A new AMS facility at the JNC Tono Geoscience Center, Japan

A new AMS facility (NEC 15SDH-2) based on a 5 MV tandem accelerator has been installed at the Japan Nuclear Cycle Development Institute (JNC) Tono Geoscience Center in March 1997. It has two cesium sputter negative ion sources (for solid and gas samples) which are connected to a fast sequential injection beam line. A number of performance tests have been done on the MC-SNICS solid source for radiocarbon. Precision measurements with modern samples have shown that a precision below 0.5% is feasible on a routine basis. The 14C background of the machine and the chemistry blank are 0.04 and 0.10 pMC respectively, indicating a 14C dating limit of 56,000 yr BP. The normalized JNC 14C values for IAEA standard materials (C-2 to C-8) agree well with the nominal values. Since October 1998, the MC-SNICS solid source is used for routine radiocarbon dating. Preliminary results of performance tests of the MGF-SNICS gas source are reported, too.

Geochemical constraints on the origin and stability of the Tono Uranium Deposit, Japan

Data characterizing the mineralogy, hydrochemistry and geomicrobiology of the Tono region of central Japan were used to interpret geochemical constraints on the origin and stability of the Tono Uranium Deposit. The derived constraints are compatible with models of deposit formation, which call for leaching of uranium from the upper weathered zone of the Toki Granite by relatively oxidizing groundwaters that are near-neutral to moderately alkaline and carboniferous. The oxidizing groundwaters then migrate into mudstones and sandstones of the overlying Toki Formation, where the uranyl species is reduced by water–rock–microbe interactions to uranous species, sorbed by various detrital and authigenic phases and eventually precipitated as uraninite, coffinite and the metastable, amorphous hydrous oxide, UO2(am). Formation of the Tono deposit may have been more or less continuous up to the present time. The modern hydrochemical system, upon which the genetic model is based, began to evolve about 15 Ma when seawater was flushed out of the sedimentary cover and basement granite by fresh, meteoric waters during a period of uplift and erosion preceding Pliocene to Pleistocene sedimentation. Recharge with meteoric water continued to the present, which suggests that palaeohydrochemical conditions were probably similar to those observed in the region today when the Tono deposit began to form about 10 Ma. Redox environments in the Tono region inferred from in-situ Eh measurements in deep boreholes and calculated potentials for the SO42−/HS− redox couple appear to be controlled by heterogeneous reactions involving Fe(III)-oxyhydroxides. Metastable equilibria and particle-size effects associated with these reactions produce a range of possible redox environments that are equally compatible with both the relatively oxidizing and reducing groundwaters of the Toki Granite. This compatibility extends to sedimentary porewaters, where the redox environment is also controlled by microbially mediated sulphate reduction, oxidation of organic matter and precipitation of sulphide minerals. Redox conditions have been stable during at least the past several tens of thousands of years based on palaeoredox indicators interpreted from the trace element contents of fracture calcites. The pH and carbonate contents of palaeogroundwaters and modern groundwaters of the Tono region were, and are, controlled mainly by calcite equilibrium.

An overview of a natural analogue study of the Tono Uranium Deposit, central Japan

The basic concept of deep geological disposal of high-level radioactive waste is to isolate the waste from the human environment for the long term. Because the Japanese islands are located in a geologically active area, geological phenomena such as exhumation and fault activity must be considered by any safety assessment connected with deep geological disposal. The Tono Uranium Deposit, central Japan, has been affected by such geological phenomena during the interval since its formation, and so it is a suitable analogue for evaluating how this might be done. The present natural analogue study of the Tono Uranium Deposit (Tono Natural Analogue Project) was started in 2001 with the main aim of studying a so-called ‘worst-case scenario’ for performance assessment (PA). The project involved characterizing the geology, hydrogeology, geochemistry and microbiology of the deposit and obtaining quantitative information about specific times in the past, as a means for developing, and building confidence in, conceptual and numerical models. This project applied systems analysis, which has been widely undertaken in PAs of deep geological isolation. Systems analysis involves a systematic identification, classification and screening of features, events and process (FEPs) that occur or have occurred in and around the deposit. Based on the site data, important FEPs were identified.

Characterizing the chemical containment properties of the deep geosphere: water-rock interactions in relation to fracture systems within deep crystalline rock in the Tono area, Japan

Abstract A water/rock interaction study has been integrated with interpretations of geological structures, in order to understand the geochemical conditions in deep granitic rock at the Tono study site, central Japan. Geological investigations show that fracture systems in the granitic rock can be classified into: intact zones; moderately fractured zones; and intensely fractured zones. This classification is based upon the frequency and width of fractures and fractured zones. Isotopic analyses of the groundwaters indicate that the groundwater at SL 80m (SL: sea level) has recharged within the last 30 years. Petrological and mineralogical observations were undertaken, and coupled with theoretical calculations using the PHREEQE geochemical computer code. The results of this approach suggest that the chemical evolution of the groundwater is generally controlled by water-rock interactions involving plagioclase, clay minerals and ferric hydroxide in the intact zones and moderately fractured zones. The geochemical condition of the groundwater is correlated closely with the nature of the fracture systems, mineral compositions and water-rock interaction processes. Different fractures contain chemically distinct groundwaters, which have differing potential capacities to mobilize radionuclides. The investigation method, based on a comparison between geological structures and groundwater chemistry, can be applied to develop a realistic hydrogeochemical model for deep, fractured granitic rocks. Such an approach is necessary if the chemical containment potential of such lithologies is to be evaluated adequately. The work demonstrates that structural controls on chemical heterogeneities in groundwaters should be understood, before the disposal of redox-sensitive wastes can be undertaken safely in fractured crystalline rocks.

Europium-binding abilities of dissolved humic substances isolated from deep groundwater in Horonobe area, Hokkaido, Japan

The binding of europium to dissolved humic substances (HSs), isolated from deep groundwater in the Horonobe area, Hokkaido, Japan, was evaluated by means of a three-dimensional fluorescence quenching technique. The Ryan and Weber model, extended by the charge neutralization model, was applied to the fluorescence quenching profiles to evaluate the conditional binding constants (K) at pH 5.0 and an ionic strength of 0.1. The K values for fulvic (FA) and humic acids (HA) in the Horonobe groundwater were comparable with each other regardless of the position of the fluorescence peaks. As compared with HSs from Lake Biwa, Aldrich, and Dando soil, the K values of the Horonobe groundwater HSs were the smallest, indicating that the Horonobe groundwater HSs have a lower affinity for the binding of europium compared to the HSs from surface environments. In addition, the results of the Aldrich-based comparison indicate that the Horonobe groundwater HSs have a lower binding affinity for trivalent lanthanides/actinides compared with other groundwater HSs obtained from different aquifer. These findings demonstrate that the characteristics and origin of deep groundwater HSs needs to be taken into consideration assessing the effects of HSs on the speciation of radionuclides in deep groundwater in geological disposal systems.