Richard Metcalfe

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.

Performance assessments for the geological storage of carbon dioxide: Learning from the radioactive waste disposal experience

The geological storage of carbon dioxide is currently being considered as a possible technology for reducing emissions to atmosphere. Although there are several operational sites where carbon dioxide is stored in this way, methods for assessing the long-term performance and safety of geological storage are at an early stage of development. In this paper the similarities and differences between this field and the geological disposal of radioactive wastes are considered. Priorities are suggested for the development of performance assessment methods for carbon dioxide storage based on areas where experience from radioactive waste disposal can be usefully applied. These include, inter alia, dealing with the various types of uncertainty, using systematic methodologies to ensure an auditable and transparent assessment process, developing whole system models and gaining confidence to model the long-term system evolution by considering information from natural systems. An important area of data shortage remains the potential impacts on humans and ecosystems.

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.

A system model for the origin and evolution of the Tono Uranium Deposit, Japan

Data from the Tono Uranium Deposit of central Japan were used to develop an improved approach for simulating uranium migration and retardation, while taking into account both long-term environmental changes and uncertainties in data. Based upon extensive field and laboratory investigations, conceptual and numerical models for environmental perturbations, including uplift, subsidence and faulting, were constructed. Model development was based on a novel adaptation of a safety assessment methodology that previously has been applied to radioactive waste repositories. A ‘reference scenario’ was developed using a systems analysis approach. This scenario is a best estimate of how the geological system and the uranium deposit reached their present states and includes descriptions of all major environmental perturbations. Uranium is mobilized from the uppermost Toki granite under relatively oxidizing conditions, and is then transported by groundwater into overlying sedimentary rocks. There, reducing conditions promote uranium deposition. A specially designed numerical model simulated the main features of this scenario. Many simulations were performed to identify key uncertainties to which the timing of ore deposition and uranium distribution are sensitive. A key finding is that retardation of U by processes other than precipitation of discrete U minerals, most probably sorption on solid phases, contributes significantly to the stability of the ore deposit. Sorption could potentially be important for confining the U within the sedimentary rocks in spite of environmental changes such as exhumation and seismic pumping. The approach could be used elsewhere, to assess the safety of deep geological high-level radioactive waste (HLW) disposal. A related application would be at potential future waste disposal sites, to prioritize site characterization so that the most safety-relevant uncertainties are reduced. There are also possible applications in other fields, most notably to assess the implications of alternative ore genetic models.

A biogeochemical assessment of the Tono site, Japan

Abstract When designing investigations of microbial populations in the subsurface, it is extremely valuable to undertake scoping calculations to estimate the likely microbial abundances and evaluate the effects of contamination during sampling. A biogeochemical assessment of the groundwater and lithologies of the Tono mine, Japan, has been made using the BGS/NAGRA computer code BGSE (Bacterial Growth in Subsurface Environments). This code enables an assessment to be made of the maximum microbial growth rates that may be achieved in ideal circumstances, based on availability of nutrients and energy calculated from mineralogical and groundwater analyses. The effect of drilling fluid/groundwater mixing on biomass was assessed using a hypothetical drilling fluid composition. The results of modelling the mixing between groundwater and drilling fluid shows that the addition of only small concentrations of drilling fluid (

Early post-mortem formation of carbonate concretions around tusk-shells over week-month timescales.

Carbonate concretions occur in sedimentary rocks of widely varying geological ages throughout the world. Many of these concretions are isolated spheres, centered on fossils. The formation of such concretions has been variously explained by diffusion of inorganic carbon and organic matter in buried marine sediments. However, details of the syn-depositional chemical processes by which the isolated spherical shape developed and the associated carbon sources are little known. Here we present evidence that spherical carbonate concretions (diameters φ : 14 ~ 37 mm) around tusk-shells (Fissidentalium spp.) were formed within weeks or months following death of the organism by the seepage of fatty acid from decaying soft body tissues. Characteristic concentrations of carbonate around the mouth of a tusk-shell reveal very rapid formation during the decay of organic matter from the tusk-shell. Available observations and geochemical evidence have enabled us to construct a ‘Diffusion-growth rate cross-plot’ that can be used to estimate the growth rate of all kinds of isolated spherical carbonate concretions identified in marine formations. Results shown here suggest that isolated spherical concretions that are not associated with fossils might also be formed from carbon sourced in the decaying soft body tissues of non-skeletal organisms with otherwise low preservation potential.

Understanding Radionuclide Migration from the D1225 Shaft, Dounreay, Caithness, UK

A 65 m vertical shaft was sunk at Dounreay in the 1950s to build a tunnel for the offshore discharge of radioactive effluent from the various nuclear facilities then under construction. In 1959, the Shaft was licensed as a disposal facility for radioactive wastes and was routinely used for the disposal of ILW until 1970. Despite the operation of a hydraulic containment scheme, some radioactivity is known to have leaked into the surrounding rocks. Detailed logging, together with mineralogical and radiochemical analysis of drillcore has revealed four distinct bedding-parallel zones of contamination. The data show that Sr-90 dominates the bulk beta/gamma contamination signal, whereas Cs-137 and Pu-248/249 are found only to be weakly mobile, leading to very low activities and distinct clustering around the Shaft. The data also suggest that all uranium seen in the geosphere is natural in origin. At the smaller scale, contamination adjacent to fracture surfaces is present within a zone of enhanced porosity created by the dissolution of carbonate cements from the Caithness flagstones during long-term rockwater interactions. Quantitative modelling of radionuclide migration, using the multiphysics computer code QPAC shows the importance of different sorption mechanisms and different mineralogical substrates in the Caithnesss flagstones in controlling radionuclide migration.

Generalized conditions of spherical carbonate concretion formation around decaying organic matter in early diagenesis

Isolated spherical carbonate concretions observed in marine sediments are fascinating natural objet trouve because of their rounded shapes and distinct sharp boundaries. They occur in varied matrices and often contain well preserved fossils. The formation process of such concretions has been explained by diffusion and rapid syn-depositional reactions with organic solutes and other pore water constituents. However, the rates, conditions and formation process of syngenetic spherical concretions are still not fully clear. Based on the examination of different kinds of spherical concretions from several locations in Japan, a diffusion based growth diagram was applied to define the generalized growth conditions of spherical concretions formed around decaying organic matter. All analytical data imply that the spherical concretions formed very rapidly, at least three to four orders of magnitude faster than previously estimated timescales. The values indicate that spherical concretions are preferentially grown within clay- to silt-grade marine sediments deposited in relatively deep (a few tens of metres) environments dominated by diffusive solute transport, very early in diagenesis.

Effects of α-radiation on a direct disposal system for spent nuclear fuel – (2) review of research into safety assessments of direct disposal of spent nuclear fuel in Europe and North America

The Japanese geological disposal programme has started researching disposal of spent nuclear fuel (SF) in deep geological strata (hereafter “direct disposal of SF”) as an alternative management option other than reprocessing followed by vitrification and deep geological disposal of high-level radioactive waste (HLW). In the case of direct disposal of SF, the radioactivity of the waste is higher and the potential effects of the radiation are greater. Specific examples of the possible effects of radiation include: increased amounts of canister corrosion; generation of oxidizing chemical species in conjunction with radiation degradation of groundwater and accompanying oxidation of reducing groundwater; and increase in the dissolution rate and the solubility of SF. Therefore, the influences of radiation, which are not expected to be significant in the case of geological disposal of vitrified waste, must be considered in safety assessments for direct disposal of SF. Focusing especially on the effects of α-radiation in safety assessment, this study has reviewed safety assessments in countries other than Japan that are planning direct disposal of SF. The review has identified issues relevant to safety assessment for the direct disposal of SF in Japan.