The experience gained in modeling the evolution, from past to present, of natural tracer profiles in geologic media can help support safety assessments of disposal concepts for radioactive wastes in deep geologic repositories because the assessments will be based on predictions using similar models of conditions that could evolve, from the present to the future, in the repository's near-field and far-field environments. Solute-transport models were developed in the present study using a forward modeling approach constrained by boundary conditions inferred from the paleo-hydrogeological evolution of the Horonobe area in Hokkaido, Japan and ranges in model parameter values determined in laboratory and field-based investigations at two deep borehole locations in the area. The models were calibrated by adjusting individual parameter values within limits imposed by the site-characterization data to optimize agreement between model predictions and observations. Results from the calibrated models were consistent with results from other studies suggesting that observed variations with sampling depth in Cl concentrations and 818O and 8D values resulted primarily from an advective transport regime that existed for a relatively brief period during the most recent glacial-interglacial cycle. Advective transport was possible during this time because thawing of a discontinuous permafrost zone starting about 18 ka allowed meteoric water to recharge the flow system and hydraulic gradients were larger than at present because sea levels were significantly lower than the current levels reached at the beginning of the Holocene (i.e., since about 12 ka). Similar climatic controls on the generation and duration of advective/diffusive transport regimes likely existed during nine previous glacial-interglacial cycles that occurred in the Horonobe area since regional uplift began about 1 Ma. If so, alternative versions of the transport models suggest the tracer profiles observed in this area may have evolved over a much longer cumulative period of advective transport and at lower Darcy velocities than assumed in the calibrated models but in a manner that is still compatible with ranges in model parameter values determined in laboratory and field investigations. Apparent differences in transport behavior at the two borehole locations considered in this study, which were situated only about 1 km apart, appear to have resulted from relatively small differences in accessible porosity and hydraulic conductivity, which in turn may have been controlled by local differences in fracture density and fracture connectivity.

NH4+ is potentially an important constituent of deep groundwater under reducing condition. The retention of cesium by sorption in geological formations may have an important role ensuring the long-term safety of hig-hlevel radioactive waste disposal systems. Cs+ sorption will be affected by competing effects due to dissolved cation likely NH4+ in groundwater, however. In the present study, a possible reaction controlling of NH4+ concentrations in deep groundwater was evaluated based on data from Japan. An investigation of mineralogy and thermodynamic modeling of groundwaters suggests that K-bearing clay minerals (smectite, illite or interstratified illite/smectite), appear to control the NH4+ concentration in groundwaters by ion exchange. Additionally, the selected groundwaters in the Horonobe area seem resemble Japanese gas and oil field groundwaters in terms of NH4+ distribution.

The polymer model provides a relatively simple and robust basis for estimating the standard Gibbs free energies of formation (ΔGfo) and standard enthalpies of formation (ΔHfo) of clay minerals and other aluminosilicates with an accuracy that is comparable to or better than can be obtained using alternative techniques. The model developed in the present study for zeolites entailed the selection of internally consistent standard thermodynamic properties for model components, calibration of adjustable model parameters using a linear regression technique constrained by ΔGfo and ΔHfo values retrieved from calorimetric, solubility, and phase-equilibrium experiments, and assessments of model accuracy based on comparisons of predicted values with experimental counterparts not included in the calibration dataset. The ΔGfo and ΔHfo predictions were found to average within ±0.2% and ±0.3%, respectively, of experimental values at 298.15 K and 1 bar. The latter result is comparable to the good accuracy that has been obtained by others using a more rigorous electronegativity-based model for ΔHfo that accounts explicitly for differences in zeolite structure based on differences in framework density and unit-cell volume. This observation is consistent with recent calorimetric studies indicating that enthalpies of transition from quartz to various pure-silica zeolite frameworks (zeosils) are small and only weakly dependent on framework type, and suggests that the effects on ΔHfo of differences in framework topology can be ignored for estimation purposes without incurring a significant loss of accuracy. The relative simplicity of the polymer model, together with its applicability to both zeolites and clay minerals, is based on a common set of experimentally determined and internally consistent thermodynamic properties for model components. These attributes are particularly well suited for studies of the effects of water-rock-barrier interactions on the long-term safety of geologic repositories for high-level nuclear waste (HLW).

Empirical, geostatistical and geochemical modeling techniques were used to determine whether chemical, isotopic and mineralogical data collected in surface-based borehole investigations at the Horonobe Underground Research Laboratory (URL) site in Hokkaido, Japan were representative of the natural environment before it was disturbed by borehole drilling, hydraulic testing and sampling. Water samples collected either by pumping groundwaters to the surface from borehole sections that had been isolated by inflatable packers or by squeezing porewaters from drillcores sampled from the same borehole and sampling depth were found to be similar in certain respects (m(Na+) m(Cl-), delta(18)O and delta D) but not others (m(SO42-), m(Ca2+), m(Mg2+) and m(K+)). The inconsistencies suggest that a majority of the porewater samples were contaminated as a result of pyrite oxidation and dissolution of carbonate minerals during sampling, storage and/or squeezing of the drillcores. Multivariate geostatistical analyses support this hypothesis, and further suggest that uncontaminated porewaters and groundwaters can be divided into three representative groups: a low salinity, low delta(18)O and low delta D group; a moderate salinity, low Ca-Mg group: and a high salinity, high K and moderate Ca-Mg group. The groundwaters also contain high concentrations of dissolved gases, including CH(4)(g), CO(2)(g), H(2)S(g) and various hydrocarbons, which exsolve from the groundwaters as they are pumped to the surface for sampling. The effects of such degassing on the chemistry of groundwater samples was evaluated by using a reaction-path model to simulate the titration of gases collected at the surface back into the samples. Results suggest that undisturbed groundwaters are slightly more acidic than their sampled counterparts, and contain roughly equivalent activities of dissolved CH(4)(aq) and carbonate species. Redox potentials calculated using the corrected groundwater compositions and assuming equilibrium for the CH(4)(aq)/CO(2)(aq) redox couple are significantly more negative than those determined in situ in one borehole using a downhole chemical probe, and those that can be inferred from the ubiquitous presence of small amounts of framboidal pyrite and siderite in siliceous biogenic sediments of the Horonobe area. The empirical, geostatistical and geochemical modeling techniques considered in this study can be adapted for use in characterizing the hydrogeochemical environment of a site that will eventually be selected to host a geologic repository for high-level nuclear waste in Japan. (C) 2011 Elsevier Ltd. All rights reserved.

Saline type groundwaters data in the Mobara area (a marine based argillaceous rock) located in the well-known “South Kanto gas field” in Japan were investigated by JNC1JNC (Japan Nuclear Cycle Development Institute) was merged in October 2005 with the Japan Atomic Energy Research Institute (JAERI) to form the Japan Atomic Energy Agency (JAEA).1 as part of a natural analogue study. Most groundwaters in the field were extracted from deep gas wells (e.g., 400–2000m below the surface), and the all data reported previously were sampled at the wellhead, where physico-chemical parameters (e.g., temperature, pH, Eh etc.) were also measured. In such cases, particular attention should be paid to whether the measured and/or analyzed results are consistent with the chemical and physical conditions in the in situ geological formation because air contamination, the temperature and pressure changes during sampling can affect the groundwater chemistry. The present study shows a test case to estimate the in situ groundwater chemistry in argillaceous rock of the Mobara area using geochemical model calculations.