H43I-01 13:40h
Determining Sorption Coefficients in Intact Rock Using an Electrical Potential Gradient as a Driving Force for Migration
The transport of radionuclides in indigenous rock is greatly affected by the sorption of cations in the porous rock matrix. Radionuclides escaping from a repository for nuclear waste can be subjected to such conditions. Because of this, accurate sorption data is needed for the performance assessment for deep geological disposal of nuclear waste. For the determination of sorption coefficients, batch experiments have traditionally been used to reduce the experimental time. For batch experiments, the rock sample is crushed into fine particles, increasing the surface area of the sample. The small particle diameter makes batch experiments time efficient but the increase of surface area has been shown to have an effect on the sorption coefficients measured (Skagius et. al., 1982, Byeg$\aa$rd et. al., 1998). It is also probable that the crushing and sieving of the rock sample causes a differentiation of the minerals present in the rock, which could further impair the measurement of sorption coefficients. A new method for measuring sorption coefficients in intact rock is being developed. The method was proposed by L$\ddot{o}$fgren and Neretnieks (2003) and uses electromigration as a mean to speed up the transport process allowing for faster equilibration between a rock sample and an electrolyte. A potential gradient acts as a driving force in addition to the concentration gradient and forces the cations through the rock sample towards the anode. By measuring the change in chemical composition of the electrolyte the sorption coefficient can be determined. Preliminary experiments using cesium as a sorbing tracer were performed using cylindrical rock samples with a diameter of 50 mm and a length of 15 mm. The results from these experiments have shown that it is possible to measure sorption capacity using this method.
H43I-02 INVITED 13:55h
Determination of In-situ Porosity and Investigation of Diffusion Processes at the Grimsel Test Site, Switzerland.
In the context of a repository for radioactive waste, 'matrix diffusion' is used to describe the process by which solute, flowing in distinct flow paths, penetrates the surrounding rock matrix. Diffusion into the matrix occurs in a connected system of pores or microfractures. Matrix diffusion provides a mechanism for greatly enlarging the area of rock surface in contact with advecting radionuclides, from that of the flow path surfaces (and infills), to a much larger portion of the bulk rock and increases the global pore volume which can retard radionuclides. In terms of a repository safety assessment, demonstration of a significant depth of diffusion-accessible pore space may result in a significant delay in the calculated release of any escaping radionuclides to the environment and a dramatic reduction in the resulting concentration released into the biosphere. For the last decade, Nagra has investigated in situ matrix diffusion at the Grimsel Test Site (GTS) in the Swiss Alps. The in situ investigations offer two distinct advantages to those performed in the lab, namely: 1. Lab-based determination of porosity and diffusivity can lead to an overestimation of matrix diffusion due to stress relief when the rock is sampled (which would overestimate the retardation in the geosphere) 2. Lab-based analysis usually examines small (cm scale) samples and cannot therefore account for any matrix heterogeneity over the hundreds or thousands of metres a typical flow path The in situ investigations described began with the Connected Porosity project, wherein a specially developed acrylic resin was injected into the rock matrix to fill the pore space and determine the depth of connected porosity. The resin was polymerised in situ and the entire rock mass removed by overcoring. The results indicated that lab-based porosity measurements may be two to three times higher than those obtained in situ. While the depth of accessible matrix from a water-conducting feature assumed in repository performance assessments is generally 1 to 10 cm, the results from the GTS in situ experiment suggested depths of several metres could be more appropriate. More recently, the Pore Space Geometry (PSG) experiment at the GTS has used a C-14 doped acrylic resin, combined with state-of-the-art digital beta autoradiography and fluorescence detection to examine a larger area of rock for determination of porosity and the degree of connected pore space. Analysis is currently ongoing and the key findings will be reported in this paper. Starting at the GTS in 2005, the Long-term Diffusion (LTD) project will investigate such processes over spatial and temporal scales more relevant to a repository than traditional lab-based experiments. In the framework of this experiment, long-term (10 to 50 years) in situ diffusion experiments and resin injection experiments are planned to verify current models for matrix diffusion as a radionuclide retardation process. This paper will discuss the findings of the first two experiments and their significance to repository safety assessments before discussing the strategy for the future in relation to the LTD project.
http://www.grimsel.com
H43I-03 14:15h
Flow and Transport of Chemical Species in a 1-m Scale Fractured Granite Block
Migration of chemical species has been studied in a single fracture of 1m scale in order to understand the transport behavior of contaminants at underground environments. The applied tracers were: tritium, anions as nonsorbing tracer and several cations and actinides as highly sorbing ones. Nine boreholes were drilled into the rock from the upper surface orthogonal to and ending at the fracture. The hydraulic conductivity in the fracture was determined from the pressure differential between pairs of boreholes. The hydraulic data were used with a variable aperture channel model to characterize the aperture distribution in the fracture. And a particle tacking method was used to simulate nuclides transport. The developed model was validated using the results obtained from the migration experiments. Results were plotted in the form of elution curves and migration plumes in the fracture. The elution curves have been explored with the transport model which takes into account diffusion into the rock matrix. The experimental study was focused on the identification of the retardation and matrix diffusion of the tracer in the fracture, thus providing a way to interpret the fracture field and the interactions between the rock and chemical species.
H43I-04 INVITED 14:30h
High-pH Plume in a Fractured Granite: Mineral Reaction and Permeability Changes
One of the objectives of the HPF experiment (Hyperalkaline Plume in Fractured Rock) at the Grimsel Test Site (Switzerland) is to study the alteration of a fractured granite due to the circulation of high-pH solutions derived from the degradation of cement. A K-Na-Ca-rich high-pH solution was injected into a fracture during 3 years. An extraction borehole was located about 80 cm away from the injection borehole. The modeling of dipole tracer tests has indicated that flow in the fracture plane was highly heterogeneous. A small-scale laboratory version of the experiment (1D core infiltration experiment) was performed at the University of Bern. Both laboratory and field experiments showed significant changes in solution chemistry and an important decrease in permeability despite the small amount of secondary mineral precipitation observed in the laboratory experiment (excavation and rock sampling are currently under way in the field at Grimsel). However, dipole tracer tests showed a decrease in first arrival and peak arrival times and an increase in peak concentrations with increased alteration. The amount of mixing with natural groundwater at extraction also decreased. Detailed reactive transport modeling coupled with flow of the laboratory experiment has shown that the evolution of the system is consistent with mineral reactive surface areas smaller than measured specific surface areas by a factor of 10 to 100. However, the initial transient period in solution chemistry seems to indicate the presence of short lived fine particles (large surface areas), which may account for the difference between measured and calculated surface areas.
http://www.grimsel.com
H43I-05 14:50h
Temperature-Profile Methods for Estimating Thermally-Driven Flow Processes in Superheated Rocks
In geologic repositories for storage of nuclear wastes, the heat generated by the decay of the radioactive waste may result in rock temperatures high enough to cause boiling conditions in the subsurface, which gives rise to strongly altered flow processes. These flow processes are characterized by (1) vapor production in the superheated zone close to the heat source, (2) pressure-driven vapor transport away from the heat source, (3) condensation in cooler regions, and (4) reflux of the condensate back to the heat source. Since the magnitude of such flow perturbation is extremely hard to measure in the field, we propose a simple temperature-profile method that uses high-resolution temperature data for deriving such information. The energy that is transmitted by the vapor-water reflux processes creates a nearly isothermal zone maintained at about the boiling temperature, referred to as a heat-pipe signature. Characteristic features of the temperature profile, such as the differences in the gradients inside and outside of this zone, can be used to derive the approximate magnitude of the vapor and water fluxes, for both steady-state and transient conditions. We present the theoretical basis for the proposed temperature-profile method, test the method in comparison with a semi-analytical solution of thermally-driven flow processes, and present a sample application using measured temperature profiles from an underground heater test.
H43I-06 15:05h
Transient Versus Seasonal Geochemical Signals in Water Dripping From Fractured Rocks and Their Related Forcing
Solute transport in fractured porous media presents a large variability in space and time due to variations in water content, in flow pathway and regime, and chemical reactions. The dynamic of these systems in response to external stimuli such as meteorological, mechanical, hydrogeological and chemical disturbances remains poorly known. To build reliable predictive models, there is a strong need to obtain data from complex but well characterized natural observatories. We set up an experiment in a 150 m long tunnel where drippings give access to ground water flowing vertically from 50 m of variably saturated fractured gneiss. This tunnel is located close to and above the artificial Lake Roselend (France) where large variations in level induce reproducible deformation and hydrogeological disturbances. This area is also characterized by contrasted infiltration regimes with alternating snow, rain, and dry periods. In addition, we performed a tracer test with the surface injection of NaCl, 50 m vertically above the dripping water collecting points. In order to get water representative of different contributions of matrix porosity and fractures, we monitored several zones with different structures and flow rates, and looked for changes in flow rates and ion concentrations in water over several years. In addition, we monitored in the tunnel atmosphere the radon activity, independently known to respond to deformation events. Water composition reflects a yearly equilibrium. In spring, concentrations decrease by inputs of rain and melted snow along fractures at high flow rates with limited reactivity. In summer, concentrations increase by diffusion of saline water from matrix porosity and increased dissolution at low flow rates. We observed transient increases of SO4 and Mg concentrations associated with radon bursts, and that are not related to meteorology. Transient enhancement of conductance, with discharges of saline water and radon from the matrix porosity to the fractures, is thus a mechanism able to produce both ground-water and gas anomalies in response to hydrogeological or mechanical processes such as increases in pore pressure or changes in crack geometry. Before the tracer injection, Na, Mg, Ca and SO4 are contributed by the host rock, whereas Cl and K are contributed by meteoric sources, soils and surface biota. After the surface injection of tracers, NaCl breakthrough (Na being late relative to Cl) is associated with an important release of K, Ca, Mg, Ba, and F that were not injected and are contributed by the host rock by ion exchange reactions. By contrast, SO4 is generated by oxidative dissolution of pyrite enhanced by air entry on drought regime and subsequent flush on recharge. Natural and artificial stimuli, even minor, are thus able to trigger major changes in solute transport in fractured rocks, with transient, seasonal or long-term evolutions.
H43I-07 15:20h
Flow and Transport in Fractured Porous Sandstone: Experimentation and Modeling at Aquifer-Analog Scale
The reliability of numerical models for the prediction of flow and contaminant solute transport in fractured porous aquifers is very limited due to the high structural complexity of these systems. Both the influence of the fracture orientation on flow and transport processes and the varying participation of advective, dispersive, diffusive, and sorptive transport mechanisms can hardly be parameterized. With the realization of in situ flow and tracer experiments at a completely sealed, natural aquifer cutout of about 200 m$^3$ volume, both flow and transport parameters can be evaluated under controlled boundary conditions. The rock at the test site is a highly porous and densely fractured Triassic sandstone. Thus, the diffusive interactions between fracture and rock matrix are intensified and can be experimentally investigated more easily. Primary field studies provide information not only about the basic rock parameters as porosity and rock permeability. Also the statistical fracture orientation, fracture distance distribution and fracture length distribution are determined. By choosing a radial symmetrical borehole arrangement in the center part of the fractured porous sandstone, comparable flow and transport experiments can be conducted at 42 different borehole-to-borehole connections and in twelve different directions. Thus a statistical rating of each flow direction concerning permeability, travel times, maximum concentration and retardation effects is made possible. The measured transport processes can also be judged in terms of dispersion lengths and the participation of diffusive processes by the application of analytical analysis. Furthermore, interactions of different preferred flow channels inside the natural fractured system may be identified. The experimentally measured breakthrough curves are finally used for the calibration and validation of different numerical model concepts.
http://www.agk.uni-karlsruhe.de/projekte/projekte_hydro/aquiferanalog.html