The CLAYTRAC project has been launched by the NEA Working Group on the Characterisation, the Understanding and the Performance of Argillaceous Rocks as Repository Host Formations (known as “Clay Club”) at the beginning of 2005. This initiative was motivated by the fact that argillaceous formations are considered as potential hosts of geological repositories for radioactive waste in several countries. A number of sites are currently being investigated, and underground research laboratories are in operation. A growing body of data pertinent to natural tracers in such formations is available, in addition to studies documented in the open scientific literature. 

Obtaining tracer data from low-permeability, argillaceous formations is a non-trivial task, and a number of new, dedicated techniques have been developed over the last decade. The experimental techniques and the analytical methods on which the tracer data presented in this Chapter are based are documented in Appendix A2.

As shown in Table 3.1-1, maximum Cl- contents vary strongly among the sites considered between values close to that of sea water (Couche Silteuse at Marcoule – MAR203 and MAR402, Opalinus Clay at Mont Russelin) to contents of less than 1 % of that of sea water (Boom Clay at Mol). Many of the formations lost most of their original salinity. Maximum Cl- contents are often found in the central parts of the low-permeability sequences, with negative concentration gradients towards both the underlying and the overlying aquifers (Table 3.1-1). Particularly high gradients for Cl- are found in the Couche Silteuse at Marcoule, in the Opalinus Clay at Mont Terri and Mont Russelin and in London Clay at Bradwell and most probably indicate geologically young interactions with the aquifers and/or high transport resistance (in particular, low diffusion coefficients) in the lowpermeability sequence. At some sites (e.g. Callovo-Oxfordian at Bure – EST311/312, Boom Clay at Essen), the Cl- concentrations in the low-permeability sequence show linear trends connecting the values in the aquifers (i.e. the concentration gradient is negative towards one aquifer and positive towards the other). As a first hypothesis, such Cl- distributions can be interpreted as steady-state diffusion profiles.

The tracer data presented in this report were generally obtained in order to better characterise the hydraulic and transport properties of clay-rich aquitards. In particular, the interest is focussed on...

Model runs are called “base-case” calculations if the simulations explain the measured data reasonably well with input parameters and scenarios that are within the independently derived ranges. All base cases consider diffusion as the only transport process – not by definition but due to the observation that adding advection does not improve the model fits to the data. In contrast, “scoping” or “alternative” models refer to cases that purposely deviate from known palaeo-hydrogeological scenarios and ranges of input parameters, or to cases that are not sufficiently well constrained by independent information and therefore remain on a hypothetical level. 

Natural tracers are powerful evidence of non-sorbing solute transport and water movement in clay-rich rocks. Moreover, the interpretation of natural tracers is in general scientifically robust and consistent with physical concepts. For the sites and clay-rich formations that have been studied, there is strong evidence that movement is controlled by diffusion. In the model cases where advective movement was also considered, the goodness of the model fits to the data could not be improved. Therefore, the advective transport rate is at best comparably slow with diffusion, if not slower or absent. 

An example of a Data Tracking Document is shown below for borehole MAR203 at Gard/Marcoule, France. The empty forms were sent to the organisations who supplied information on their site(s) and also used the forms as checklists and general guides to what is needed. The Core Group then evaluated all information received and filled in the Data Tracking Document for each site (areas shaded grey in the example below). For each item, the name of the underlying electronic document was noted (if applicable), together with the references to sources of the information. Very importantly, screening procedures and all changes done to the data were recorded, together with comments.

The mass of pore water present in an argillaceous rock formation is significant and varies between a few % to several tens of % depending on the geologic history of the formation. Due to the low permeability of argillaceous formations, pore water cannot be sampled by conventional groundwater sampling techniques. Thus, information about tracer concentrations in pore water can only be obtained by applying indirect extraction techniques based on fresh, saturated rock material, or directly by long-term and cost-intensive in-situ experiments in underground facilities. Indirect techniques are described in Sections A2.4 to A2.9, and direct techniques follow in Section A2.10. 

Most values for diffusion coefficients were obtained on the basis of laboratory experiments conducted at ambient temperature. In-situ temperatures may deviate significantly from the laboratory temperature. In the case studies considered here, the most significant deviations are identified in borehole MAR501 in the Couche Silteuse de Marcoule (29.5 °C) and in Opalinus Clay at Benken (34.5 °C). In a generic way, the temperature dependence of diffusion coefficients can be described by the Stokes-Einstein equation (Li & Gregory 1974)

In order to constrain the evolution of the boundary conditions for the clay pore waters over time, the stable isotopic composition of meteoric recharge that feeds the aquifers needs to be known over the considered evolution time. The temporal variability of the stable isotopic signature of recharge water at a given site depends mainly on temperature and therefore on climate. Provided the evolution of surface temperature can be constrained on the basis of independent evidence and the relationship between temperature and isotopic composition of recharge is known, the evolution of the latter can be estimated for the periods of interest. For this purpose, previous works (e.g. Marivoet et al. 2000) used the world-wide correlation between mean annual temperature and O, H isotope compositions in precipitation (Dansgaard 1964, Fricke & O’Neil 1999:

The simulations presented in this report were carried out with the computer code FLOTRAN, written by Peter Lichtner (Lichtner 2004). FLOTRAN can simulate time-dependent, coupled thermalhydrologic- chemical processes in variably saturated, non-isothermal porous media in up to three spatial dimensions. FLOTRAN calculates multi-component reactive transport involving aqueous, mineral or gaseous species. It can also be used for systems involving two-phase flow, that is systems containing a liquid and a gaseous phase, such as in the unsaturated zone above the water table, and for systems consisting of fractured porous media. Both Cartesian and cylindrical model geometries can be used.