María Jesús Turrero

Generation and stability of bentonite colloids at the bentonite/granite interface of a deep geological radioactive waste repository

The possible mechanisms of colloid generation at the near field/far field interface of a radioactive repository have been investigated by means of novel column experiments simulating the granite/bentonite boundary, both in dynamic and in quasi-static water flow conditions. It has been shown that solid particles and colloids can be detached from the bulk and mobilised by the water flow. The higher the flow rate, the higher the concentration of particles found in the water, according to an erosion process. However, the gel formation and the intrinsic tactoid structure of the clay play an important role in the submicron particle generation even in the compacted clay and in a confined system. In fact, once a bentonite gel is formed, in the regions where the clay is contacted with water, clay colloids can be formed even in quasi-static flow conditions. The potential relevance of these colloids in radionuclide transport has been studied by evaluating their stability in different chemical environments. The coagulation kinetics of natural bentonite colloids was experimentally studied as a function of the ionic strength and pH, by means of time-resolved light scattering techniques. It has been shown that these colloids are very stable in low saline (approximately 1 x 10(-3) M) and alkaline (pH > or = 8) waters.

Study of the pore water chemistry through an argillaceous formation: a paleohydrochemical approach

The spatial and temporal changes of the pore water along an argillaceous formation were studied on the basis of the chemistries of the fluids collected through the system. The study is based on chemical characterisation results of the pore water, which requires careful sampling and 4 years monitoring. The concept was applied to the water sampled from several boreholes drilled through the Opalinus Clay formation in the anticline at Mont Terri, Canton Jura, Switzerland. Asymmetrical gradients from the clay formation toward the limestone overburden are observed. In this upper formation, recharge with low mineralised water has caused the diffusion of species from the saline pore waters in the clay formation and their depletion. These species form concentration profiles from the underlying Jurensis Marl formation containing hydrophobic organic material toward the upper Lower Dogger Limestone formation. Non-sorbing species such as chlorides, bromides, iodides and sulphates as well as Na as a weakly sorbing cation form increasing concentration profiles from the Lower Dogger Limestone into the Opalinus Clay formation. Heavy water isotopes display similar profiles. The pH increases slightly downward through the system (from about 7 to 8) together with the total organic C (TOC) concentration, while the total inorganic C (TIC) concentration decreases from the Lower Dogger Limestone through the Opalinus Clay formation. A similar profile is observed for pe which decreases from the limestone groundwater (+2.5) toward the underlying marl water (−2.5). The water composition is discussed taking into account in-situ pore water dilution from recharge water by mean of a diffusion mechanism. Quantification of this transport process and of the consequent concentration profile is carried out through the 165 m thick Opalinus Clay layer, which was impregnated by fossil pore water derived from Tethys Ocean, considering a diffusion process that started about 10 Ma ago. The apparent diffusion coefficients estimated in the Opalinus Clay for Cl−, Br−, I– and Na+, on one hand, and, for 2H and 18O on the other are 2.6±0.8 and 5.2±1.5·10−11 m2 s−1, respectively. These values are compared with data gained from other argillaceous systems. The Opalinus Clay formation is likely to have acted as a geological nanoporous barrier for 10 Ma.

Diagenetic processes influencing porosity in sandstones from the Triassic Buntsandstein of the Iberian Range, Spain

Abstract Sandstones of the Triassic Buntsandstein of Spain were studied in an attempt to quantify effects of diagenetic processes such as compaction, cementation and leaching on reservoir properties. Samples were taken from Shell well Siguenza 44-3 some 110 km northeast of Madrid, drilled on the Siguenza High in the Iberian Range. The sandstones are arkoses to sublitharenites of a typical Triassic redbed facies characteristic of the whole region. The present-day depth of the base Buntsandstein is 538 m, whilst a maximum burial depth of ca. 2600 m was estimated from vitrinite reflectance (0.94 to 1.14), kaolinite temperature (76°C) and illite crystallinity (Kuebler index 4.9). From an estimated average Initial Porosity of 38%, porosity was reduced to the present value of 13%. The strongest process to reduce porosity was found to be mechanical compaction causing a loss of 15%, followed by chemical compaction (pressure solution) with a loss of about 6%. The cement (11.4% R.V.) apparently reduced the Initial Porosity by about only 5%. Early carbonate and sulphate cements are interpreted to have stabili sed the rock framework working against mechanical compaction and pressure solution. This is supported by an increase in sutured grain contacts where early cements are absent or rare. The later quartz cement, interpreted to be largely the product from pressure solution, has reduced porosity in a ratio of 0.5:1. This suggests that compaction continued after the formation of the late quartz cement. Although leaching was observed especially in feldspar grains, clay and K-feldspar neomorphosis is interpreted to have counteracted the formation of substantial secondary porosity. Permeability in the Buntsandstein is generally low (average 17.4 mD) and has been strongly affected by the alteration of feldspars to clay causing a wide range of permeability from 0.11 to 73.0 mD.

Mineralogical Control of the REE Distribution in the Fracture Fillings of an Uranium-Ore (Caceres-Spain)

The behavior of actinides in a deeply buried radioactive waste cannot be predicted from evidence of their movement in geological environments in the geological past because these elements do not occur naturally in appreciable concentration. A useful means of reducing the uncertainty is to observe the behavior, both past and present, of chemical analogues in geological environment. In this sense, the rare earth elements (REE) have been used to predict the behaviour of actinide series elements in water-rock systems on account of the similar valence and ionic radii and high similarity in its electronic structure. This paper describes the factors that control REE signature in the fracture fillings of an ancient uranium mine (dug granite) located in the Central-Iberian Zone of the Hesperian Massif (Caceres, Spain). The study of REE distribution in the fracture fillings provides interesting information about the REE distribution in the hydrothermal fluids that flow through the system. In order to obtain this information, it is necessary to evaluate the control of different minerals in the total REE content of the fracture fillings. The fracture fillings display a large mineralogical variety dominated by K-feldspars, quartz, plagioclase, micas, monazites, xenotimes, apatites, uraninites, inherited of the granites, and carbonates, quartz, apatites, phyllosilicates, sulphides and Fe-oxihydroxydes, neoformed in successive hydrothermal stages. In general, the fracture fillings present different kind of REE patterns, just as positive or negative Eu anomalies. The REE final distribution in the fracture fillings is controlled by the presence of REE-minerals, both inherited and neoformed, and of carbonates, with great capacity to fractionate HREE in the system. On the other hand, the chemical composition of the carbonates and accessory minerals is responsible, as a last resort, for the Eu anomalie (positive or negative) observed in the fracture fillings.