Effect of the pore water composition on the diffusive anion transport in argillaceous, low permeability sedimentary rocks

ABSTRACTIn Belgium, the Boom Clay is studied as the reference formation for geological disposal of high-level radioactive waste and spent fuel. As the Boom Clay is considered as the main barrier for radionuclide migration/retention, a thorough characterisation of the clay and its pore water was done. This facilitates better understanding of the long-term geological processes and the distribution of the trace elements and radionuclides.From a mineralogical/geochemical point of view, the Boom Clay is considered as a rather homogeneous sediment, vertically as well as laterally. It is composed of detrital minerals, organic matter and fossils. Minerals are mainly clay minerals, quartz and feldspars. Minor amounts of pyrite and carbonates are also present. Small variations in mineralogical/geochemical composition are related to granulometrical variations. The radiochemical study indicates that the Boom Clay is in a state of secular radioactive equilibrium, meaning that the Boom Clay has not been disturbed for a very long time.Pore water sampling is done in situ from various piezometers, or by the squeezing or leaching of clay cores in the laboratory. These three pore water sampling techniques have been compared and evaluated. Boom Clay pore water is a NaHCO3 solution of 15 mM, containing 115 mg·1−1 of dissolved natural organic carbon. Some slight variations in pore water composition have been observed and can be explained by principles of chemical equilibrium.

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The occurrence of gas hydrates in Eastern Mediterranean mud dome structures as indicated by pore-water composition

Geological Society London Special Publications ◽

10.1144/gsl.sp.1998.137.01.14 ◽

1998 ◽

Vol 137 (1) ◽

pp. 167-175 ◽

Cited By ~ 11

Author(s):

G. J. De Lange ◽

H.-J. Brumsack

Keyword(s):

Pore Water ◽

Gas Hydrates ◽

Eastern Mediterranean ◽

Water Composition

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Pore Water Chemistry of the Febex Bentonite

MRS Proceedings ◽

10.1557/proc-663-573 ◽

2000 ◽

Vol 663 ◽

Cited By ~ 27

Author(s):

A.M. Fernández ◽

J. Cuevas ◽

P. Rivas

Keyword(s):

Water Chemistry ◽

Pore Water ◽

Nuclear Waste Repository ◽

Exchange Reactions ◽

Data Set ◽

Mineralogical Characterization ◽

Sulphate Content ◽

Water Interaction ◽

Water Composition ◽

Pore Water Chemistry

ABSTRACTThe knowledge of pore water chemistry in the clay barrier is essential for performance assessment purposes in a nuclear waste repository, since the pore water composition controls the processes involved in the release and transport of the radionuclides. The methodology followed to define the representative composition of the FEBEX bentonite pore water is presented in this paper.A series of bentonite-water interaction tests have been performed with the aim of providing a database on the main chemical parameters of the bentonite. These tests were carried out both with high solid to liquid (s:l) ratios (squeezing tests) and low s:l ratios (aqueous extracts tests). The exchangeable cations have also been analyzed to determine the selectivity coefficient of the exchange reactions. To complete the data set, a physical and mineralogical characterization of the bentonite was made.The most significant bentonite-water interaction processes controlling the chemistry of the system was identified. The ion concentrations basically depend on the s:l ratio of the system, and the pore water composition is controlled by the dissolution of chlorides, dissolution/precipitation of carbonates and sulphates and the cation exchange reactions in the smectite.The bentonite/water system was modelled with the PHREEQC2 program to obtain the best possible estimation of the pore water composition for initial conditions of water content (≍14%), after checking the conceptual model with the experimental results. The model predictions fitted satisfactorily with the experimental data at low s:l ratios. At high s:l ratios, the modelled results agree adequately, except for the sulphate content, which could be affected by the effective porosity, anion exclusion or stagnant zones not taken into account in the model. According to the model, the FEBEX bentonite pore water at 14% moisture is a sodium-chloride type, with an ionic strength of 0.25 M and pH of 7.78.

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