title: Pore water colloid properties in argillaceous sedimentary rocks.
abstract: The focus of this work is to evaluate the colloid nature, concentration and size distribution in the pore water of Opalinus Clay and other sedimentary host rocks identified for a potential radioactive waste repository in Switzerland. Because colloids could not be measured in representative undisturbed porewater of these host rocks, predictive modelling based on data from field and laboratory studies is applied. This approach allowed estimating the nature, concentration and size distributions of the colloids in the pore water of these host rocks. As a result of field campaigns, groundwater colloid concentrations are investigated on the basis of their size distribution quantified experimentally using single particle counting techniques. The colloid properties are estimated considering data gained from analogue hydrogeochemical systems ranging from mylonite features in crystalline fissures to sedimentary formations. The colloid concentrations were analysed as a function of the alkaline and alkaline earth element concentrations. Laboratory batch results on clay colloid generation from compacted pellets in quasi-stagnant water are also reported. Experiments with colloids in batch containers indicate that the size distribution of a colloidal suspension evolves toward a common particle size distribution independently of initial conditions. The final suspension size distribution was found to be a function of the attachment factor of the colloids. Finally, calculations were performed using a novel colloid distribution model based on colloid generation, aggregation and sedimentation rates to predict under in-situ conditions what makes colloid concentrations and size distributions batch- or fracture-size dependent. The data presented so far are compared with the field and laboratory data. The colloid occurrence, stability and mobility have been evaluated for the water of the considered potential host rocks. In the pore water of the considered sedimentary host rocks, the clay colloid concentration is expected to be very low (<1ppb, for 10-100nm) which restricts their relevance for radionuclide transport.
authors.affiliation: Engineering Department, University of Lancaster, LA1 4YW Lancaster, UK; ChiAM & Institute of Environment, University of Geneva, 1211 Genève 4, Swizerland; Earlier, NES, Paul Scherrer Institute, 5232 Villigen, Switzerland. Electronic address: c.degueldre@lancaster.ac.uk.
authors.forename: Claude
authors.initials: C
authors.lastname: Degueldre
authors.affiliation: NAGRA, Hardstrasse 73, 5430 Wettingen, Switzerland.
authors.forename: Veerle
authors.initials: V
authors.lastname: Cloet
keywords: Clay colloids; Groundwater colloids; Sedimentary pore water