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The Influence of Groundwater on the Stability of Silica Colloids Pirkko Holtta1, Mari Lahtinen1, Martti Hakanen1, Jukka Lehto1 and Piia Juhola2 1 University of Helsinki, Department of Chemistry, P.O. Box 55, 00014 University of Helsinki, Finland 2 Posiva Oy, Olkiluoto, 27160 Eurajoki, Finland ABSTRACT Non-cementitious grouts have been tested in Olkiluoto for the sealing of fractures with the small hydraulic apertures. A promising non-cementitious inorganic grout material for sealing the fractures with the apertures less than 0.05 mm is commercial colloidal silica called silica sol. The potential relevance of colloid-mediated radionuclide transport is highly dependent on their stability in different geochemical environments. The objective of this work was to follow stability of silica sol colloids in low salinity Allard and saline OLSO reference groundwater (pH 7–11) and in deionized milliQ water. Stability of silica sol colloids was followed by measuring particle size distribution, zeta potential, colloidal and reactive silica concentrations. The particle size distributions were determined applying the dynamic light scattering (DLS) method and zeta potential based on dynamic electrophoretic mobility. The colloidal silica concentration was calculated from DLS measurements applying a calibration using a standard series of silica sol. Dissolved reactive silica concentration was determined using the molybdate blue (MoO4) method. These results confirmed that the stability of silica colloids dependent significantly on groundwater salinity. In deionized water, particle size distribution and zeta potential was rather stable except the most diluted solution. In low salinity Allard, particle size distribution was rather constant and the mean particle diameter remained less than 100 nm. High negative zeta potential values indicated the existence of stable silica colloids. In saline OLSO, particle size distribution was wide from a nanometer scale to thousands of nanometers. The disappearance of large particles, decrease in colloidal particle concentration and zeta potential near zero suggest flocculation or coagulation. Under prevailing saline groundwater conditions in Olkiluoto silica colloids released from silica sol are expected to be instable but the possible influence of low salinity glacial melt water has to be considered. INTRODUCTION In Finland the final disposal of spent nuclear fuel is investigated by Posiva Oy in Olkiluoto utilizing the underground rock characterization facility ONKALO. The project seeks to obtain information about the bedrock at the site planned for the final disposal repository and assess its safety and test final disposal technology in actual deep underground conditions. Cement is typically used for permeation grouting in hard rock. Because high pH can be harmful for the Engineered Barrier System (EBS) and cement-based grouts may have limited penetration, noncementitious grouts have been tested for the sealing of fractures with a small hydraulic aperture. A promising non-cementitious inorganic gro