Natural colloids in El Berrocal Site (Spain): the effect of ionic strength in their stability properties

In this paper we characterize the colloids naturally occurring in both reduced and oxidized groundwaters and in fracture-related clayey materials of El Berrocal granitic media. In all samples silica, aluminosilicates (K-feldspars and clays as beidellite) and Fe, Ti and Al oxy-hydroxides have been detected, these being interpreted as the result from mechanical disintegration or lixiviation of the media. In the reduced groundwater samples, sulphides and/or carbonates are also present. These latter do not seem to be originated by mechanical processes but by precipitation or neo-formation. Furthermore, the stability of the natural colloids from El Berrocal has been studied and then compared with that of the colloids extracted from the bentonites that will be used as backfill and sealing materials in high-level repositories. These stability studies strongly suggest that natural variations in the ionic strength are capable of changing the natural colloid stability producing aggregates of larger size (>1μm) than the original sizes in colloids from the natural media as well as in the backfill and sealing clays. (authors). 24 refs., 9 figs., 3 tabs