Colloid properties in granitic groundwater systems .2. Stability and transport study

The groundwater colloid concentrations previously measured in the Grimsel area (Grimsel Test Site, Transitgas Tunnel), Northern Switzerland (Leuggern, Zurzach) and the Black Forest (Bad Sackingen, Menzenschwand) are explained on the basis of the colloid stability properties for their composition and the chemistry of the investigated groundwaters. If reversible attachment of the colloids onto the rock is assumed, colloid transport could take place when the colloid population is stable. The question considered in connection with colloid transport and its modelling is that of colloid attachment. Natural colloids, and the surface of the rock on which they may be collected, generally have negative surface charge so that colloid attachment may be difficult. Since it has been shown that a theory like DLVO is inapplicable because of inherent shortcomings which lead to unrealistic predictions, attachment factors were determined experimentally for systems which correspond as closely as possible to the natural system. Montmorillonite colloids are used as model material because colloids in granitic groundwaters comprise phyllosilicates (Part I), Under constant hydrogeochemical conditions, the colloid concentration in a granitic groundwater was found to be less than 100 ng . ml(-1). (10(7) pt . ml(-1) > 100 nm) when the Ca concentration is larger than 10(-4) M and the Na concentration is larger than 10(-2) M. However, when the chemical or physical conditions of the system are not in a steady state, such as with hydrothermal activity or tectonic events, colloid generation may be enhanced. The small colloid concentration in a representative deep crystalline water (< 100 ng . ml(-1), or <10(7) pt . ml(-1) for sizes > 100 nm) is a consequence of an attachment factor for clay colloids (e.g. montmorillonite) close to 1, and limits the colloids transport. Copyright (C) 1996 Elsevier Science Ltd