MODELING COLLOID TRANSPORT IN GROUNDWATER - THE PREDICTION OF COLLOID STABILITY AND RETENTION BEHAVIOR

The association of contaminants with mobile colloidal particles present in groundwaters has been recognised as a potentially important mass transfer mechanism for contaminant migration in the environment. To predict the fate of environmental contaminants there is a need to develop numerical models which include colloid-mediated transport. The mobility of groundwater colloids is controlled by their stability towards aggregation and attachment to rock surfaces. For inorganic particles, the conceptual framework for predicting their stability and deposition behaviour is provided by the DLVO theory. However, under conditions unfavourable to coagulation or surface attachment (ie. when particles and surfaces are of like charge) there are significant discrepancies between theory and experimentally measured coagulation and deposition rates. Predictive shortcomings of the DLVO theory arise from the simplicity of the original model, which was formulated for smooth bodies with ideal geometries and uniform surface properties. However, surfaces are by nature rough, non-uniform and heterogeneous in composition. In addition, the theory does not consider the dynamics of particle interactions. Furthermore, the presence of additional forces, which may be either attractive or repulsive, acting at short range, which arise from interactions between surfaces and water, are not accounted for. Significant developments have been made to extend and modify the DLVO model to account for the discrepancies between theory and experiment. In this paper the prediction of colloid stability and deposition behaviour under unfavourable conditions is reviewed. Emphasis is placed on the phenomenological behaviour of inorganic colloids in aqueous systems that may need to be accounted for in a transport model.