The stability of colloids in Yucca Mountain groundwaters is of great importance in assessing colloid-facilitated radionuclide transport at the potential nuclear waste repository. Aggregation experiments were performed to evaluate colloid stability of silica and clay colloids as a function of ionic strength in a carbonate rich synthetic groundwater. When the concentration of electrolyte is increased to induce aggregation, the aggregate growth proceeds at an exponential rate, is irreversible, and the rate of aggregation increases with increasing electrolyte strength. The rate of particle aggregation for both types of colloids was estimated using autocorrelation photon spectroscopy, (APS). The stability ratio, W, was determined using the Smoluchowski rate expression for irreversible aggregation. Aggregation of silica particles and Kaolinite clay particles was not detected for an electrolyte concentration, CNaCl, below 300 mM and 100 mM, respectively. However, it is possible that reversible aggregation does occur in this concentration range.
