Physico-chemical factors affecting the transport of colloidal particles in groundwater systems

Increasing attention has been given to the role of colloids in accelerating contaminant transport as evidence of significant subsurface transport of contaminants continues to be presented. Colloid migration models typically include the processes of advection, dispersion, deposition and release. Present models are limited in their ability to predict the transport and fate of colloids due to limited understanding of the interaction between these processes. The objective of this research is to gain a better understanding of the physical/chemical factors which affect the transport of colloidal particles. Laboratory-scale experiments were conducted in two phases to examine the effects of different chemical and physical factors on colloid transport. In the first phase, pulses of 1.09 and 0.28 $\mu$m fluorescent particles were injected into sand-packed columns under constant chemical conditions, and the response monitored over time. The second phase consisted of making a step change in solution chemistry (reduction in ionic strength or increase in pH) or flowrate to the system and monitoring the fluorescence response over time. The relative impact of various processes including advection, deposition/release, retardation and hydrodynamic chromatography on colloidal transport was evaluated using experimental results. Various mathematical models were evaluated for their effectiveness in describing the observed colloid migration. Results of finite pulse experiments conducted under constant chemical conditions confirm that solution chemistry is a key factor affecting particle deposition and release. Deposition results show evidence of the effects of hydrodynamic chromatography and retardation. As expected, deposition increases with increasing ionic strength. Perturbations in solution chemistry caused the subsequent release of particles. However, the amount released was only a fraction of the total deposit. The release process warrants further research