A review of colloid transport in fractured rocks

Recent recognition of colloid and colloida-ssociated transport of strongly sorbing contaminants in fractured rocks highlights the importance of exploring the transport behavior of colloids under conditions prevailing in the field. The rapid transport of colloids through fractured rocks-as affected by the hydraulic properties of the flow system, the properties of fracture surface and the geochemical conditions-has not been sufficiently elucidated, and predictions of colloid transport through fractures have encountered difficulties, particularly at the field scale. This article reviews the current understanding of the mechanisms and modeling of colloid transport and retention in fractured rocks. Commonly used experimental techniques and approaches for conducting colloid transport experiments at different scales, ranging from the laboratory to the field scale, are summarized and commented upon. The importance of various interactions (e.g., dissolution, colloid deposition, generation, mobilization and deposition of filling materials within fractures) between the flowing solution and the fracture walls (in many cases, with skin or coating on the host rock at the liquid ?? solid interface) has been stressed. Colloid transport through fractures of high heterogeneity has not yet been well understood and modeled at the field scale. Here, we summarize the current knowledge and understanding accumulated in the last two decades in regard to colloid and colloid-associated transport through fractures. Future research needs are also discussed.