Laboratory observations and mathematical modeling of colloid‐facilitated contaminant transport in chemically heterogeneous systems

In the work reported here, I examine colloid‐facilitated solute transport through a natural porous medium that exhibits grain‐scale heterogeneity in chemical composition. I conducted six column experiments using the contaminant hydroxyatrazine (HA) and either homogeneous clay‐colloid suspensions or mixtures of clay colloids and dissolved organic matter. The transport of colloids composed of illite and montmorillonite, although not conservative, is only slightly affected by rate‐limited mass transfer reactions with the oxide‐coated aquifer material. Coinjection of HA with monomineralogic suspensions of illite colloids produces modest increases in effluent concentrations of HA relative to experiments performed with colloid‐free influent solutions. Montmorillonite colloids have a much higher capacity to bind HA than illite colloids and substantially increase the transport rate of HA in the column experiments. Suwannee River humic acid (SRHA) does not strongly complex aqueous phase HA, but its addition to column influent mixtures that contain HA and montmorillonite suppresses HA adsorption to the clay colloids and promotes a decline in HA mobility. A model for colloid‐facilitated transport, formulated to account for nonuniformity in the kinetics of colloid deposition and HA adsorption, closely describes HA transport in experiments performed with monomineralogic clay‐colloid suspensions, heterogeneous suspensions made of both illite and montmorillonite, and mixtures of montmorillonite and SRHA.