Effects of dissolved organic matter on the co-transport of mineral colloids and sorptive contaminants

• NOM alone did not increase Cs-137 transport.
• Illite and kaolinite colloids increased Cs-137 transport.
• NOM substantially increased mineral-colloid facilitated Cs-137 transport.

Colloid-facilitated transport of contaminants in the vadose zone has important implications to groundwater quality, and has received considerable attention. Natural organic matter (NOM) is ubiquitous in subsurface environments, and its influence on mineral colloids and solute transport has been well documented. However, research on the influence of NOM on colloid-facilitated transport is limited. The objective of this paper is to elucidate the effects of NOM on colloid-facilitated transport of a radioactive contaminant (Cs-137) within partially-saturated sediments. Measurements made with re-packed columns reveal that Cs-137 mobility was low when mineral colloids were absent and was unaffected by the presence of NOM. The addition of mineral colloids to influent increased Cs-137 mobility, and effluent Cs-137 was dominated by the colloid-associated form. When NOM was added to systems that contained mineral colloids and Cs-137, the mobility of Cs-137 further increased. A mathematical model simulating colloid-facilitated transport showed that NOM increases Cs-137 transport by increasing colloid mobility and reducing the rate of Cs-137 adsorption to the porous medium.