Modeling Transport of Colloid-Bound Herbicides and Heavy Metals to Groundwater

Recent studies have suggested that contaminant transport to groundwater may be enhanced by association with colloidal particles. This study evaluated the role of water dispersible colloids with diverse mineralogical composition in co-transporting selected herbicides and heavy metals through intact soil columns. Colloid recovery in the eluents ranged from 45-90% for the herbicides and 10-60% for the heavy metals. The presence of colloids enhanced the transport of atrazine by 2-18%, and metolachlor by 8-30%. The corresponding increase for Cu and Zn was 2-150 and 5-30 times, respectively. For Pb, there was essentially no elution in the absence of colloids, suggesting nearly complete sorption by the column matrix and 8-3,000-fold increase in the presence of colloids. The extent of the colloid-induced transport was dependent on colloid, soil column, and contaminant properties. Colloid surface area and charge, pH, organic carbon, and soil macroporosity had a positive effect on co-transportability, while Fe and Al oxyhydroxides and kaolinite appeared to be inhibiting. Also inhibiting were \u3e 10 mg/L influent metal concentrations due to flocculation phenomena. In most cases, the increased transport of contaminants in the presence of colloids exceeded their sorption capacity, indicating the involvement of physical exclusion and binding mechanisms. This suggests that the colloids play a dual role as carriers and facilitators in the contaminant transport process