Nonideal transport of solute and colloidal tracers through reactive zeolite/iron pellets

Abstract

Solute and colloidal tracer tests were conducted in laboratory columns to examine the hydraulic properties of a foamed zeolite/iron pellet material that was developed for in situ remediation of contaminated groundwater. The colloidal tracer (1 μm polystyrene microspheres) moved through the columns much faster than the nonreactive solute tracer tritiated water, reflecting the interpellet preferential flow paths in the packed material. Flow interruption experiments with tritium and bromide showed concentration rebound of both tracers after the interruption (during elution), indicating the existence of nonadvective zones inside the pellets. Inverse modeling of microsphere data using a physical nonequilibrium transport model yielded immobile water content (θim) equivalent to the intrapellet porosity (0.40), suggesting that the microspheres were excluded from the small intrapellet pores and could only move through the large interpellet pore spaces. Inverse modeling of tritium data using physical nonequilibrium and dual‐permeability dual‐porosity models yielded θim values of 0.1–0.2, confirming the existence of nonadvective zones inside the pellets as suggested by the flow interruption experiments. The dual‐permeability dual‐porosity model also indicated that 6–11% of the total porosity was preferential flow porosity, consistent with the observation of enhanced microsphere transport with respect to tritiated water. Forward modeling with the dual‐permeability dual‐porosity model suggested that the immobile water in the pellets would not significantly affect the removal efficiency of contaminants subject to sorption and reduction. In contrast, the preferential flow porosity would drastically lower the contaminant removal efficiency.