Relating mechanistic fate with spatial positioning for colloid transport in surface heterogeneous porous media

Hypotheses: The transport behavior of colloids in subsurface porous media is altered by surface chemical and physical heterogeneities. Understanding the mechanisms involved and distribution outcomes is cru-cial to assess and control groundwater contamination. The multi-scale processes that broaden residence time distribution for particles in the medium are here succinctly described with an upscaling model. Experiments/model: The spatial distribution of silver particles along glass bead-packed columns obtained from X-ray micro-computed tomography and a mechanistic upscaling model were used to study colloid retention across interface-, collector-, pore-, and Darcy-scales. Simulated energy profiles considering variable colloid-grain interactions were used to determine collector efficiencies from particle trajectories via full force-torque balance. Rate coefficients were determined from collector efficiencies to parameterize the advective-dispersive-re active model that reports breakthrough curves and depth pro-files. Findings: Our results indicate that: (i) with surface heterogeneity, individual colloid-grain interac-tions are non-unique and span from repulsive to attractive extremes; (ii) experimentally observed spatial positioning of retention at grain-water interfaces and grain-to-grain contacts is governed respectively by mechanistic attachment to the grain surface and retention without contact at rear-flow stagnation zones, and (iii) experimentally observed non-monotonic retention profiles and heavy-tailed breakthrough curves can be modeled with explicit implementation of heterogeneity at smaller scales.(c) 2023 Elsevier Inc. All rights reserved.