Interception History: A Paradigm Shift for Particle Transport in Porous Media

Over the past several decades, the focus of colloid transport in groundwater has expanded from pathogens and radionuclide-bearing clays to include engineered nanomaterials and most recently micro- and nano-plastics.  For all of these and other colloid types, the following variances from expectations of Colloid Filtration Theory (CFT) have been well-demonstrated under unfavorable conditions where a repulsive barrier exists in colloid-surface interactions: a) extended tailing of low concentrations in breakthrough-elution concentration histories (BTEC) following initial elution; b) retention profiles (RP) that are non-exponential (multiexponential or nonmonotonic).  We present recent experiments and simulations demonstrating that these variances from CFT arise from variations in interception history among the attached colloids.  Specifically, we show that the fraction of the colloid population that attaches after a single interception is the majority under favorable conditions whereas it is the minority under unfavorable conditions. We show that colloid concentrations decrease exponentially only for colloids that attach after a single interception, whereas colloids that attach following multiple interceptions assume gamma distributions down gradient from the source with maxima at transport distances that increase with interception order.  We show that all these distributions are governed by the collector and attachment efficiencies ( and ).  The well-observed non exponential RPs result from superposition of the RPs for single- and multiple- interception attachers, wherein decreases or increases in  or  with interception order yields multiexponential or nonmonotonic RPs, respectively.  Extended tailing in BTECs reflects colloids that eluted after many repeated interceptions without attachment.  We speculate on the origin of changes in  or  with interception order.  We emphasize that these variances reflect a fundamental aspect of transport under unfavorable conditions, i.e., the stochastics of attachment to nanoscale heterogeneity, as they arise in the absence of variations in colloid size, surface properties, and density, as well as in the absence of straining and detachment.