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This study examines the dynamics of colloid mobilization and leaching from macroporous soil columns by means of laboratory experiments and numerical modeling. On the basis of a previous column study involving high and low water flow rates in structured soil, we designed a novel experiment emphasizing the time‐dependence of the colloid release process. Intact macroporous soil columns were exposed to variable pauses in irrigation (flow interruption for 30 min, 1 d, or 7 d) followed by resumed infiltration. The experiments showed that (i) there was a seemingly unlimited source of in situ colloids even after prolonged leaching and (ii) the peak concentration of colloids in the effluent after the flow interruption increased with increasing length of the preceding pause. The results demonstrated that colloid mobilization is not controlled by hydrodynamic shear induced by the flowing water but is a time‐dependent and possibly diffusion‐limited process. We developed a simple, equivalent macropore model to investigate the hypothesis that colloid release to the flowing water is governed by two diffusion processes, one in a uniform water film lining the macropore and one in the crust of the macropore. The model was capable of reproducing and explaining the characteristic results of our soil column experiments and required no recalibration of exchange process parameters to simulate the particle mobilization after a flow interruption. However, model calibration yielded unexpected results with respect to the size of the diffusion coefficient of the crust and did not warrant accepting the dual diffusion model hypothesis. Using a simpler mass transfer concept to quantify the mobilization of colloids in 21 soil columns, we found mass transfer coefficients to be about 30 times higher in the water film than in the crust.

The binding and speciation of hydrophobic organic chemicals (HOCs) in aqueous solutions were determined by controlling chemical activity and measuring total concentrations. Passive dosing was applied to control chemical activities of HOCs in aqueous solutions by equilibrium partitioning from a poly(dimethylsiloxane) polymer preloaded with the chemicals. The HOC concentrations in the equilibrated solutions [C(solution(eq))] and water [C(water(eq))] were then measured. Free fractions of the HOCs were determined as C(water(eq))/C(solution(eq)), whereas enhanced capacities (E) of the solutions for HOCs were determined as C(solution(eq))/C(water(eq)). A mixture of polycyclic aromatic hydrocarbons served as model analytes, while humic acid, sodium dodecyl sulfate, hydroxypropyl-β-cyclodextrin, and NaCl served as model medium constituents. The enhanced capacities were plotted versus the concentrations of medium constituents, and simple linear regression provided precise partition ratios, salting out constants, and critical micelle concentrations. These parameters were generally in good agreement with published values obtained by solid phase microextraction and fluorescence quenching. The very good precision was indicated by the low relative standard errors for the partition ratios of 0.5-8%, equivalent to 0.002-0.03 log unit. This passive dosing approach allows binding and speciation of HOCs to be studied without any phase separation steps or mass balance assumptions.