Visualization of Colloid Transport Pathways in Mineral Soils Using Titanium(IV) Oxide as a Tracer

In soils, colloidal transport has been identified as the most important pathway for strong adsorbing, environmental contaminants like pesticides, heavy metals, and phosphorus. We conducted a comparative dye tracer experiment using a Brilliant Blue (BB) solution and a Titanium(IV) oxide (TiO2) colloid suspension (average particle size 0.3 μm), aiming to visualize and quantify colloid pathways in soils. Both dye tracers showed comparable general flow patterns with preferred transport over the deepest part of the soil profile, independent of clay content. The stained area was generally smaller for TiO2 than for BB by a factor of ten, however, and there was no TiO2 to be found at all in the low clay content soil. The travel distance was almost identical for the solution and the suspension (0.7 m) giving evidence that environmentally critical compounds bound to microparticles may be vertically transported over longer distances in soils, even within single rainfall events. The spatial variability of the dye patterns was large on a small scale with a range of 0.35 m for TiO2 in the horizontal plane, which was taken as a general proof for a pronounced preferential transport situation. The study indicates that TiO2 is transported exclusively through singular macropores of biogenetic nature, while BB passes also through the soil matrix of coarse‐bedded soils, the secondary pore system or interaggregate pore space. The results emphasize the general suitability of TiO2 for the visualization of colloid transport pathways in soils, opening up new research opportunities for contaminant transport in soils. Core Ideas Colloidal transport is the most important pathway for strong adsorbing contaminants in soils. TiO2 is a suitable tracer to visualize colloid pathways in mineral soils. The distribution of colloid dye tracer can be explained as a function of clay and silt content. TiO2 is exclusively transported in singular macropores rather than along soil ped interfaces.