Colloid and Bromide Transport in Undisturbed Soil Columns: Application of Two‐Region Model

Abstract

Bromide tracer breakthrough and natural soil colloid leaching curves for undisturbed soil columns were used to characterize dissolved and suspended matter transport at the field scale. Data from 33 columns of 20‐cm diameter and 20‐cm height were used. Columns were collected in a grid of 25 by 30 m at an agricultural field. A two‐region (mobile–immobile water phase, MIM) solute transport model was fitted to data. The model was used to estimate bromide and colloid transport parameters including mobile and immobile water contents (θm, θim), bromide and colloid advective velocities (vBr, vColl), and mobile–immobile mass transfer coefficients (αbr, αcoll). Both soil physical properties and transport parameters were highly variable across the sampling field. Comparison of bromide transport parameters with basic soil physical properties revealed that vBr was proportional to soil clay content and bulk density (ρb), but αBr was inversely proportional to these parameters. Colloid transport parameters, vColl and αColl, however, showed only a weak correlation with clay content and ρb. Also, vColl was typically three to four times higher than vBr. The colloid velocity was generally higher than the bromide velocity, implying size exclusion of colloids. The spatial distributions of soil physical properties, bromide and colloid transport parameters, and leached quantities of particles were compared. The results suggested that bromide and colloid mass transfer (diffusion) were not controlled by the same soil physical conditions, and that soil structure and macropore flow are more important than the quantity of dispersible colloids in controlling colloid leaching.