Bromide and herbicide transport under steady‐state and transient flow conditions

Summary The dynamics of water flow in soils influences the transport behaviour of solutes. Transport of bromide and herbicides through undisturbed soil columns was investigated under conditions of unsaturated steady‐state and transient water flow. Effective transport parameters were obtained from fitting the convection–dispersion equation to curves of concentration against cumulative drainage, and these enabled us to interpret the observed behaviour. Under both steady‐state and transient flow bromide and herbicides were transported through similar parts of the pore volume of a homogeneous single grain soil (Bv horizon). However, in aggregated Ah and Ap horizons preferential transport occurred during transient flow but not during steady‐state flow. For preferential flow the mean transport volume seemed to depend on the prevailing pore system and the fraction of preferentially flowing water. Solute leaching was more efficient under steady‐state than under transient flow for bromide in all soils and for herbicides in the Bv horizon. However, when transient flow caused preferential transport, herbicide loss was greater under transient flow than during steady‐state flow. Under preferential flow conditions a three‐step herbicide concentration development recurred in successive drainage events. This behaviour was not observed for the non‐reactive tracer. It seemed to be caused by sorption. A steady‐state model with cumulative drainage as independent variable instead of time can predict the transport of non‐reactive and adsorbed solutes in homogeneous soils without features of preferential flow. Otherwise constant effective input parameters cannot be assessed a priori.