Solute transport for steady-state and transient flow in soils with and without macropores

The effect of flow rate and flow regime on solute transport in two soils, a sandy-loam (Glossudalf) and loam (Udifluvent), was investigated. For each soil type, leaching experiments were carried out in two large undisturbed soil columns (0.3-m i.d., 1-m length) for three different steady-state flow rates and three (sandy loam), or two (loam) transient now regimes. Solute concentrations were measured in the drain water, c(f), and in situ, c(r), using time domain reflectometry (TDR). In order to approximate the transient by a steady-state flow transport process, a solute penetration depth coordinate, zeta, was used. Breakthrough curves (BTCs) of c(r) and c(f) were used to optimize parameters of the convection-dispersion equation (CDE). In the sandy loam, the CDE described transport for steady-state and transient flow conditions well and relevant CDE model parameters could be derived from BTCs of c(r). In the loam soil, due to the activation of macropores, lateral solute mixing decreased with increasing flow rate, which resulted in an increase of dispersivity with increasing depth for higher now rates. Since bypass now and transport through macropores is barely apparent in time series of concentrations measured in situ, c(r), CDE parameters derived from BTCs of c(r) were inconsistent with parameters derived from BTCs of c(f) when bypass flow was important. The dispersivity increased with increasing flow rate in both soil types and an effective or flux-weighted average flow rate rather than a time-averaged flow rate was used to derive the relation between the dispersivity and the flow rate.