LYSIMETER STUDY OF ANION TRANSPORT DURING STEADY FLOW THROUGH LAYERED COARSE-TEXTURED SOIL PROFILES

Transport of Cl and NO3 was studied during steady-state unsaturated water flow in several lysimeters filled with a coarse sand or sandy loam. Breakthrough curves were obtained with suction cups at six different depths. The transport parameters were determined by fitting the classical convection-dispersion equation to the data assuming a homogeneous or a two-layer soil profile. As expected, the two-layer solution described the data better than the solution assuming a homogeneous soil, especially for the sand where the fitted pore-water velocities were about 60% higher in the subsoil compared with the topsoil. The average dispersivity for the sandy loam was 1.9 cm for the topsoil and 1.3 cm for the subsoil, whereas a value of 0.58 cm could be used for the entire profile for the coarse sand. Solute outflow from the lysimeters was reasonably well predicted using the transport parameters estimated from breakthrough curves obtained with the suction cups. The dispersion coefficients estimated from the effluent concentrations were generally somewhat larger than those derived from suction cup data. The total concentration had little effect on the dispersion coefficient and the pore-water velocity. No significant differences were found between NO3 and Cl, although dispersivities for Cl were always slightly higher than those for NO3.