Nonequilibrium Transport of Atrazine through Large Intact Soil Cores

Preferential flow in heterogeneous soils may result in more rapid leaching of pollutants through soils than would be predicted using transport models based on the local equilibrium assumption (LEA). Our objectives were to evaluate nonequilibrium processes important to the transport of tritiated water (³H₂O) and atrazine under varying pore water velocities and soil water contents, and to distinguish between transport-related nonequilibrium (TNE) and sorption-related nonequilibrium (SNE). Column experiments were performed using a ³H₂O-¹⁴C-labeled atrazine pulse through intact soil cores (15.24-cm diam., 30-cm length) at pore water velocities of 0.12, 0.69, and 2.16 cm h⁻¹ (θᵥ ≈ 0.39) and at 0.74 cm h⁻¹ (θᵥ ≈ 0.44). The asymmetrical shape and the left-handed displacement of ³H₂O breakthrough curves (BTCs) as a function of pore water velocity and soil water content (θᵥ) indicated that ³H₂O was subject to TNE at only the 0.74 (θᵥ = 0.44) and 2.16 cm h⁻¹ pore water velocities. The asymmetrical shape and increased tailing of atrazine BTCs at all pore water velocities indicated that atrazine was influenced primarily by SNE at pore water velocities of 0.12 and 0.69 cm h⁻¹, and a combination of both TNE and SNE at pore water velocities of 0.74 (θᵥ ≈ 0.44) and 2.16 cm h⁻¹. The convection-dispersion equation based on the LEA was unable to predict atrazine BTCs at any pore water velocity. Although the nonequilibrium bicontinuum (two-site/two-region) model provided excellent fit to all atrazine BTCs, fits to the model cannot be used to separate between TNE and SNE when both mechanisms are operative. Results of this study confirm that TNE and SNE are important transport processes in naturally structured soils under conditions of relatively high pore water velocities and volumetric water contents.