Modeling Physical and Chemical Nonequilibrium Transport of Herbicide in Soils From Different Tillage Systems.

Abstract

The physical and chemical nonequilibrium transport of alachlor were studied in a surface Gigger soil from different tillages through tracer studies, and batch and miscible displacement experiments. Batch experiments indicated initially fast reaction followed by slow adsorption. Adsorption and desorption results indicated time dependent hysteretic behavior and was best described by a multireaction model incorporating nonlinear equilibrium reaction, a reversible kinetic mechanism, and a consecutive irreversible mechanism. The model predicted alachlor hysteresis and adsorption-desorption kinetics satisfactorily based on parameters obtained from adsorption experiments. Tracer (Eosin Y and Blue FCF dyes) studies showed non-uniformly stained areas in undisturbed soil cores (6.4 cm i.d, 15 cm length) and indicated more pronounced preferential flow and physical nonequilibrium solute transport in no-till than in conventional tillage. Tritium breakthrough curves (BTCs) indicated earlier breakthrough associated with bimodal peaks in short pulses for no-till. The shape of BTCs were also dependent on flow direction. The superimposed experimental data from short pulses well predicted the data of long pulses. The classical convective-dispersive equation was inadequate and there was no improvement in describing tritium BTCs using physical nonequilibrium models (mobile-immobile and stochastic models) for soils from no-till. Miscible displacement results indicated that alachlor BTCs in soils of no-till were more asymmetrical, with earlier breakthrough and longer tailing than soils from conventional tillage. A multireaction transport model (MRTM) was not satisfactory for alachlor prediction using independently measured parameters from batch experiments. However, MRTM successfully described alachlor BTCs in a calibration mode where physical and chemical nonequilibrium were dominant. Best-fit parameters indicated the dominance of kinetic reactions compared with parameters from batch experiments and may be attributed to soil heterogeneity. Although no-till increased alachlor retention in batch experiments, an overall estimation based on the sum of kinetic and equilibrium retention showed no significant influence on retention by tillage. High pressure liquid chromatography (HPLC) chromatograms, fitted transport parameters, flow interruptions and percent recoveries indicated a significant consecutive irreversible reaction in soils of conventional tillage. Moreover, no-till increased alachlor transport based on breakthrough time compared with conventional tillage.