Acifluorfen Sorption, Degradation, and Mobility in a Mississippi Delta Soil

Abstract

Potential surface water and groundwater contaminants include herbicides that are applied postemergence. Although applied to the plant canopy, a portion of any application reaches the soil either directly or via subsequent foliar washoff. This study examined sorption, degradation, and mobility of the postemergence herbicide acifluorfen (5‐[2‐chloro‐4‐(trifluoromethyl)phenoxy]‐2‐nitrobenzoic acid) in Dundee silty clay loam (fine‐silty, mixed, thermic, Aeric Ochraqualf) taken from conventional till (CT) and no‐till (NT) field plots. Homogeneous surface and subsurface samples were used in the sorption and degradation studies; intact soil columns (30 cm long and 10 cm diam.) were used in the mobility study. Batch sorption isotherms were nonlinear (Freundlich model) and sorption paralleled organic C (OC) content. All tillage by depth combinations of soil exhibited a time‐dependent approach to sorption equilibrium that was well described by a two‐site equilibrium–kinetic model. Acifluorfen degradation followed first‐order kinetics. No more than about 6% of applied 14C‐acifluorfen was mineralized by 49‐d incubation. Extracts of incubated soil gave little indication of degradation products; however, 14C did accumulate in an unextractable fraction. Degradation was faster in the surface soils compared to subsurface soils and faster in CT surface soil compared to NT surface soil. Tillage did not affect acifluorfen degradation in subsurface samples. Elution of Br pulses from the intact soil columns under steady‐state, unsaturated flow indicated preferential water flow. Nonequilibrium transport of Br was well described using a two‐region, mobile–immobile water model. Inclusion of sorption kinetics in the transport model rather than assuming equilibrium sorption led to improved predictions of acifluorfen retardation. Column effluent contained negligible concentrations of acifluorfen degradation products and, as in the incubation study, an unextractable residue developed in the soil columns. However, unlike results from the incubation study, a greater fraction of applied acifluorfen was apparently bound and there was also evidence of extractable degradation products. Furthermore, first‐order rate constants obtained from the batch study underestimated acifluorfen degradation during transport. Faster acifluorfen degradation in the soil columns may have been due to poorer aeration compared to the batch systems.