Bentazon Mobility through Intact, Unsaturated Columns of Conventional‐ and No‐till Dundee Soil

Potential ground and surface water 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 the mobility of the postemergence herbicide bentazon [3‐(1‐methylethyl)‐(1 H )‐2,1,3‐benzothiadiazin‐4(3 H )‐one 2,2 dioxide] through intact columns of Dundee silty clay loam (fine‐silty, mixed, thermic Aeric Ochraqualf) taken from conventional‐till (CT) and no‐till (NT) field plots. Effects of sorption, biodegradation, and physical nonequilibrium on bentazon fate and transport were studied using miscible displacement experiments. Steady‐state, unsaturated flow was established in 30 cm long by 10 cm in diam. columns, then narrow pulses of Br tracer and 14 C‐labeled bentazon were applied and displaced through the columns. Once bentazon pulses were eluted, the columns were sectioned and soil extracted for bentazon and metabolites. Despite development of bound residue typical of bentazon degradation, HPLC analysis gave no evidence of bentazon metabolites in solution. Preferential flow occurred in all soil columns and was well‐described using a two‐region, mobile‐immobile water model. Use of batch sorption and degradation data led to predictions of bentazon mobility and residual 14 C in the soil columns that were generally consistent with the experimental data. Attempts to fit the transport data using different degradation rate constants for conducting and nonconducting regions offered no better description of the data than use of single rate constants.