Chlorsulfuron {2‐chloro‐N‐[(4‐methoxy‐6‐methyl‐1,3,5‐triazin‐2‐yl) amino carbonyl] benzenesulfonamide} selectively controls broad‐leaf weeds in winter wheat (Triticum aestivum L.), but also is extremely persistent in soil, resulting in residual injury to succeeding crops that are susceptible to chlorsulfuron. To determine the factors affecting chlorsulfuron degradation, the effect of environmental variables and method of application on chlorsulfuron persistence was measured in a loam and sandy loam soil using a corn (Zea mays L.) root bioassay. Increasing soil temperature from 20 to 40°C decreased chlorsulfuron persistence, with significant differences in chlorsulfuron concentration occurring 31 d after application to the sandy loam soil. Soil water level affected chlorsulfuron persistence only in the loam soil, where increasing the soil water level decreased persistence. Adding wheat residue to the soils increased chlorsulfuron persistence in the sandy loam, but not in the loam soil. One possible explanation for this reduced degradation in the sandy loam was a shift in pH toward basicity, as the addition of 0.050 kg kg−1 wheat residue raised the pH from 6.1 to 7.2. In the loam soil, the wheat residue did not affect soil I~H. Chlorsulfuron persistence was affected by texture, with longer activity occurring in the loam soil in all studies. Incorporating chlorsulfuron caused a greater loss of chlorsulfuron activity in the sandy loam, indicating that incorporation may influence chlorsulfuron persistence.

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The U.S. Environmental Protection Agency (USEPA) Office of Pesticide Programs (OPP) has completed an evaluation of three watershed‐scale simulation models for potential use in Food Quality Protection Act pesticide drinking water exposure assessments. The evaluation may also guide OPP in identifying computer simulation tools that can be used in performing aquatic ecological exposure assessments. Models selected for evaluation were the Soil Water Assessment Tool (SWAT), the Nonpoint Source Model (NPSM), a modified version of the Hydrologic Simulation Program‐Fortran (HSPF), and the Pesticide Root Zone Model‐Riverine Water Quality (PRZM‐RIVWQ) model. Simulated concentrations of the pesticides atrazine, metolachlor, and trifluralin in surface water were compared with field data monitored in the Sugar Creek watershed of Indiana’s White River basin by the National Water Quality Assessment (NAWQA) program. The evaluation not only provided USEPA with experience in using watershed models for estimating pesticide concentration in flowing water but also led to the development of improved statistical techniques for assessing model accuracy. Further, it demonstrated the difficulty of representing spatially and temporally variable soil, weather, and pesticide applications with relatively infrequent, spatially fixed, point estimates. It also demonstrated the value of using monitoring and modeling as mutually supporting tools and pointed to the need to design monitoring programs that support modeling.

Laboratory and field studies were conducted to evaluate pendimethalin [N‐(1‐ethylpropyl)‐3,4‐dimethyl‐2,6‐dinitroaniline] dissipation over time in a Crowley silt loam (Typic Albaqualfs). In the laboratory, dissipation rates approximately followed pseudo first‐order kinetics over a 56‐d period, except in air‐dried soil that had no significant loss of pendimethalin detected after 56 d. Half‐lives for 30 kPa, continuous flood, and alternately flooded and dried treatments averaged 59, 63, and 30 d, respectively, under laboratory conditions. Application rates of 0.5, 1.0, and 2.0 µg/g of soil did not have a significant influence on the half‐lives. The ratio of pendimethalin residues in laboratory systems detected by a root bioassay with grain sorghum [Sorghum bicolor (L.) Moench.] to those detected with gas‐liquid chromatography (GLC) declined by 17% from 0 to 56 d after treatment. Dissipation in the field in each of 2 y was studied with lowland rice (Oryza sativa L.) (flush irrigated then flooded 2–3 weeks after application), upland rice (flush irrigated throughout the season), and soybean [Glycine max (L.) Merr.] (furrow irrigated as needed) management systems. Soil water content had a strong influence on the amount of pendimethalin that dissipated, especially for about the first 2 weeks after herbicide application. Half‐lives in the field were much shorter during the initial 2 weeks than after 2 weeks, with >50% of the applied herbicide having disappeared in 1 week for all treatments except in soybeans the first year. Soil persistence of pendimethalin was greater under soybean culture (soil‐incorporated herbicide, low irrigation frequency) than under rice culture (surface‐applied herbicide, high irrigation frequency, and/or flooded conditions).