Pesticides can be transported to groundwater more rapidly through preferential flow paths than would be predicted from their physicochemical properties. The leaching rates of the herbicides 2,4-D, bromoxynil, clopyralid, dicamba, diclofop, MCPA, and mecoprop were compared on plots at a site in Saskatchewan, Canada, tilled after harvest (conventional till, CT) and those that were not (autumn tillage operation omitted, NT). The soil-incorporated herbicides tri-allate and trifluralin were applied to the CT plot only. Herbicide was applied immediately prior to a leaching irrigation for salt removal, which represents a 'worst-case' scenario for pesticide leaching. Direct evidence of preferential flow was obtained when the herbicides, with the exception of tri-allate and trifluralin, were detected in the first water reaching the tile drains. Although the non-incorporated herbicides were transported preferentially at the same rate, the amounts transported depended on the solubility and adsorption coefficient ( Koc) of the herbicide. Only 0.01% of the application of the least soluble herbicide, diclofop, was transported, compared with 0.46% of the most soluble herbicide, dicamba. Preferential flow was only slightly reduced by the tillage pass. The amounts of herbicide transported to the tile drain, however, were substantially reduced on the CT plot. The tillage effect was greatest for the more soluble and less strongly absorbed herbicides. There was no clear relationship between amounts transported in the year after application and reported persistence but herbicides with the longer half-lives persisted in relatively greater amounts than the other herbicides.
