Leaching rates and preferential flow of selected herbicides through tilledand untilled soil

Pesticides can be transported to ground water more rapidly through preferen tial flowpaths than would be predicted from their physicochemical propertie s, The leaching rates of the herbicides 2,4-D, bromoxynil, clopyralid, dica mba, diclofop, MCPA, and mecoprop were compared in this study on plots till ed after harvest (conventional till, CT) and those that were not (fail till age operation omitted, NT), The soil-incorporated herbicides triallate and trifluralin were applied to the CT plot only, Herbicide was applied immedia tely prior to a leaching irrigation for salt removal, which represents a "w orst-case" scenario for pesticide leaching. Direct evidence of preferential now was obtained when the herbicides, with the exception of triallate and trifluralin, were detected in the first water reaching the tile drains. Alt hough the nonincorporated herbicides were transported preferentially at the same rate, the amounts transported depended on the solubility and adsorpti on coefficient (K-oc) of the herbicide. Only 0.01% of the application of th e least soluble herbicide, diclofop, was transported, compared with 0.46% o f the most soluble herbicide, dicamba, Preferential now was only slightly r educed by the tillage pass, The amounts of herbicide transported to the til e drain, however, were substantially reduced on the CT plot. The tillage ef fect was greatest for the more soluble and less strongly absorbed herbicide s. There was no clear relationship between amounts transported in the year after application and reported persistence but herbicides with the longer h alf-lives persisted in relatively greater amounts than the other herbicides .