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
.