Herbicides are applied in greater amounts than insecticides in the dryland drainage basins. Some herbicides, triallate and MCPA, for instance, were detected only in the dryland drainage basins. Terbacil and pendimethalin are two herbicides that were detected only in the irrigated drainage basins. Variations of concentrations Variations of concentrations of pesticides in surface waters sometimes provide clues about how the pesticides are transported to surface- water systems. Highest concentrations of some pesticides were observed during or shortly after their application period.
Pesticides enter surface waters in runoff from fields and through the ground-water system.
Highest concentrations of pendimethalin were detected during or shortly after the period when it is typically applied. Also, all dimethoate values were deleted from the data base due to this compound's poor performance in analytical tests. While making these adjustments to the data base, it was discovered that some compounds detected at very low concentrations had been inadvertently reported as nondetections. These are now reported as detections.