Without having to site anyone I can tell that suspended particles tend to settle out as opposed to dissolve chemicals.

Not sure how this fact is bent to explain how chemicals stay on the land where it is sprayed. The dissolve chemicals even seep into the underground water polluting drinking water and also seep into main drainage lines at lower elevations from where it was sprayed. Not to even mention runoff from a storm event.

Solutions range from contour plowing to strategically placed geosynthetics.