There is evidence that glycphosates can linger in the soil. The chemical is broken down by manganese in the soil. However, if it binds with copper, the glyphosate doesn't break down, and then can be washed away.
A researcher in this field made this comment to me:
"glyphosate preferentially complexes with copper, making it more soluble and therefore leachable. There may be some risk that , over time with repeated Roundup application, that some soils could have their available copper depleted by this process."
This point is important and should make those who use glyphosates think twice. Most importantly, they should be checking their soil for bio-available copper, not just total copper. In fact, the copper in the soil may be bound to the glyphosate molecule, thus short changing the crops grown on this treated land.