Mark,
Rain water will, in the absence of other forces, travel straight down into the water table. There is some small amount of lateral diffusion as a result of capillary action, but it's almost negligible compared to the motion of the water. If you receive such a deluge that the ground saturates, the water will run off, but in that case it's not going to extract glyphosate out of the soil to any significant extent, as the glyphosate isn't on the surface of the soil. I actually modeled this very situation computationally when I was in the biz, so I can say this with some certainty.
Mark Lipton