I LOVE the common misconception the general public gives to the dangers of agricultural chemicals. You will more than likely do more damage to the lake in the form of runoff due to tillage than you would with glyphosate / 2-4,D applications. Paraquat is a different story because it does have a high toxicity but the other two are safe. But with any ag chemical, absorption within the plant will be quick (all herbicides should be safe as far as runoff) within a 24 hour period but even after a couple of hours it would take a quick downpur to wah it off. JMO