I was talking with a guy that helped develop Agent Orange. He said the active ingredient that kills the vegetation would be great in lakes to kill millfoil. And after a short period in the water (3 days?) it is reduced to harmless chemicals. I almost want to say that it would only kill the stuff near the surface where the sunlight is - but not sure.
And he said the nasty chemicals (for humans) was in the “sticky” stuff they added to it so it would stick to the leaves in Vietnam. But the herbicide itself wasn’t too bad.
Copper sulphate is one of the active ingredients in the spray they use.