Let me put your mind at ease. While CS is no fun for anyone in acute exposure levels (I don't know PPB protocols but probably 1 ppm over 10 to 30 minutes) your tadpoles are going to be just fine. By the time it gets to the river it has already started degrading. And even if PPB got their kicks by shooting dozens of grenades all night long it would still be nowhere near the levels of concentration that would harm a river discharging something like 10K feet of water per second.