I think you might need to just remember that what you've watched is a little 10 minute simplified video for public consumption. We didn't see any of the planning that went into this, we just saw a bit of footage of some drones, some other equipment and a rifle. So as complete outsiders it is a bit naive of any of us to assume they're making basic mistakes (like - they haven't considered alternative methods, whether their solutions are making it worse, whether they're even using lead pellets ...) and we know a better course of action.
It's not a marsh, it's a deep pit left over from mining
The area is so toxic, a few lead rounds is the least if concern.
Lead shot has been federally banned for almost three decades.