Maybe you are being sarcastic. I'm not sure and my apologies if so.
They have two choices in this situation:
The water WILL NOT recede on its own. It has to be pumped out. They cannot save the ones still alive nor remove the bodies until the area is pumped out (the bodies also have to be removed from NO because they cannot create mass graves there). Leaving it the way it is for any length of time will create disease not just for the immediate area but for the entire south and even the rescuers -- who are from all parts of the country.
Or... they could worry about the environmental impact of pumping that goop -- not water but goop -- into the lake and allow every sort of deadly disease to spread in New Orleans. Dysentery is only the first and mildest disease that will spread.
There are only two choices and neither is good but one is worse than the other.
They would be far better off pumping it out and down the Mississippi than into the lake. Right now those pumps won't work, they have to be dried out first. A hundred smaller pumps would be better than no pumps.
No I was not being sarcastic, I have observed the method and minds of liberals.