How salty is the floodwater?

Before the storm hit Lake Pontchartrain and flooded the city, salts were present in the lake water at a concentration of between 5 parts and 10 parts per thousand. For comparison, freshwater salt is less than 1 ppt; seawater salt is 35 ppt. In general, any concentration over 7 or 8 is too salty to drink; rehydration fluids tend to top out at 4 or 5 ppt. The strong rehydrating brew doled out by aid workers from the U.N. has a salt content of about 6.4 ppt.

Since saltwater is heavier than freshwater, the bottom of Lake Pontchartrain is likely to be saltier than the top. That means that if the average salinity of the lake were a dangerous 7 ppt, you might still find a more palatable rating of 4 ppt near the surface. It's likely that the water that flooded over the levees came from the top of the lake, where the water tends to be fresher.

Rainwater might have further diluted the flood. In addition to blowing a surge of water off the lake, Hurricane Katrina deposited several inches of precipitation into both the lake and the city. So, whatever brackish water flooded into New Orleans would have been mixed with a reasonable quantity of fresh rainwater.

Even so, the water on the surface of the lake may very well have increased its salt content to 10 ppt or more in the run-up to the flood. Katrina could have blown very salty (20 or 25 ppt) water into the lake from the Gulf of Mexico and churned up the salty depths of the lake with a powerful storm surge. If this mixture overtopped the levees to the north of the city while some gulf water washed in from the east, the resulting floodwaters could have a salinity as high as 15 ppt—far too salty for human consumption.