From this:
Coast Guard Vice Adm. Thad Allen said federal officials have worked with Mayor Ray Nagin and support his vision for repopulating the city, but he called Nagin's idea to return up to 180,000 people to New Orleans in the next week both ``extremely ambitious'' and ``extremely problematic.''
...Allen said a prime public health concern is the tap water, which in most of the city remains unfit for drinking and bathing. He said he was concerned about the difficulties of communicating the risk of using that water to people who return and might run out of the bottled water they brought along...
...Another concern, Allen said, was the risk of another storm hitting the region, threatening an already delicate levee system and possibly requiring residents to be evacuated again...