A Good Time To Visiy Marsnews.com for H2O News
http://www.marsnews.com/ has links to many Mars H2O articles. Most are virtual copies of the NASA PR, but space.com has a little extra and some of the others have commentary.
The 500m rumor appears to come from extrapolating 1 m concentrations down a full km. Still very much an educated guess. The good news is that there is a lot of water readily accessible for humans and potential life.
Regards,
Mark