Local tap water and human hair

A geochemist and ecologist from The University of Utah state that people can be tracked from where they have been by the water they have drank. The isotope ratios of the local tap water and of human hair correlated with one another.

The two researchers took human hair clippings from hair salons and collected local tap water from 65 cities of 18 states. The isotopes found were 2H and 18O. This information could be useful as forensic evidence or archaeological studies.

Questions I thought of:
How long does water retain in a person's hair after they pass away?
Also, obtaining a sample of hair from a suspect would be difficult to do if they are on the run; so is this technique/finding very helpful to forensic scientists?

http://pubs.acs.org/isubscribe/journals/cen/86/i09/html/8609news6.html

http://www.pnas.org/gca?submit=Get+All+Checked+Abstracts&gca=pnas%3B105%2F8%2F2788