You would need a reverse osmosis pump as carried on ship's liferaft kits. The water would still be very suspect. Everytime it rains here our beaches aren't even safe for surfers because of contaminant runoff. There are local problems of sewage, excess nitrogen from farming and equestrian manure and the accumulated filth washed down from adjoining communities. We did very well after the Northridge earthquake. Bottled water was easily trucked in and though rationed in the immediate area was readily available. The nice thing about California (so far) is that as destructive as our own earthquake,fire,rain and landslide cycles are they are local. If THE BIG ONE comes and overwhelms even adjoining regional resources a supply in place makes better sense.