sorry I got off the OP's topic with the deionizing resins and following up on Izzys comments... I think Acropolis5 is on the right track for urban use..

a .1micron absolute tubular filter and chemical purification is probably what my reality would be, however for one carrying a couple of filter bottles, one tubular and one charcoal might be an good option for posssible groundwater contamination

good call on the sillcock key.. we used them at school, but never crossed my mind to carry one...