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There are so many potential questions raised with respect to runoff water, so I'll just stick with one:

When I was a kid I was taught to let the water of a drinking fountain run for 10 seconds before drinking. This was to let the water run cold, and, we were always told, to remove toxins.

Would running that extra water be a problem of Bal Tashhith (prohibition against wasting)?

What about just the water that runs off while you're drinking?

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In Shabbos 129a and 140b, the Gemara points out that "bal tashchis" of one's own body outweighs other kinds. So for your first question, if indeed letting the water run gives you purer water (which is better for you), then indeed this should apply. If it's just to get colder water (which presumably does the same thing for your body as if it were lukewarm), I don't know.

I'm also not sure about the second case, though it may well be that since that's the normal way of drinking from such a fountain, it's not a problem. [It might be analogized to the cases - before four major Yamim Tovim - where a butcher can be compelled to slaughter an animal for a customer who wants some meat, even if he may not be able to sell the rest of it before Yom Tov (Chullin 83a); the importance of honoring Yom Tov outweighs the potential bal tashchis.]

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Additionally, bal tashchis might not apply to water (see Sh"A OC 170:22). – Fred Nov 7 '13 at 18:47

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