I'm writing a story set in the immediate (and eventually distant) aftermath of the human race globally being given access to a runic magic system in the modern day. This... eventually goes very, very wrong. It's not entirely humanity's fault, but eventually the dangerous powers humanity is given access to, combined with the main villain causing all technology running on electricity to be unusable for the rest of her life (however long that winds up being), cause the complete and total collapse of human society.
Now, one of the things I want to do with this concept is show the evolution of a single location from quiet, modern, pre-magic New Jersey suburb, to different but still relatively stable post-magic New Jersey suburb, to ash-risen, self-sufficient, magipunk post-apocalyptic city-state. The problem with this, of course, is that usually when the apocalypse hits, 99% of people on the planet die, and most of the remaining 1% who survive had to pack up and move somewhere more conducive to life without modern infrastructure. So if I want my story's pre-collapse setting to be the same as its post-collapse setting, I'm tasked with creating a location where staying put is actually a wise and viable idea: a place that has everything it needs to function post-collapse, as long as its residents can get their act together in time.
Designing this location is a complicated task, so I've decided to split it up into several parts. And to start, I'd like to focus on the third-highest tier on the rule of threes of survival, right after oxygen and shelter: water.
What would be the best source of water for a soon-to-be post-apocalyptic settlement that used to be a quiet American suburban town?
The criteria for best answer:
1: Believability. If this is something I can put in this town without it qualifying as a plot contrivance, awesome, that's ideal. The fewer rare things this town just so happens to simultaneously coincidentally have before the apocalypse happens, the better for reader suspension of disbelief. This should ideally be something that wouldn't remotely be out of place in our average middle to upper middle class suburban town.
2: Reliability. While this can be supplemented with other efforts to gather supplies, for the most part this will be what the town depends on to stay hydrated, so it needs to be consistent and not prone to things going wrong. This needs to provide the post-apocalyptic water needs for the town with enough consistency that they can compensate for any shortfalls.
3: Quantity. Obviously. More water provided by this source is better.
4: Potability. While the magic humans have access to means that most of the town isn't susceptible to disease anymore and could conceivably make do with less sanitary water sources, everyone under the age of 13 has no such luck, and so the closer this water is to being drinkable, and the easier it can be made so, the better.