So, they have a team of mad scientists who clean tap water. These people are very good. They work hard at removing every possible impurity from the local tap water. However, they aren't able to get it all. One chemical in particular, a contaminant that probably results from the manufacture of fun stuff like rocket fuel, was still at a measurable level (half a part per million or so). This wasn't acceptable, especially since that's one of the chemicals they have to test for.
Fortunately the stuff is broken down by sunlight. So, every day they hoist huge containers of water up to the roof and let them sit for a day in the bright sun. Problem solved; nasty stuff gone.