You provided the answer yourself: there already is a list of limits for things we definitely don't want in our water. If we find new things that we didn't notic

You provided the answer yourself: there already is a list of limits for things we definitely don't want in our water. If we find new things that we didn't notice before, we ASAP figure out what their health impact is, and introduce appropriate limits and procedures to mitigate the hazard. And such checks are already applied to water sources and distribution facilities to ensure their safety.