It depends on different nations.
In one case, it's because of the issue concerning the intersection between disinfecting drinking water and organic material in the source of the fresh water. It's very difficult to find a different source. I can think of a couple of communities, in particular. You can't just boil it in those cases. The issue is that you must disinfect the water if it's from surface water. You must have some system of disinfection. Then it becomes a question of whether we find a different method for disinfection instead of, for example, chlorination. Perhaps there can be different technologies. Perhaps it's time to think about a filtration system.
I can't speak to the specifics because I'm not an engineer, and I'm not acting for communities. I'll just say that, in principle, when you find you have a long-standing, intractable problem, you need to start to look for other solutions.
If I may, I'll quickly add that it's also a question of where the first nations' own infrastructure is. It may be that they need senior governments to think about helping them relocate. For example, the location of their lagoon, waste-water system outflow, intake or landfill site.... Some of those source-water plans we did with first nations, with grant funding, noted those issues. For example, in some communities in northern Ontario, it costs big capital dollars to change that around so you don't have the outflow from the waste-water plant ahead of the intake or a landfill site.