Ive studied this amazing vitamin and im pretty sure if we drank natural water we would have plenty of it. We may have parasites and disease, but we would have plenty of b12.
There was a doctor in Australia that tested the local river water and said it had enough b12 in one glass for a healthy daily dose.
I think we are going in the right direction to seek off-grid water supplies. Rain water is distilled until it drains into the ground water/ aquifer. It picks up minerals there.
Then you are required to purify it. Unless you have an artisan spring with a subterranean rock flow, all water needs purified.
Although there are a couple of drawbacks, I have no regrets with using my well.
Are there therapeutic doses of B12 in the water supplies you've looked into?
In Australia, perhaps there was an algae bloom at the time? That's one source of plant material with cobalamin.
Australia is also known for the cattle industry. If the samples were taken downstream from a ranch, I can only imagine that the amount of cobalamin/ B12 from their feces in the water would be trace amounts. B12 is produced in their rumens and in the intestines of other animals. I don't know if the proper fermentation would occur from human waste piped into the rivers to produce cobalamin.