From lots of neutrons hitting water. Regular water yields some, heavy water (like the CANDU units) generate more.
It is a very rare fission product coming directly from U fission, but the quantity is negligible.
I think the US used to make it by bombarding lithium in reactors, we don’t any more.
What do you think about the “100x normal” claim? Is it believable that a leak in the cooling jacket would release that much tritium that there would be a 100x level in the wells? Maybe that might come from natural sources. Granite is famous for having trapped radioactives. I would think that any tritium produced in the cooling water would quickly go downstream.