They use reverse osmosis for purification of a local drinking water distribution source and they *may* supplement the final product because reverse osmosis produces a very bland water.

On flights inbound to the US one will often find products other than Dasani present; e.g. on flights from Europe one will often find Scottish or French spring water. I once did a blind tasting test with an FA and she could easily tell the difference between Dasani and a Scottish spring water and preferred the spring water!