Natural water — water from a natural source or minimally treated. Often seen on bottled mineral water labels.
登山中に湧き水を『天然水』として飲む場合は、衛生状態に注意する必要がある。
If you drink spring water labeled 'natural water' while hiking, you need to pay attention to hygiene conditions.
コンビニで天然水を一本買って、喉を潤した。
I bought a bottle of natural water at the convenience store and quenched my thirst.
その地域の天然水はミネラル分が豊富だと評判だ。
The natural water in that area is reputed to be rich in minerals.