to be safe as Evian

What does it mean? I haven't found the meaning of this phrase on the internet. I took it from Upstream textbook C1 level on 37 page.

The whole sentense is "l stooped at a rivulet to lake a drink - safe as Evian - then stripped off my clothes and threw myself naked into a
chilly lake."

Question

to be safe as Evian

What does it mean? I haven't found the meaning of this phrase on the internet. I took it from Upstream textbook C1 level on 37 page.

The whole sentense is "l stooped at a rivulet to lake a drink - safe as Evian - then stripped off my clothes and threw myself naked into a
chilly lake."

Answer

Evian is a well-known brand of mineral water sold mostly in France (but also exported elsewhere...)

As it is pure and natural spring water, the speaker is saying that the river water was equally pure and safe to drink. 🙂