After reading the Czechvar website, here is what they say about water:
Fresh soft water is essential for brewing great lager. And no brewery in the world has a source as pristine and perfect as ours. Drawn from an Ice Age aquifer 300m underneath our brewery in České Budějovice, our water is absolutely pure and dates from a time before pollution or agriculture. Unlike other breweries, our natural water doesn’t need chlorinating or softening before brewing; it rises from our artesian wells clean, clear and ready for brewing.
By the above it seems that it is not treated in any way, as it is "ready for brewing" from the well.
I emailed the brewery asking about this.
The next big brew day at Bel Air is a Premium Pale Czech Lager.
edit: If we use all RO water, will this work?