While I don't know the particulars, there's been some interesting news filtering up from the McCloud area with regards to a new NESTLE bottling plant there, to tap the water from Mt. Shasta, which similar to your Rathdrum water is filtered through volcanic rock. Very low mineral levels as well...
A few years back I wandered into the McCloud Rwy. shop and spent a few hours BSing about steam with their old master mechanic, name's on the tip of my tongue but I can't quite remember it right now. Anyway at the time I was still "involved" in the steam locomnotive thing, rebuilding a very scaled-up boiler and we got to talking about McCloud's water treatment program for their boilers. In short, they didn't really use any! He said the Shasta water was surprisingly mineral-free, they'd not noticed nearly as much scaling as I'd have expected.
Anyway...