I'm no expert, but this seems like a clear cut case of bacteria infection to me. If it's just alkilinity, then why didn't this happen before? And why does this happen at other springs too? And why would lemon juice applied quickly afterwards cure the rash? There must be some way to completely drain those other tubs. You could always put a hose in there, start sucking and let the laws of physics take over to create a vacuum. Or just shovel out the water with a bucket and big spoon. But again, I'm sure the USFS would have some ideas. I think a couple days of no water in the tubs would cure this, although if the bacteria is coming from the source then I guess it wouldn't work.