Well, the springs originating from the Bei Tou/Yangmingshan area do contain the sulfur that has a strong smell. If you go to a spring in one of these places, or Wulai, I imagine, and it’s clear warm water, I seriously doubt they are 100% hot springs.
Based on my extensive experience in Northern Taiwan, I stand by what I say. And yes, there are fakes in YMS, Jinshan, Wulai, and Beitou. Don’t be naive and believe unscrupulous hotel/resort owners who tell you that their springs are “different” and are supposed to be clear and odor free.
The hotsprings museum in Beitou clearly shows the source of springs in Northern Taiwan, and there aren’t very many. Every hotel/resort draws from a fairly small number of sources.
My experience in Ilan holds the same about the color/smell. Further south than that, I’m not so sure, so you may be right.
Perhaps what you’re experiencing in some other areas (“natural,” free, or whatever) are hot springs with little mineral content. If that’s the case, there isn’t really much benefit to bathing in them except for enjoying a nice hour or so outdoors, which I suppose is fine in its own way.
Regardless, though, people should be very careful about choosing a place to enjoy the springs. Fakes and filthy pools abound. It really bothers me, since the spa/spring scene is one of the things I love most about Taiwan.
Recycling the springs is not necessary or practiced by every hotel. Again, the cheap ones will scam you.
My advice is to spend 5,000 or so to stay at a full resort that includes springs, sauna, steam bath, etc. In fact, I just came back from one, and it was fantastic. Hey, once every couple of months won’t break you. A lot of sloshes here spend that much in one night at a pub.
Believe me, I’ve tried many cheap places, and the experiences were really awful.
But I would like to look a bit more into some outdoor naturally occuring springs in Wulai or elsewhere - away from the crowds and hotels, of course. That sounds quite nice.