We visited Hirugami–onsen in southern Nagano prefecture(our province).
It is a quiet rural area along the river Achi–gawa and surrounded by calm mountains.
Though I am fond of onsen so much and had known many other hot spring resorts in Nagano as well as other areas in Japan, I can definitely tell you ”Hirugami” is at the top of my list in terms of water quality–alkaline sulfurous of PH 9.6.
The abundant waters of the hot springs are just crystal clear and velvety smooth. You will know what I mean as soon as you scoop some water out of the tub — you may even double-check if you have not any soap on your palms.
It is called ” Onsen of Beautiful Skin”
You will just relax soaking in the water with a slight smell of sulfur.
It is also good for diseases such as rheumatism and diabetes.
Here is some additional info on PH —
According to the following page, the highest onsen PH is 11.3 and the lowest 1.2.
Among the lowest are famous Kusatsu (Gunma) and Zao (Yamagata), and Hakuba–Happo onsen is among the highest.
Please refer to the following multi-language page for more info on Hirugami and some other hot springs in Nagano prefecture which are some of my other recommendations in terms its smooth water quality–
Tazawa onsen: http://www.mountaintrad.co.jp/nagano/tzw/hi0/guide.html,