Are they true 'Onsen' or really just 'Sento'?

An ongoing, unresolved 'discussion' with my wife concerns how to refer to these local bathing places.

I call them 'onsen' which is part of their outside, public business title but she prefers to call them 'sento' since many of the local, traditional 'onsen' lack real, naturally-heated, mineral water.

Instead of natural mineral water, some local onsens pump in artisan well water. Then heat it up with either wood or city gas. Often, their smokestacks conspicuously point out their locations. An advantage to heating the water up on site is better temperature regulation.

Some older onsen, to better compete with more modern, natural onsen, add various minerals to enhance their water's therapeutic effect.

Most people may not realize this but the area between Nishinomiya and Sannomiya has long been a favored spot to produce sake, mostly because of the good and plentiful amount of artisan water that the nearby Rokko Mountains store.