GENERAL:
There isn't much background on these hot springs.  They aren't advertised, and there is no sign "out front".  A search of the internet mentions them at: http://www.arenal.net/tour/eco-thermales-hot-springs, and a few others.  Our guide said you could only come to the springs with a reservation.  "Eco" as in ecological.  The story goes that this area was once a cattle farm, but eventually the spring water turned too hot for the cattle to drink.  So, the farmer sold the land to the present developer not knowing its potential!  See orientation map (HERE).