What is the ground water temperature in Costa Rica?

It depends on which part of the country are you measuring the temperature (because of the altitude) and the aquifer´s depth you are measuring the temperature at. Rule of thumb in this case is that for shallow aquifers, ground water is going to be some degrees (celsius) lower than the mean environ. temperature for the site. From own experience in Alajuela or Heredia, GW temperature might be 20 °C, but as I said, it depends on altitude, GW depth among other factors.