Thermal Waters The mineral waters are one of the most precious natural resources in Bulgaria. There are more than 500 hydrothermal sources with a total flow rate of 3000 L/s, having different temperature and mineral composition. Almost all of them are thermal as the temperature varies in a wide range from 20oC to 100oC. About 33 % of the total discovered flow rate is of temperature between 20oC-30oC, and 43%-between 40oC-60oC (Petrov, et al., 1998) (Figure 1).