The thermal waters in Sofia City (Bulgaria): importance, condition and problems

Sofia, the capital of Bulgaria, is situated in a region rich in thermal waters, which were a major factor for the establishment and development of the city. The thermal spring in the city center of Sofia has been in use since ancient times. As the city expanded rapidly in the 19^th and 20^th centuries, it absorbed many other sources of thermal water. In addition, the available volume of usable thermal water increased with the construction of abstraction wells. The prevailing types of thermal waters are sourced from Mesozoic rocks at the base of the Sofia graben. The water temperatures are in the range between 21 °C and 54 °C, and the mineralization is up to approximately 1 g/L total dissolved solids (TDS). There is only one source in deeper rock formations with approximately 4 g/L TDS. The population of the city utilizes the thermal water resources to a various degree, mainly for bottling of natural mineral water. Some sources are used for balneology, sport, recreation and sanitation; however, their potential is not sufficiently utilized. Considering the importance of the thermal water resources for the city and its population, a numerical model was set up to study the impact of subway construction on the thermal spring in the center of Sofia.