Geochemistry of thermal springs and associated gases along the Strymon River Valley (Bulgaria and Greece)

This paper presents and discusses the water and gas geochemistry of a large number of thermal springs occurring along the N-S trending Strymon Valley, from its source, near Sofia (Bulgaria), to the Aegean Sea (Greece). In Bulgaria springs have markedly alkaline pH, relatively low Total Dissolved Solids and prevalent Na-HCO3 to Na-Cl(SO4) in composition while the associated gas phase is mostly N2-dominated. When moving to the Greek sector, the thermal springs, Ca(Mg)-HCO3 to Na-HCO3, become less alkaline and more saline whereas the associated gas phase is CO2-dominated. The abrupt geochemical change in the Greek sector is caused by a variation in the thickness and nature of the sediments filling the Strymon Valley, the latter being characterized by a relevant amount of Neogene marine material. Such changes occur south of an important E-W lineament named Middle Mesta, south of which marble formations extensively crop out and are likely occurring below the sedimentary succession. The presence of these carbonate sequences embedded in the Neogene sediments is explaining the CO2-rich gases associated to the Greek springs. Water isotopes indicate a meteoric origin for the studied waters. From a geothermometric point of view, solute (previous studies) and gas (this work) geothermometers suggest that no high enthalpy systems occur in the Bulgaria and northern sector of Greece with estimated temperatures <120 C. Consequently, these thermal springs can be regarded as tectonically-derived along the many fault systems that border the Strymon Valley. The convective circuits are thus originated from rainfall in the crys-talline massifs that border the valley, i.e. the Serbo-Macedonian to the west and the Rhodope to the east.