Hydrochemical characterization of thermal waters in Shipkovo thermal field

Abstract

“Shipkovo” thermal water field is one of the few in the country formed in karstified carbonate rocks. Initially, there was a natural spring with elevated temperature, located at the boundary of Triassic karstified limestones and dolomites and Lower Cretaceous terrigenous rocks, sinking in depth to the east. Later, drilling operations were carried out in the area, which resulted in the discovery of additional quantities of water with different temperatures and chemical composition, which led to the discovery of additional water of different temperature and chemical composition. Eight wells were drilled east of the natural spring with a depth from 174 to 1315 m, with thermal waters from different depths found in the Triassic karst aquifer and the fissured Jurassic terrigenous rocks. The characterization of the chemical composition of the waters was made based on 55 water samples analyzes. It is established that the chemical composition is specific to the individual water sources and varies over time. The natural spring is characterized with the most significant fluctuations in the concentrations of some hydrochemical indicators – the maximum and minimum values of the total dissolved solids (TDS) differ approximately twice, and the pH – by about 2 units. As the flow rate of the spring increases and its temperature decreases, the type of water changes from HCO3-SO4–Ca-Mg to HCO3–Ca, and the concentrations of indicators characterizing the deep origin of thermal water decrease – fluorine (from 1.5÷1.6 to 0.1÷0.3 mg/l) and metasilicic acid (from 20÷25 to 10 mg/l). In periods when mainly deep mineral water flows from the spring, rather than mixed thermal and atmospheric waters, the concentrations of Ba, Sr, Al, Li are significantly higher. Comparison of the chemical composition of groundwater from individual wells shows the influence of geological and hydrogeological factors on its formation and changes in depth. The mixing of the waters formed by active water exchange in the karst environment of the Triassic limestones and dolomites with HCO3-Ca and HCO3-Ca-Mg type with the water formed in fractured rocks with stagnant character and SO4-Ca-Mg type in the Jurassic deposits and within the reach of the Shipkovo Fault, the fieldspecific SO4-HCO3-Ca-Mg waters are formed. With respect to the microcomponents, there is a slight tendency to increase the concentration of F, Sr, Ti, Ba, Mo, Ni, Mn in depth, but in the deepest wells they are relatively lower. The results obtained are useful in support of management decisions for the utilization of thermal waters from the Shipkovo field.