Scholarly record

INFLUENCE OF WATER LEVEL OF THE KARST SPRING BULAZ ON BALNEOLOGICAL PROPERTIES OF THE THERMAL SPRING SVETI STJEPAN IN ISTARSKE TOPLICE SPA (CROATIA)

Mladen Kuhta

Abstract

The waters of the thermal spring Sveti Stjepan in Istarske Toplice spa in Croatia has been used for medical purposes since 1817. Given the physicochemical indicators, the water of the Sveti Stjepan spring is classified into a group of highly sulfuric-radioactive-mineralized Na-Ca-Cl hypo to isothermal waters. It should be noted that rich sulfuric content and at the same time radioactive water occur very rarely. The unique mineral composition significantly contributes to the high balneological quality of the spa. The dynamics and levels of cold groundwater in this area are related to the large karst spring Bulaz, which is about 450 m NE from the thermal spring. Bulaz drains the majority of groundwater accumulated in the carbonate hinterland. The spring discharge varies from 50 L/s in extreme drought up to 32.4 m3/s during high water conditions. The average annual yield of the sprig is 1.38 m3/s. From the earlier known fact, the high water conditions at the spring Bulaz manifested by the decrease of water temperature in Istarske Toplice spa. The previous changes in the thermal water temperature were of a feedback character, i.e. shortly after the high waters, there was a rise in temperature to the initial value of 34-36 °C. However, during the construction of new water supply intake structure the spring spillway was raised approximately 1.2 m above natural level, which resulted in a permanent reduction of the thermal spring temperature to the 20-22 °C only, with a decrease of all significant mineral contents. In order to demonstrate the causal relationship between the performed construction works and the poor condition of the thermal spring, experimental lowering of the level at the Bulaz spring for approximately 1 m (by opening a protective embankment) was performed. In addition to the increase in water temperature, which increased from 22-23 to 33.9-35.4 °C, that resulted in a very rapid increase in concentrations of all characteristic chemical parameters of thermomineral water. Thus, the electrical conductivity increased from about 2 to 4.82 mS/cm, the sulphide content (H2S) increased from initial 1.34 to more than 30 mg/L, and the content of sulphate, chlorine and sodium increased several times. Likewise, the total radioactivity of thermal water increased from the initial 2 884 to 16 328 Bq/m3. The results obtained have shown that water levels at the karst spring Bulaz directly affect the balneological characteristics of the thermal water spring Sveti Stjepan in Istarske Toplice spa.

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