A conceptual model of mildly alkaline water discharging from the Zlatibor ultramafic massif, western Serbia

Abstract

Ultramafic rocks are generally taken to be ‘waterless’ or bearing little water. The mountain Zlatibor, western Serbia, largely built of Upper Jurassic ultramafics, is without perennial springs. However, in Gruda, an area on the NE side of Zlatibor, there are two perennial ascending springs with uniform discharge: Bijela Cesma and Hajducko Vrelo. The water from both springs is naturally mildly alkaline (pH ∼8.4), of Mg–HCO3 type, and temperature ∼11 °C. The springs have been investigated with respect to derivation, dynamics and chemistry with a view to commercial use of the water. The results indicate zones of rocks fractured during tectonic events and/or under lithostatic pressure. Deep ultramafic rocks, equivalent to abyssal peridotites (the least depleted rocks of the upper mantle, rich in magnesium), include fractured aquifers characterized by heterogeneity and anisotropy. Groundwater arriving at the land surface derives from atmospheric precipitation, and its quality is converted through the process of serpentine acid hydrolysis into mildly alkaline Mg–HCO3 water. Both quantity and quality of water from these springs are stable and are unaffected by atmospheric precipitation or other external influences. A conceptual model of the structure, hydrogeological character of the aquifer, and the mechanism of groundwater derivation is developed for both springs.