An estimate of deep groundwater temperature is necessary for the research and utilization of this geothermal resource. Geothermometers are based on the temperature relation of some chemical reactions or the solubility of some minerals. Researchers mostly use silicon-based (quartz, chalcedony, amorphous silica) and cation-based (Na-K, Na-K-Ca, Na-K-Mg, and so forth) geothermometers. Temperatures of some prospectively abundant geothermal water resources in Serbia are estimated using the silicon-based geothermometers. In the absence of hot water resources, temperature of deep thermal groundwater has to be estimated and considered for power generation with the Kalina or the Rankine binary cycle. Best thermal waters (temperatures from 130°C to 160°C) for the purpose are located in the spa of Vranjska Banja, followed by Kuršumlijska, Sijarinska, and Jošani~ka spas and Bogati} of Ma~va. Pumped at the present rate of 200 l/s, the mentioned sources may generate 70 MW t , of which some 30 MW t the Vranjska Banja alone. Total power (for the five tested resources) is estimated at about 2200 TJ per year.

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Magnesium is chemical element commonly found in the environment and the main constituent of many types of minerals and rocks. This element is also essential to man. Owing to its abundance in nature, magnesium is present in all water resources and generally occur as the dominant cation, with calcium, in those that feature low TDS levels, whose origin is associated with large formations of sedimentary rocks (limestones, dolomites), and to a lesser extent with the degradation of silicate minerals that contain Mg. Magnesium concentrations in groundwater of Serbia vary over a wide range and their distribution is not uniform, but certain laws of nature do apply. The variation in the concentrations of this ion depends on the considered hydrogeological province, while within a single province it is a consequence of Serbia?s highly complex geology. The best examples are the Carpatho-Balkanides, with predominant karstified rock formations, and the Vardar Zone where ophiolites prevail but the makeup is much more complex than that of the Carpatho-Balkanides. [Projekat Ministarstva nauke Republike Srbije, br. 43004]

The City of Belgrade receives most of its drinking water supply from the alluvial aquifer of the Sava River. The wells are radial, placed in the lower part of the aquifer, so they partly run below the Sava riverbed. However, the groundwater quality of the wells in one part of the source (near the confluence of the Sava and Danube rivers) is found to differ somewhat from the groundwater quality of the other wells. The finding gave rise to additional investigations. The results revealed the existence of a deeper, limestone aquifer which is isolated from upper alluvial sediments by a thick layer of clay in most of the terrain. The naturally potential hydraulic contact of the two aquifers was additionally maintained by well operation in this part of the source. According to multiple analyses of groundwater flow using a hydrodynamic mathematical model, a hydrogeological and hydraulic system of groundwater flow was defined. Although the wells are situated adjacent to the river, and some well laterals are below the riverbed, most of the groundwater that flows to the wells is partly from the wider zone of the alluvial aquifer, and partly from the deeper aquifer. The initial results of hydrochemical investigations also showed an unexpected, inverse oxic character of the groundwater in these two aquifers.