Current utilization and hydrochemical characteristics of geothermal aquifers in the Bjelovar sub-depression

The Bjelovar sub-depression is situated in the north-western part of Croatia, on the southwestern margin of the Pannonian Basin System where favourable geothermal conditions exist. Thermal waters are used for recreation, balneotherapy, space heating, directly as sanitary water and electricity production. Geophysical, geological and borehole data were used to determine the types of geothermal reservoirs. In addition, several campaigns were conducted to sample geothermal waters from the Daruvar spa, Velika-1 and Krečaves locations for isotope (δ18Ο and δ2H) and physico- chemical (EC, T, pH, DO, Na+, K+, Mg2+, NH4+, Ca2+, SO42-, Cl-, Br-, F-, SiO2 and H2S) analyses to determine their hydrochemical characteristics. Two major types of geothermal reservoirs were determined: (i) ‘basement – BM’ reservoir, (ii) ‘basin fill – BF’ reservoir. The BM reservoir consists of Palaeozoic and Mesozoic sediments composed of: (i) fractured/karstified carbonate sediments and/or (ii) fractured/fissured crystalline/metamorphic rocks. The BF reservoirs are ‘Lower Pannonian’ and ‘Upper Pannonian’ sediments composed of coarse and fine-grained sand, sandstones and marls. The BM geothermal aquifers are the most important ones in the study area. The stable isotope δ2H and δ18O indicate that the monitored thermal waters have a meteoric origin, but without recent replenishment. Monitored waters belong to mixed hydrochemical types, from Na-ClHCO3 to Na-HCO3 types in the deep basin thermal waters and a CaMg-HCO3 type in the thermal waters from shallower parts. The study area has great geothermal potential. The estimated total available thermal power from Križevci, Velika-1 and the Daruvar spa is 70.5 MWt, but only 28 % of this thermal power is used. Since the predominant activity in the study area is agriculture, the geothermal resources available could lead to modern agricultural development and consequently contribute to increasing the standard of living of the local population. However, additional geophysical, geological, hydrogeological and hydrochemical investigations at a number of new potential locations are required to estimate the total available geothermal resources.