Transboundary geothermal resources of the Mura-Zala basin: a need for joint thermal aquifer management of Slovenia and Hungary

Large transboundary Upper Miocene geothermal sandy aquifers which are widely utilized by both countries forbalneological and direct heat purposes exist in the Slovenian-Hungarian border region. In NE Slovenia the totaldirect heat use was 382 TJ in 2010, while in SW Hungary it was 648 TJ, including utilization from basement reservoirs.The total installed capacity of the 13 Slovenian users was 38.8 MWt, while that of the 29 Hungarian userswas 70.6 MWt. Utilisation takes place without harmonized management strategies which might endanger the longtermsustainability of these systems. We aimed to overcome this by delineating a transboundary thermal groundwaterbody (TTGWB) Mura-Zala with an aerial extent of 4,974 km2 and with vertical extent between depths 500–2,200 m, which was done based on detailed geological, hydrological, geochemical and geothermal models as wellas numerical modelling. The regional groundwater flow in the Mura-Zala TTGWB is from west to east in general,the modeled cross-border flow is approximately 50 l/s. At present, thermal water abstraction rates from the Mura/újfalu Fm. (61.8 l/s in the Slovenian and 67.3 l/s in the Hungarian part of the TTGWB) does not endanger the goodregional quantity status of the water body, and this should be maintained by allowing a maximum increase of thermalwater abstraction 3.5 times higher than today. However, to achieve target numbers for an increased proportionof geothermal energy in the total energy mix in both countries, we suggest that increase of thermal efficiency andre-injection should be prioritized apart from the higher thermal water abstraction with setting up limit of themaximum allowable drawdown.