Transboundary fresh and thermal groundwater flows in the west part of the Pannonian Basin

In the paper, a new transboundary Upper Pannonian thermal groundwater body was identified which extends over 22,128 km2 in Austria, Hungary, Slovakia, Slovenia and Croatia. The presented joint numerical simulation of freshwater and geothermal aquifers in the Pannonian sedimentary basin and weathered basement rocks provided new insights into the regional water balance and cross-border groundwater flow rates. It is highlighted that predominant thermal water flow directions in the pre-exploitation state were from Hungary to Austria, and from Slovenia and Slovakia to Hungary. The study, intended to simulate changes in regional flow patterns, revealed that the current production rates of thermal water dramatically decreased the cross-border flows in all cases, and even reversed the flow direction to be now from Hungary to Slovakia. Simulated drawdowns at the state borders are in the range of 2-10 m, and they penetrate far into the neighboring countries. The expected future production, if increased for a factor of 3.5, should maintain the regional drawdown below 30 m; however, flow reversals would occur. Although the current regional exploitation cannot be called unsustainable, the quantity status of some geothermal aquifers is deteriorating locally and demands fast management actions. The joint transboundary management should focus on regular exchange of information, on increasing energy efficiency, and on obligatory use of geothermal doublets for geothermal heat production.