Characterization of the regional groundwater flow systems in south Transdanubia (Hungary) to understand karst evolution

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Characterization of the regional groundwater flow systems in south Transdanubia (Hungary) to understand karst evolution and development of hydrocarbon and geothermal resources Katalin Csondor 1 & Brigitta Czauner 1 & Lehel Csobaji 2 & Orsolya Győri 3 & Anita Erőss 1 Received: 10 February 2020 / Accepted: 14 July 2020 # The Author(s) 2020

Abstract In South Transdanubia (Hungary), the remarkable geothermal and hydrocarbon resources in the Drava Basin and the hypogene caves at the margin of outcropping carbonate hills were usually investigated separately and their interactions were hitherto neglected. The aim of this study is to give all these groundwater-related resources and phenomena a common framework applying the concept of regional hydraulic continuity, and to complete the regional (i.e., basin-scale) hydraulic assessment of the area based on preproduction archival measured data. Pressure-elevation profiles, tomographic fluid-potential maps and hydraulic cross-sections were constructed to determine the vertical and horizontal fluid-flow conditions. As a result, two kinds of fluid flow systems could be identified. Within the gravitational flow systems, horizontal flow conditions are dominant and the regional flow direction tends toward the S–SE. In deeper basin regions, an overpressured flow system is prevalent, where fluids are driven laterally from the deeper sub-basins towards their margins. Based on the regional-scale evaluation of fluid flow systems, conclusions could be drawn regarding the geothermal and hydrocarbon potential of the area. Additionally, local-scale phenomena could be explained, particularly in the southern foreland of the Villány Hills. Cave formation cannot be related to the present-day flow systems here. In the Harkány area, groundwater chemistry could be explained by fluid contribution from the Drava Basin. A comparison with the marginal Buda Thermal Karst area allows for generalized conclusions regarding the connections between marginal karst reservoirs and the Pannonian Basin. Keywords Basin-scale groundwater hydraulics . Karst . Hypogene cave . Thermal water . Hungary

Introduction Hungary has favorable geothermal conditions related to the formation of the Pannonian Basin. During the early–middle Miocene basin formation, the lithosphere was thinned out due to extension, Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10040-020-02216-9) contains supplementary material, which is available to authorized users. * Anita Erőss [email protected] 1

József & Erzsébet Tóth Endowed Hydrogeology Chair, Department of Geology, Institute of Geography and Earth Sciences, Faculty of Science, Eötvös Loránd University, Pázmány Péter sétány 1/c, Budapest 1117, Hungary

2

Central Transdanubian Water Management Directorate, Balatoni út 6, Székesfehérvár 8000, Hungary

3

TDE ITS Ltd., Pasaréti út 46, Budapest 1026, Hungary

resulting in an elevated geothermal gradient in the lithosphere (45 °C km−1), and thus high surface heat flow (Royden