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

Renewable and Sustainable Energy Reviews 57 (2016) 439–454

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Transboundary fresh and thermal groundwater flows in the west part of the Pannonian Basin György Tóth a, Nina Rman b,n, Ágnes Rotár-Szalkaia, Tamás Kerékgyártó a, Teodóra Szőcs a, Andrej Lapanje b, Radovan Černák c, Anton Remsík c, Gerhard Schubert d, Annamária Nádor a a

Geological and Geophysical Institute of Hungary, Stefánia út 14, H-1143 Budapest, Hungary Geological Survey of Slovenia, Dimičeva ulica 14, SI-1000 Ljubljana, Slovenia c State Geological Institute of Dionýz Štúr, Mlynská dolina 1, SK-817 04 Bratislava 11, Slovakia d Geological Survey of Austria, Neulinggasse 38, A-1030 Wien, Austria b

art ic l e i nf o

a b s t r a c t

Article history: Received 11 March 2015 Received in revised form 21 July 2015 Accepted 10 December 2015 Available online 5 January 2016

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 preexploitation 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. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Cross-border flow Geothermal aquifer Thermal water Aquifer depletion Water management

Contents 1.

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3.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Investigated area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Geographical settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Hydrogeological settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Hydrogeological data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Transboundary flow model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Software, model grid and boundary conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Phase development of the model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Calibration of the model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Regional groundwater flow dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Regional flow directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Corresponding author. E-mail address: [email protected] (N. Rman).

http://dx.doi.org/10.1016/j.rser.2015.12.021 1364-0321/& 2015 Elsevier Ltd. All rights reserved.

440 440 440 441 441 441 442 442 442 443 443 445 445

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4.2. 4.3.

Regional water budgets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447 Cross-border flows and budgets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447 4.3.1. Austria–Slovenia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447 4.3.2. Austria–Hungary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447 4.3.3. Austria–Slovakia. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 448 4.3.4. Hungary–Slovenia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 448 4.3.5. Hungary–Slovakia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 449 4.4. Model limitations and future studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450 5. Recommendations for strategic transboundary groundwater management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450 5.1. Sustainability of thermal water production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451 5.2. Benchmarking of geothermal management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451 5.3. Regular exchange of information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451 6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452

1. Introduction 1.1. Background Combining water management and sustainable development of water resources is a challenging issue not only because of the complexity of natural systems and uncertainty in available datasets, but also due to limitations of the various tools applied. This is especially demanding when dealing with resources that extend across boundaries of several countries. A number of regional numerical studies of freshwater aquifers were published [1–7], and management of transboundary drinking water aquifers was also investigated [8–13]. However, a large gap becomes evident when addressing transboundary geothermal resources. Investigations of high-enthalpy geothermal systems are based on local numerical models of their restricted natural settings [14], and there has usually been no need for joint multinational research. On the other hand, low-enthalpy systems can evolve in large sedimentary basins [15,16] which often extend over several countries. Very few examples of joint research or management of such hydrogeothermal resources are known to the authors, notably the Austrian–German Jurassic thermal karstic reservoir [17], the South American Guaraní aquifer system [18,19], the Slovenian–Hungarian Upper Pannonian aquifer [20], and the French–German–Swiss Rhine Graben [21]. Sustainable use of groundwaters is a worldwide issue [22–27] for which most of management tools are primarily available for drinking water resources [11,28,29]. In the field of geothermal energy use, lack of management is noticed [30], and renewability, exergetic efficiency and equitability are important issues when addressing the sustainability of resource exploitation [31]. When discussing transboundary resources, a combined approach of aquifer knowledge, interpretation of long monitoring series, and spatiotemporal numerical simulation have been suggested to evaluate the state of exploitation [32,33]. The use of numerical models [23] is not a priori a requirement of the Water Framework Directive (WFD) [34], but it should nevertheless be considered when developing a water management plan. Groundwater management can be carried out in practice at different levels, from local [35] to multinational. The Convention on the Protection and Use of Transboundary Watercourses and International Lakes (Water Convention) promotes and encourages worldwide activities [36], while the Danube River Protection Convention (DRPC) is focused on the Danube River basin area [37]. The latter also provides a platform for transboundary activities regarding the WFD [34]. The framework for this paper is furnished by the Pannonian basin extending across eight countries in Central and Eastern Europe. Its geothermal conditions [38] are favorable for exploiting

thermal waters which are widely used for balneology as well as for direct heating purposes [39–43]. This deep sedimentary basin contains a number of geothermal aquifers that are shared by the neighboring countries [39,40,44–48], and significant basin-scale groundwater flows [49,50] can be expected to occur across national boundaries. Geological mapping of the ancient Austrian-Hungarian Empire in the 19th century provided a first basis for the interpretation of regional geology [51], but later, the political isolation of these countries prevented scientific cooperation for almost a century after its abolition. Limited international cooperation was based on bilateral agreements on water management which mostly date back to the 1970s [52], and joint projects focused on groundwaters were established only after the late 1990s [20,53–57]. Local deterioration of the quantity and/or quality state of geothermal aquifers in the Pannonian basin was identified [58–61] together with a fundamental lack of administrative management [20,62]. However, no systematic measures have yet been taken to deal with the risk of regional over-exploitation. 1.2. Objective The Renewable Energy Action Plans of the four countries, Austria, Hungary, Slovakia and Slovenia, are based on fulfilling aims of the Directive on the Promotion of the Use of Energy from Renewable Sources in Europe [63], and they foresee a 3 to 3.5-fold increase of geothermal heat production from 2010 to 2020 [64–66]. Before implementing these ambitious energy goals, however, the sustainability of various approaches needs to be tested with a special focus on the transboundary geothermal aquifers. Existing studies imply that a change in outflow temperature of thermal water is unlikely to occur despite its potential over-exploitation [43,60,67]. Many of depleted geothermal aquifers are transboundary and it appears essential to investigate the regional water budgets and the impacts of thermal water production on flow rates between the countries. Induced recharge and leakage can be estimated on the basis of chemical analyses [59,68] or, even more effectively, by application of numerical models. The present paper deals with research on a numerical model on a scale of 1: 500,000 that has been used to gain information on large-scale thermal water flow systems in the western Pannonian basin. The objectives of the study have been:


To develop the methodology by setting up a joint numerical


model of intergranular aquifers in a sedimentary basin and in fissured and karstic aquifers in the basement of the basin. To quantify the water balance and cross-border flow rates of multiple transboundary aquifers in the pre-development state.

G. Tóth et al. / Renewable and Sustainable Energy Reviews 57 (2016) 439–454

441


To investigate the effects of present and foreseen geothermal developments on the quantity status of the aquifers.


To provide scientific support for joint cross-border management and protection policy of the identified transboundary geothermal aquifers.

2. Investigated area 2.1. Geographical settings The investigated region covers in total an area of approximately 47,700 km2 in east Austria (AT), north-west Hungary (HU), southwest Slovakia (SK) and north-east Slovenia (SI). It is surrounded by heights belonging to the Central and Eastern Alps in the west, the Western Carpathian Mountains in the north, the Transdanubian Range in the east, and the Haloze and Zala hills in the south. Lowlands of the Styrian, Vienna, Danube and Mura-Zala basins are positioned between the hilly areas (Fig. 1). The majority of the region (71%) is lower than 300 m above the sea level (a.s.l.), while mountains with elevations above 500 m cover 12% of the area, reaching the highest peaks at around 2000 m. The population exceeds four million inhabitants, and is concentrated in a few major cities, such as Vienna and Graz (AT), Bratislava (SK), Győr (HU) and Maribor (SI). The humid temperate climate evolved at a junction of three climate regions: maritime temperate, dry summer temperate and rather humid continental climate, which are additionally modified by topographical effects. The average air temperature is comparable all over the region, being between  2 and  4 °C in January, and between 23 and 25 °C in July. The mean annual precipitation decreases from above 1500 mm in the south-west to 500 mm in the north-east, and it is influenced by topographical effects. The entire area belongs to the Danube River drainage basin, and its most important tributaries are the Drava, Morava, Vah, Hron, Ipel and Raba Rivers, while the Zala River first discharges into Lake Balaton and joins the Danube outside the study area, through the Sió Canal. The largest tributary of Drava is the Mura River [69]. 2.2. Hydrogeological settings The Pannonian basin is situated on a positive geothermal anomaly with an average heat flow of 100 mW/m2 [70]. It is a result of Miocene crustal extension following the ongoing subduction along the Carpathians, and extension later transformed to compression [71,72]. The maximum temperature may reach 150 °C at a depth of 2500 m. In 2009, 149 users produced thermal water with an outflow temperature of above 20 °C in the investigated area, its total abstraction exceeding 33 million m3 [43]. Basement of the basin consists of the Paleozoic and Mesozoic crystalline and carbonates, which were covered by siliciclastic or andesitic deposits in the Paleogene. The role of faults on groundwater flow can be varying [73], and local studies imply that some act as hydraulic barriers while others enable free convection [71,74–76]. However, their anisotropy in the poorly lithified sedimentary sequence above [77] has not yet been investigated. The permeability of carbonate rocks is strongly controlled by karstification, while regional tectonic zones enhance the fissure permeability of the crystalline. The basement outcrops in hilly areas enable the recharge for regional groundwater flow through a fewten-meters thick weathered zone at the top of the crystalline. This zone forms a common hydraulic unit with the overlying Miocene basal coarse-grained sediments [20]. The thermal karst systems in carbonate rocks store important amounts of thermal water in the Vienna basin [78,79], Slovakia [80] and Hungary [81–83]. The hydrogeochemical information is extremely important to consider

Fig. 1. Study area with main topographic features. Coordinates are in WGS 84 UTM 33 N coordinate system. Position of two hydrogeological cross-sections is marked by 1-1' and 2-2'.

when investigating this type of regional flows for identification of mixing processes and over-pressured areas [84–86]. The predominantly marine sedimentation ceased about 12 million years (Ma) ago. A huge inland lake, Lake Pannon, formed, and it deepened until approximately 9.8 Ma before present. Sub-basins were filled rapidly by huge deltaic systems of rivers originating in the surrounding Alps and Carpathians [87]. The resulting siliciclastic deposits can be up to several thousand meters thick in the deepest sub-basins. Turbidity currents deposited coarse material on the basin floor, while, at the same time, silt and argillaceous marl were deposited on the slopes. Turbiditic sand bodies contain rather isolated aquifers which are often exploited for oil or gas production [88]. Prograding shelf sedimentation resulted in the deposition of thick sand bodies in the delta front environment. This was overlain by fine-grained sediments of the delta and alluvial plain deposition, representing the latest stages of the filling up of the lake basin [87,89]. The most favorable geothermal aquifers formed in the Upper Pannonian delta front sands at depths from 500 to 2000 m. The 50–300 m thick units consist of regionally connected intercalations of 10–20 m thick sand lobes, which are separated by pelitic layers [20,74,76]. Basin-scale groundwater flow penetrated as far as these highly anisotropic sands, storing thermal water of the Pleistocene age [46,90]. At greater depths, density- and heat-driven flow components interfere with gravitational forces, and thermal water with an outflow temperature of 90 °C can be produced. Free convection cells are not expected to occur in these sands due to their strong anisotropy [91]. These two regional flow systems are separated by thick Carpathian to Lower Pannonian aquitards. Local connections may occur, for example at Lake Hévíz (HU) [92], and in the Lutzmannsburg-Zsira area (AT-HU) [58]. Intermediate flow systems evolved in few hundred meters deep Upper Pannonian delta plain and deeper Quaternary formations, from where freshwater is being produced. Local gravity-driven freshwater flow systems occur at margins of the basin and in the mountainous areas. They are most often identified in the outcropping Quaternary alluvial and fluvial sediments [49,86], and form unconfined aquifers connected with the surface water bodies. Distinction between the flow systems is also evident from the vertical stratification of waters. The Ca–Mg–HCO3 water type

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evolves with depth to Na–HCO3 and Na–Cl waters, while the Ca– Mg–HCO3 waters occur in the Hungarian thermal karst system [46,74,90]. 2.3. Hydrogeological data Geological surveys of the four countries were involved in elaboration of the transboundary flow model. This model represents one integral part of a much wider approach to establish a joint management of transboundary geothermal aquifers within the project TRANSENERGY. It was extremely important to provide data that were harmonized and comparable. A joint database was set up consisting of 11 parameter groups, from the geological and hydrogeological characteristics of the area to the utilization parameters of thermal waters. In total, 1615 geothermal objects (e.g. wells or springs) with 233,971 connected records have been stored in the database [93], and public access to information on 529 geothermal objects and 28,415 records has been made possible. The Open Street Maps (OSM), licensed under the Creative Commons Attribution-ShareAlike 2.0 license, were used as topographic background and to set the positions of surface water bodies. Topography was accounted for by applying a digital elevation model derived from the SRTM Version 4 Digital Elevation Data with a resolution of 90 m at the equator [94]. The altitudes of boreholes and springs as well as groundwater levels and their maps are originally stored in different vertical datums in the four countries. The Adriatic Datum is used in Austria, the Baltic in Hungary, the adjusted Baltic in Slovakia, and the Trieste Vertical Datum in Slovenia. It was extremely important to transform all information into a single system, for which the Baltic Vertical Datum was used. Previous hydrogeological models distinguished the main aquifer and aquitard types, but could not sufficiently address their spatial extent. Therefore, the new regional geological model was prepared on a scale of 1:500,000 [76] in the form of a series of harmonized geological base maps. Because altogether 219 geological units were identified, a simplification of the flow model structure was essential. However, one must consider various prediction errors [95,96]. Each hydrostratigraphic unit is a composite of various geological formations with similar hydrogeological properties [97,98], and seven units were used in this case study (Table 1). Information on the regional hydrogeological parameters was taken from the joint database [93] and publications [79]. In general, the horizontal hydraulic conductivities range from 10  2 to 10  4 m s  1 in the Quaternary freshwater aquifers, from 10  5 to 10  6 m s  1 in the Upper Pannonian geothermal aquifers and from 10  5 to 10  8 m s  1 in the thermal karst aquifers. The anisotropy (ratio of horizontal versus vertical hydraulic conductivity) of karst

aquifers is estimated to be approximately 10, and between 2000 and 5000 for the Upper Pannonian sands. The simulated anisotropy in sedimentary basins worldwide can vary from 10 [99] to 100,000 [7], depending on the setting [100]. Experience shows that framework models should be used with great care in simulations of trended and stratified heterogeneity with parallel flow direction [98]. Besides, a few magnitudes higher regional than measured hydraulic conductivity can often be applied for carbonate and sandstone aquifers in order to obtain properly calibrated models [101]. Based on previous experience of simulating groundwater flow in the Pannonian basin [57,102], an effective porosity of 0.15 was applied to all model layers in the sedimentary basin, and of 0.03 to the basement rocks. The effects of thermal water production may cause significant changes in the regional groundwater flow pattern [103]. Production information for 169 geothermal wells open in the Upper Pannonian aquifers was gathered for the years 2007–2010 [43,104]. Summarized production data were reported for 83% of them, yielding a total of 11.4 million m3 of thermal water in 2009. Thermal water from the crystalline and carbonate basement aquifers was produced from 100 wells and springs situated mostly in Austria and Hungary, but production information was available only for 59% of them in Hungary, and none from Austria. The Hungarian production exceeded 19.4 million m3 in 2009, of which 65% appertained to the natural discharge of Lake Hévíz. The local effects of excessive production were studied in details with higherresolution numerical models [105,106], and were therefore not included in the present study.

3. Transboundary flow model 3.1. Software, model grid and boundary conditions A three-dimensional multiple-layer, heterogeneous and anisotropic groundwater flow model of the large transboundary area was built by using the U.S. Geological Survey code MODFLOW v.2011, and the Visual MODFLOW interface [107]. The size of the model area was 283 km  314 km horizontally and 8 km vertically, with a regular cell size of 1 km  1 km. The grid was discretized into 11 numerical layers representing seven hydrostratigraphical units (Table 1), resulting into the total number of 88,862 cells. Since the layers in the model had to be continuous, their minimum thickness was assigned where reasonable. One layer could contain several hydrostratigraphic units in order to accurately represent the geological settings. The production models had the same grid and boundary conditions as the pre-exploitation model, except

Table 1 Comparison of delineated geological and hydrostratigraphic units of the flow model. No. of layer in the flow model

Hydrostratigraphic unit

Layer of the geological model

Horizontal hydraulic conductivity [m/s]

Vertical hydraulic conductivity [m/s]

1 2

Quaternary unconfined aquifers Deeper quaternary confined freshwater aquifers Upper Pannonian confined freshwater aquifers Upper Pannonian confined geothermal aquifers Lower Miocene to lower Pannonian aquitards Weathered and/or karstified geothermal basement aquifers Low permeable unweathered basement rocks

Surface geology map Base of the quaternary

4.1E-06 – 2.5E-04 1.6E-06

2.1E-09 – 1.4E-06 1.6E-09

Arbitrarily divided

4.0E-06 – 5.0E-05

6.0E-10 – 7.0E-08

Base of the upper Pannonian formations Base of the Cenozoic formations

4.0E-06 – 5.0E-05

6.0E-10 – 7.0E-08

1.0E-09

1.0E-10

2.4E-06 – 2.7E-05

7.2E-09 – 1.7E-07

1.0E-09 – 1.1E-07

1.0E-09 – 1.1E-09

3–5 6 7–9 10 11

Parallel and below the base of the Cenozoic formations Base of the model (8 km b.s.l.)

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that an additional “well” boundary condition was activated to simulate production. Various boundary conditions (BC) were specified. A “no-flow” BC was prescribed for the lateral and bottom limits of the model. Inactive cells were assigned to the surroundings of the delineated area within the rectangular setup of the model, and simulation was carried out only in the active area. A “constant flux” BC was prescribed to the top faces of the first flow model of the shallowest unconfined aquifers. Recharge zones with similar hydrogeological characteristics were delineated according to the regional surface geological map on a scale of 1:200,000 [76] to control the recharge process [108]. Because insufficient spatial information was available, only one average value was applied as the recharge rate of the whole model, namely that of 70 mm year  1. Natural discharge was simulated as a “drain” BC (head-dependent flux) which was prescribed to all top faces corresponding to the topographic surface of the model in order to force the deepest parts of the valleys to allow for discharge. A “constant head” BC was prescribed to the top nodes of all confined flow models. This represents the calibrated freshwater heads in the shallowest unconfined aquifers, and accounts for recharge of the confined models. The two steady state production models, for the present and the forecast production, had a “well (constant flow rate)” BC assigned to the locations of geothermal wells open in the Upper Pannonian aquifers. Only the constant pumping rates were taken into account because there was not enough detailed information available on time variation of production or on time series of groundwater levels for building a reliable transient production model. The average annual production rates in the period 2007–2009 were used in the model of the present production, being 0 m3 day  1 for Austria having no geothermal wells, 4628 m3 day  1 for 21 wells in south–west Hungary, 5854 m3 day  1 for 19 wells in north–west Hungary, 7726 m3 day  1 for 27 wells in Slovakia and 5336 m3 day  1 for 17 wells in Slovenia. These values were multiplied by five in the forecast model. 3.2. Phase development of the model A multi-phase approach was applied to develop the transboundary flow model. Production models were constructed first, based on an assumption that the regional flow systems had hydraulically reached a quasi-steady state at the beginning of the 20th century, when the observed groundwater levels in many shallow freshwater and deeper geothermal wells in Hungary were relatively constant. Later, a pre-exploitation natural steady state model was elaborated after eliminating the thermal water production, while a forecast simulation with increased production was performed in the last step. First, a simple two-dimensional horizontal unconfined flow model was constructed to simulate the regional groundwater table. This was important because the recharge and flows in a confined model with high temporal variability are completely determined by prescribing the water table as a top BC [107]. Since the simulated geothermal aquifers store thermal water from the Pleistocene [46,108], they are probably not very time-sensitive. Similar systems in the Great Hungarian Plain and the Paris basin display a long-term transient behavior due to the last climate transition, and are therefore not in equilibrium with their present BC [109]. However, this had to be disregarded also due to lack of information. Topography was applied on top of the model and the thickness of simulated layer reached few thousand meters to eliminate its effects. Because this numerical layer represented an equivalent of the realistic 10–50 m thick shallow water table aquifers, their transmissivities were identical, and the simulated hydraulic conductivity was reduced by approximately two orders of magnitude. The calibrated heads of the first model served as a

443

top BC for improving the numerical stability of the subsequently constructed confined flow models. Secondly, a three-dimensional two-layer confined model was developed. A 30 m thick layer was added parallel to the groundwater table, representing the regional unconfined shallow aquifers (layer 1 in the final model). The layer below represented all deeper intergranular aquifers. Thirdly, the model was extended to contain six layers which were able to represent the regional groundwater flow system of intergranular aquifers in the sedimentary basin. Elevation of the bottom of the Upper Pannonian sequence was assigned as the bottom of the model, followed by assigning the bottom of the Quaternary sequence as the bottom of the second layer. The area in between was split into three evenly thick layers (the bottom two were layers 5 and 6 in the final model) so as to be able to account for heterogeneity of formations and abstraction of fresh- and thermal waters from different depths. Additionally, the uppermost Upper Pannonian layer was again split vertically into two even layers (layers 3 and 4 in the final model) because this part is widely used for freshwater abstraction in Hungary and Slovenia [20]. Unfortunately, insufficient information on the production of freshwater was available, and therefore it was not possible to simulate it. Fourthly, the model was extended to its final depth, to 8 km below the sea level. A homogeneous and strongly anisotropic layer of low hydraulic conductivity was added below the intergranular aquifer system, representing the Lower Pannonian, Sarmatian, Badenian, Lower Miocene and Upper Cretaceous layers that consist of predominant argillaceous sediments. This layer was additionally split into three equally thick layers (layers 7 and 8 in the final model) to account for a more permeable sequence in Austria (layer 9 in the final model). Fifthly, the area below the Neogene beds was subdivided into two layers. The top one (layer 10 in the final model) represented weathered, fissured and/or karstified basement rocks. Their thickness is estimated at 50–100 m in reality, but a thickness of 500 m was uniformly applied in the model in order to avoid its numerical instability because of very variable elevation of the bedrock. Of course, the hydraulic conductivity was adjusted accordingly to keep the representative transmissivity of the system. The lowermost layer (layer 11) accounted for the fresh, mostly crystalline basement rocks of very low permeability. The layer 10 was locally connected to the lowest Upper Pannonian one (layer 6) in order to permit simulation of the extremely important mixing processes. Sixthly, the thermal water production rates were removed from the production model, so that the flow systems were allowed to recover into a pre-exploitation natural steady state. The latter model was not calibrated to any measurements because of scarcity of information on the pre-exploitation quantity state. In the last step, a steady state forecast simulation with five times increased production rates of thermal water was performed by applying the natural steady state model as the initial conditions for the simulation run. 3.3. Calibration of the model Both regional groundwater flows, in the sedimentary basin and in the basement, discharge into surface water bodies, and have also hidden seepage into shallow freshwater aquifers. Their mixing processes are extremely important for many hydraulic features (see Section 2.2), and they were accounted for in the model. It is again worth noting that the pre-exploitation steady state flow model was not calibrated because it was derived by deducting all thermal water abstraction rates from the calibrated steady state production model. The averaged time series of measurements in

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Fig. 2. Distribution of simulated groundwater heads in pre-exploitation state with locations of calibration points (CP) for: (a) shallow unconfined aquifers, model layer 1, CP¼ black circle, (b) confined freshwater Upper Pannonian aquifers, model layer 3, CP¼ empty circle, (c) geothermal Upper Pannonian aquifers, model layer 6, CP ¼ empty triangle and (d) karstic basement aquifers, model layer 10, CP¼ rotated black triangle.

observation wells from the last two decades were applied as calibration targets in the production model. These were for intergranular aquifers: the observed heads in unconfined freshwater aquifers, the observed heads in confined geothermal aquifers which were corrected for density variation to get comparable freshwater heads at a reference temperature of 10 °C [110–112], the groundwater level contour maps in smaller alluvial plains of the Mura and Drava Rivers [113], and the locations of permanent rivers, creeks and seepages. Two additional parameters were used to calibrate the model with respect to basement aquifers: discharge rate of major springs, and mixing of groundwaters in larger thermal and lukewarm springs. The latter can occur due to connectivity of sand channels in multiple-aquifer systems in sedimentary basins [114] as well as at the contact of two regional flow systems [92]. The mixing was accounted for by local adjustments of hydraulic conductivity in the vicinity of major springs. Calibration involved a manual trial and error method. A single sub-region in a layer was calibrated at each calibration step. The first general values of hydraulic conductivity and its anisotropy

were laterally varied to distinguish among the following hydrogeological categories: alluvium, Quaternary terrace sediments, carbonate sands, intergranular aquifers, fissured aquifers, karstified aquifers, double porosity aquifers, karstified and fissured aquifers and aquitards. The vertical hydraulic conductivity was usually considered three orders of magnitude lower than the horizontal one, and the model was found to be sensitive to anisotropy. The calibrated present production flow model has a mosaic-like structure with a realistic range of hydrogeological parameters. The goodness of fit test between the simulated and measured groundwater levels shows that the mean error (ME) between the two values of the shallow unconfined aquifers (model layer 1) is in the range of 0.3–0.5 m, depending on the sub-region. The root mean square error (RMSE) is 22.1 m for 1692 observation points in Austria, 3.4 m for 88 observation points in Hungary and 2.4 m for 72 observation points in Slovenia. The ME of the confined Upper Pannonian geothermal aquifers (model layer 6) is in the range of 0.1–3.5 m and the RMSE is 10.7 m for 22 observation points in

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Hungary, 1.6 m for 174 observation points in Slovakia and 13.2 m for 10 observation points in Slovenia. The ME of the basement geothermal aquifers (model layer 10) is in the range of 1.2–4.4 m, and the RMSE is 6.8 m for 30 observation points in the west Transdanubian range, 23.3 m for 52 sites in its north-east part, and 21.6 m for 21 sites in its central part in Hungary. As these errors are rather small relative to the maximum regional groundwater level variations of  200 m in the unconfined aquifers, 100 m in the Upper Pannonian geothermal aquifers, and  200 m in the basement aquifers, the model calibration was considered reasonable.

4. Regional groundwater flow dynamics Previous research on the Upper Pannonian aquifers [20] did not provide sufficient cross-border information on groundwater flows for all countries to which they extend. Since aquifer depletion is often the most critical factor in sustainable use of low temperature geothermal systems [115,116], the research was focused on the magnitude of its occurrence. The simulation results are presented in a sequential order, from the pre-exploitation model to the model of present production, and the increased production one. 4.1. Regional flow directions The simulated pre-exploitation groundwater table elevation followed the main surface water bodies which are their most important discharge areas (Fig. 2a). In the southern part, groundwater level decreased in the NW–SE direction from approximately 350–150 m a.s.l. along the Mura and Drava Rivers (SI), and to 108 m a.s.l. around Lake Hévíz (HU) (Figs. 2 and 3). In the central part of the model, the hydraulic gradient was found to be almost twice as high as in the alluvial plains due to the surrounding mountains, and the elevation of groundwater table decreased toward the Raba River (HU) from 300 to 115 m a.s.l. The

445

groundwater flow direction changed along it toward north from the SW–NE to NW–E direction due to the inflow of the Morava River (AT-SK) into the Danube River (HU-SK), and groundwater levels decreased to 105 m a.s.l. in this area. Groundwater levels in geothermal aquifers are less prone to topographical effects and have much lower hydraulic gradients. The regional thermal water flow direction was from west to east in the southern part of the Upper Pannonian geothermal aquifers (Fig. 2c). Thermal water flowed from SW to NE in the south part of the Danube basin, and from NE to SW in its northern part, toward the Danube River in both cases. The heads were in the range from 220 to 110 m a.s.l. Major discharges occurred west of Lake Balaton and near Győr (HU), and south–east of Lake Neusiedl (AT). Thermal water production from the Upper Pannonian aquifers has caused transboundary drawdown cones of more than 0.5 m (in the model layer 6), and they extend as far as 60 km in the neighboring countries (Fig. 4a). Drawdowns vary from 1 to 6 m due to production in each individual state, but are between 2 and 10 m in total when the regional production is simulated (Fig. 4e). Locally, these values exceed 20 m at the production sites. The inspection of impacts of individual countries has shown that the Slovenian production has caused a maximum drawdown of approximately 6 m at the HU-SI border, and a negligible effect toward Austria (Fig. 4a). The production in south–western Hungary has resulted in a maximum drawdown of approximately 2 m at both, the HU-SI and AT-HU borders (Fig. 4b). The drawdowns are in accordance with previous flow models of the Mura-Zala basin [20]. The production in north-western Hungary has had a pronounced effect in Slovakia: a maximum drawdown of approximately 2.5 m is simulated at the border, and one just below 2 m at the AT-HU border (Fig. 4c). Thermal water production in Slovakia has had a minor effect on Austria, as the maximum drawdown was below 1.5 m, while it could cause a drawdown of approximately 3 m at the HU-SK border (Fig. 4d). In practice, all countries exploit aquifers simultaneously, and therefore the effects

Fig. 3. Characteristic transboundary hydrogeological cross-Sections 1–1' and 2–2' showing simulated directions of regional groundwater flows in pre-exploitation state (location in Fig. 1). Thicker lines stand for upper 100 m of the sedimentary sequence.

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Fig. 4. Simulated cross-border steady state groundwater drawdown in the Upper Pannonian geothermal aquifer (model layer 6) caused by thermal water production: (a) in Slovenia, (b) in southwest Hungary, (c) in northwest Hungary, (d) in Slovakia, (e) in all countries together, and (f) at five-fold increase in production in all countries. Black dots indicate locations of production wells.

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447

of total production have to be summed up (Fig. 4e). The most exploited and, consequently, affected transboundary areas are the Mura-Zala basin (HU-SI), with the maximum drawdown of approximately 10 m, and the Danube basin (HU-SK), with the approximate drawdown of 6 m. The drawdown rarely exceeds 2 m at other transboundary areas. The drawdowns are expected to be approximately 2 m higher if freshwater production is accounted for in the flow model [20], but it is within the measured regional drawdown of above 15 m in north–east Slovenia [60]. In Hungary, an average regional drawdown of 10 m was measured in intergranular aquifers [117], while values up to 90 m were recorded at production sites in the Great Hungarian Plain [61]. If thermal water production were to increase to five times the present rate, the drawdown effects would become even more pronounced. The maximum drawdown could increase to over 40 m at the HU-SI border, and to almost 30 m at the HU-SK border (Fig. 4f). The Austrian side would be affected in three different ways. The southernmost part would have a drawdown of 20 m, a drawdown of approximately 15 m would occur along the AT-HU border, and reduced discharge rates to the Lake Neusiedl area are also expected.

170 m3 day  1. The unconfined aquifers (zone 1) are forced to provide 4–12% more recharge to freshwater confined ones (zones 2, 4, 6, 8), or 15,920 m3 day  1 in total. The latter provide an additional 2–41% of induced recharge to other confined aquifers (zones 2, 4, 6, 8), or 13,060 m3 day  1 in total. The extremely large increase in percentage reflects the newly established flow paths. In geothermal aquifers (zones 3, 5, 7, 9) the discharge has been reduced by 3–86%, or 24,980 m3 day  1 in total. If the dramatic five-fold increase in production rate were achieved, the unconfined aquifers (zone 1) would provide an induced recharge of 50,100 m3 day  1 in total, which is 19–55% more than in the pre-exploitation state, and new flow connections would be established in Hungary and Slovakia (Table 4). Total discharge to shallow unconfined aquifers (zone 1) would decrease by 82,230 m3 day  1, from 31 to 100% in different zones. The present production has induced the recharge rate for only approximately 6%, while the previewed increase would cause an induced recharge of almost twice the natural rate. The discharge of thermal waters (zones 3, 5, 7, 9) would be reduced by 31–68%, or 37,500 m3 day  1 in total, with the exception of Austrian aquifers which would provide much additional recharge to Hungary.

4.2. Regional water budgets

4.3. Cross-border flows and budgets

Regarding the pre-exploitation water budget, the cold unconfined aquifers (zone 1 in Fig. 5; all subsequent zones are shown in Fig. 5 and listed in Tables 2–4) provided a recharge of 204,170 m3 day  1 to the confined freshwater aquifers (zones 2, 4, 6, 8; Table 2). The confined geothermal aquifers (zones 3, 5, 7, 9) received 86,910 m3 day  1 of groundwater and discharged 94,270 m3 day  1. The difference was attributed to the fact that not all layers of the flow model (the surrounding aquitards) were included in the reported budget calculation. Owing to present production, the discharge into the shallow unconfined aquifers (zone 1) has been decreased to 28,340 m3 day  1, from 9 to 33% in different zones. Discharge from geothermal aquifers (zones 3, 5, 7, 9) is the most restricted if looking at the percentages (Table 3), but the decrease of the total rate is only

4.3.1. Austria–Slovenia Freshwaters in confined aquifers (zones 6 and 8) flowed from Austria to Slovenia in the NW–SE direction at a flow rate of 450 m3 day  1 (Figs. 2 and 3, Table 2), but thermal waters did not have appreciable transboundary flow (zones 7 and 9). The reasons are in the Upper Pannonian geothermal aquifers that extend to Austria only in small patches, while the karstic basement aquifers are not of transboundary character in this area. The present production has increased the transboundary freshwater flow rate from Austria to Slovenia (zones 6 and 8) by 20%, but no change is evident in the opposite direction (Table 3). The total transboundary freshwater flow rate between the countries has increased by 40%, from 450 to 630 m3 day  1. An increased production would boost these effects (Table 4).

Fig. 5. Zones of the Upper Pannonian geothermal aquifers which were used in water budget calculation. Zone 1 extends on top of the whole study area and stands for cold unconfined aquifers. Others stand for following aquifers: zone 2 - HU cold confined, zone 3 - HU geothermal, zone 4 - SK cold confined, zone 5 - SK geothermal, zone 6 - SI cold confined, zone 7 - SI geothermal, zone 8 - AT cold confined, zone 9 - AT geothermal.

4.3.2. Austria–Hungary Shallow freshwaters in Hungary (zone 2) received much recharge from the Austrian mountains (zone 8) in the preexploitation state. The total transboundary flow rate was simulated to had been 15,750 m3 day  1, and a minor surplus of 300 m3 day  1 was evident as a freshwater recharge to the Hungarian geothermal aquifers (zones 3 and 8; Figs. 2 and 3, Table 2). The alignment of the latter is almost parallel to the AT-HU border and only the westernmost part of the sequence is located in Austria. Consequently, recharge occurred along the southern part of the state border in Austria, and the prevalent flow direction was toward (south-)east, and therefore the majority of regional flow had been evolved in Hungary. Regional discharge of both groundwaters occurred in the north part of this transboundary area, in the surroundings of Lake Neusiedl (AT). Here, the flow direction was reversed from Hungary to Austria, north–westward. The total thermal water budget between Austria and Hungary (zones 3 and 9) was positive for the first country with a flow rate of 730 m3 day  1. The same is valid for discharge of thermal waters into freshwater aquifers (zones 2, 3 and 8, 9), where Austria benefited from a surplus rate of 110 m3 day  1. The hydraulic gradient in geothermal aquifers was simulated to be approximately 1.9  10  3. Austria and Hungary do not share karstic basement aquifers. The total flow rate from the Hungarian (zone 2) to Austrian (zone 8) confined freshwater aquifers has increased by approximately 6% due to thermal water production, by 910 m3 day  1

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Table 2 Water budget of the transboundary Upper Pannonian aquifers in the pre-exploitation model. Names of the countries are abbreviated. Slash stands for not connected aquifers. Numbers in brackets denote zones in Fig. 5.

HU geothermal (3)

SK freshwater confined (4)

SK geothermal (5)

SI freshwater confined (6)

SI geothermal (7)

AT freshwater confined (8)

AT geothermal (9)

From

Quaternary freshwater unconfined (1)

HU freshwater confined (2)

[m3/d]

Quaternary freshwater unconfined (1)

To

101,410

0

63,250

0

15,330

/

24,180

/

30,470

3,410

60

9,170

1

6,570

90

170

2,560

550

2,720

170

2,690

16,960

/

/

150

/

/

/

0

/

11,690

460

/

/

/

HU freshwater confined (2)

144,660

HU geothermal (3)

330

36,700

SK freshwater confined (4)

73,620

610

100

SK geothermal (5)

180

130

8,560

20,510

SI freshwater confined (6)

14,540

9,800

440

/

/

SI geothermal (7)

/

0

6,650

/

/

8,360

AT freshwater confined (8)

10,840

22,320

390

390

2

910

/

AT geothermal (9)

/

60

1,960

/

/

/

/

(Table 3). The Austrian geothermal aquifers (zone 9) provide 28% more recharge to the Hungarian ones (zone 3), which sums to an additional 550 m3 day  1, and the flow rate in the opposite direction has reduced by 120 m3 day  1. The total transboundary flow in geothermal aquifers (zones 3 and 9) is still positive for Austria, but it dramatically declined to only 60 m3 day  1. The surplus ratio of discharge of thermal waters into freshwater aquifers is not changed and is still positive for Austria at a rate of 110 m3 day  1, but both quantities are much reduced. Here, the drawdown does not exceed 2 m in general (Fig. 4e). Even though the flow rate of thermal waters from Hungary (zone 3) to Austria (zone 9) in the model of increased production were slightly increased and exceeded the pre-exploitation rate by 90 m3 day  1, the flow in the opposite direction would become prevalent because it would be triple the original rate (Fig. 4f, Table 4). In this case, the transboundary water budget of thermal waters would sum to 3240 m3 day  1, now being positive for Hungary. A drawdown of approximately 15 m would be expected, and reduced discharge to the Lake Neusiedl area would occur. 4.3.3. Austria–Slovakia The AT–SK state border mainly follows the course of Morava and Danube Rivers (Figs. 1 and 2). Two Upper Pannonian sequences occur in this transboundary area, one in the Vienna basin and the other in the Danube basin, but only the latter was investigated as it is also being shared with Hungary and Slovenia. The direction of transboundary freshwater flow (zones 4 and 8) was along the border and toward south–east in the preexploitation state, with a total surplus flow rate of 240 m3 day  1

1,570 2,480

for Slovakia (Table 2). The transboundary thermal water flow between the countries was negligible. The production has increased the transboundary flows as the total freshwater flow rate (zones 4 and 8) has increased by 25%, being 300 m3 day  1, and is still positive for Slovakia (Table 3). This has also caused a new flow connection to occur: discharge of the Slovakian thermal water (zone 5) to the Austrian confined freshwater aquifers (zone 8) has been established at a rate of 6 m3 day  1. The effects will become more evident if the production rate is increased (Fig. 4f, Table 4). 4.3.4. Hungary–Slovenia Shallow freshwaters in unconfined transboundary aquifers (zone 1) discharged mainly in the Mura River (SI) which also gained some water from the Zala hills (HU) (Figs. 2 and 3). Deeper freshwaters (zones 2 and 6) flowed south-eastward along the SIHU border, with a transboundary flow rate surplus of 630 m3 day  1 for Hungary (Table 2). Thermal waters had a southeastward flow direction from Slovenia (zone 7) to Hungary (zone 3), and the transboundary flow was strongly positive for Hungary at a rate of 3930 m3 day  1. Its hydraulic gradient was simulated to be approximately 8.6  10  4. Much of this water discharged west of Lake Balaton, in the surroundings of Lake Hévíz (HU). An additional 440 m3 day  1 were gained there as a freshwater recharge from Slovenia (zone 6), but, at the same time, thermal water from Hungary (zone 3) had recharged the Slovenian freshwaters (zone 6) of a similar magnitude, at a rate of 550 m3 day  1. Slovenia and Hungary do not share karstic basement aquifers. At present production, the total transboundary flow rate in confined freshwater aquifers (zones 2 and 6) is 580 m3 day  1. It is

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449

Table 3 Change in the water budget of the transboundary Upper Pannonian aquifers in the flow model of current production. Names of the countries are abbreviated. Applied production rates are listed in the last column. Slash stands for not connected aquifers. The value of 4 4100% stands for a change in flow rates which were 0 m3/day in the pre-exploitation model. Numbers in brackets denote zones in Fig. 5.

HU geothermal (3)

SK freshwater confined (4)

SK geothermal (5)

SI freshwater confined (6)

SI geothermal (7)

AT freshwater confined (8)

AT geothermal (9)

Geothermal wells [m3/d]

From

Quaternary freshwater unconfined (1)

HU freshwater confined (2)

[%]

Quaternary freshwater unconfined (1)

To

9

0

4

0

12

/

9

/

0

18

2

17

-1

100

-2

0

495

-12

30

-31

-3

-12

-4

9,981

20

/

/

40

/

237

/

/

>>100

/

7,488

24

0

/

88

/

/

5,038

18

0

HU freshwater confined (2)

-12

HU geothermal (3)

-33

-26

SK freshwater confined (4)

-9

-5

10

SK geothermal (5)

-33

-23

-86

-26

SI freshwater confined (6)

-18

-1

-11

/

/

SI geothermal (7)

/

0

-15

/

/

-22

AT freshwater confined (8)

-19

3

41

31

-100

20

/

AT geothermal (9)

/

-33

28

/

/

/

/

still positive for Hungary, but it has been reduced by 8% (Table 3). The decreased recharge of the Hungarian geothermal aquifers (zone 3) with Slovenian freshwaters (zone 6) sums to 390 m3 day  1, and the same trend is valid for the discharge from the first aquifers to the second ones, as a decrease of 31% has been simulated. The total flow rate between the two geothermal aquifers (zones 3 and 7) is reduced by 24%, being 2980 m3 day  1 in total, but the flow direction has not changed. The maximum simulated drawdown at the state border is approximately 10 m (Fig. 4e). The total transboundary freshwater flow from Slovenia to Hungary (zones 6 and 2) would change its direction to the first country if the production were to dramatically increase, to estimated 170 m3 day  1 in the future (Table 4). The induced freshwater recharge from Slovenia (zone 6) to the Hungarian geothermal aquifers (zone 3) would provide almost as much water as in the pre-exploitation state, but the thermal water flow direction would be reversed and it would provide a surplus rate of 1560 m3 day  1 for Slovenia. A drawdown of 40 m would be expected in this area (Fig. 4f). A comparison with previous numerical flow models [20,67] indicates that the simulated transboundary flow rates are of similar magnitude. The drawdowns are probably a few meters larger in reality due to hydraulic interconnection of fresh- and thermal water aquifers [20]. 4.3.5. Hungary–Slovakia The state border between Hungary and Slovakia follows the Danube River, and, consequently, the freshwater flow in the unconfined aquifers (zone 1) is directed along the border, to southeast (Figs. 2 and 3). The freshwater budget of the Upper Pannonian

-32

0

geothermal aquifers in the pre-exploitation state was positive for Slovakia (zones 2 and 4) at a flow rate of 2800 m3 day  1, but some freshwaters from Slovakia (zone 4) recharged the Hungarian geothermal aquifers (zone 3) at a flow rate of 40 m3 day  1 (Table 2). The geothermal aquifers had the lowest simulated hydraulic gradient in the region, of approximately 1.5  10  4. The total transboundary flow rate (zones 3 and 5) was found very positive for Hungary, and it summed up to 6000 m3 day  1 of thermal water. On the opposite side, thermal waters from Hungary (zone 3) discharged into the Slovakian freshwater aquifers (zone 4) at a much lower rate of 40 m3 day  1. The production of thermal water has resulted in an induced recharge of fresh- and thermal waters from Hungary to Slovakia, as the freshwater budget of confined aquifers (zones 2 and 4) has had an additional surplus of 110 m3 day  1 (Table 3). The total transboundary flow rates between freshwater and geothermal aquifers did not change (zones 2, 4, 3, 5), still being 40 m3 day  1, but the magnitude increased. The thermal water budget (zones 3 and 5) changed dramatically as the flow rate of thermal waters from Slovakia to Hungary declined by 86%, being only 1180 m3 day  1, and it increased in the opposite direction by 30%, being 3320 m3 day  1. This resulted in a reversal of the transboundary flow direction towards Slovakia, with a new total flow rate of 2140 m3 day  1 for this country. The maximum simulated drawdown is approximately 6 m (Fig. 4e). The reversal of thermal water flow direction from Hungary to Slovakia would become even more pronounced with a dramatic increase in production. The transboundary flow rate would double (Table 4), and the drawdown would increase to almost 30 m (Fig. 4f).

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Table 4 Change in the water budget of the transboundary Upper Pannonian aquifers in the flow model of five-times increased production. Names of the countries are abbreviated. Applied production rates are listed in the last column. Slash stands for not connected aquifers. The value of 4 4100% stands for a change in flow rates which were 0 m3/day in the pre-exploitation model. Numbers in brackets denote zones in Fig. 5.

SK freshwater confined (4)

SK geothermal (5)

SI freshwater confined (6)

SI geothermal (7)

AT freshwater confined (8)

AT geothermal (9)

23

>>100

19

>>100

55

/

27

/

0

115

17

67

3

300

-3

11

2,473

-35

223

-80

94

-18

3

49,903

123

/

/

193

/

1,187

/

/

=

/

37,441

152

9

/

438

/

/

25,192

135

0

HU freshwater confined (2)

-31

HU geothermal (3)

-98

-60

SK freshwater confined (4)

-33

-3

90

SK geothermal (5)

-100

-85

-57

-72

SI freshwater confined (6)

-58

-6

-7

/

/

SI geothermal (7)

/

0

-44

/

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AT freshwater confined (8)

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16

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4.4. Model limitations and future studies The presented models have several limitations connected to the applied space and time scales. They emerge partly from a disability to implement a huge amount of spatially variable boundary conditions to the flow model and partly from a lack of sufficient and/ or high quality data on production and monitoring. This was partly caused by various restrictive policies regarding the data use, and by the non-existence of monitoring networks. One of the major disadvantages was also the non-inclusion of the Croatian territory into the simulation even though the Upper Pannonian geothermal aquifers are developed to some extent there. The Međimurje hills (CRO) control transboundary groundwater flows between Croatia, Hungary and Slovenia, even though the rates are not expected to be large. Further geothermal development is foreseen also in this part of Croatia, increasing the number of active geothermal wells from one to several [48], and therefore all future investigation and management of these transboundary geothermal aquifers will have to be conducted with the Croatian participation. The pre-exploitation flow model provided the first quantitative estimation of the regional and transboundary groundwater flows, and it confirmed the importance of gravitational forces on their evolution. The steady state production models could not account for transient effects caused by implementation of new geothermal sites or closure of old ones, nor for seasonal production of thermal water which is very characteristic for the applied direct use schemes [60]. Since freshwater production was not accounted for, the interference between the two aquifer systems could not be investigated and will have to be performed at a further stage of

-78

Geothermal wells [m3/d]

HU geothermal (3)

From

Quaternary freshwater unconfined (1)

HU freshwater confined (2)

[%]

Quaternary freshwater unconfined (1)

To

0

research. However, the accomplished evaluation of the averaged effects of production seems to be reasonable. Modeling and monitoring can be jointly used for status assessment [5], and if transboundary monitoring is implemented, the models will be able to simulate the observed transient state. Additional calibration should also be performed by the use of transport simulation [3,102]. It is essential that production from thermal karst aquifers is implemented in the improved model, and that the latter is set up in a code which also supports simulation of heat transfer to account for free convection processes.

5. Recommendations for strategic transboundary groundwater management Delineation of groundwater bodies can be subjective and expensive in practice, but the use of hydrogeological numerical models can help to mitigate this issue [23]. Many methodologies agree in the opinion that several steps are needed to assess the transboundary groundwater resources [summarized in 5,33,118]: delineation and description of the transboundary groundwater body; classification, diagnostic analysis and zoning, and data harmonization and information management. The present research highlights that these very important steps have already been taken by the four neighboring countries in the western part of the Pannonian basin. In 2011, there were efforts to introduce a transboundary thermal groundwater body (TTGWB) Mura-Zala, delineated at an extent of 4974 km2 between Hungary and Slovenia, to the Permanent Hungarian–Slovenian Water

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Management Committee [20]. But a much wider research, as presented in this paper, was needed to provide solid scientific grounds to delineate the Upper Pannonian transboundary thermal groundwater body shared by Hungary, Slovakia, Slovenia and Austria. Its extent is assessed to be approximately 22,128 km2 based on GIS layers of a three-dimensional geological model [76]. The first steps of delineation and characterization were taken in the frame of the Transboundary Waters Assessment program in 2014, when Slovenia, Slovakia and Hungary provided their unilateral characterizations of the Upper Pannonian geothermal aquifer according to the pre-set template designed by the International Groundwater Center. Further on, joint investigation and management need to be extended to Croatia, where a minor part of the same geothermal aquifers is also exploited (see Section 4.4). 5.1. Sustainability of thermal water production Many approaches address the sustainability of water withdrawal following the basic principle that the present development should not endanger the future possibilities [26]. Natural capabilities of hydrogeothermal systems should be determined by using different methods to evaluate the quantity status of aquifers according to the WFD. The “sustainable yield” approach [27] enables a long-term dynamic equilibrium to be maintained between the natural and induced recharge rates and (decreased) discharge. The minimum impacts on environmental and social benefits should be aimed at [31], and these are extremely important also in the investigated transboundary area. This region provides economic and social benefits at 149 user sites, and several ecological areas with unique hydrogeochemical and temperature characteristics of natural discharges [58,92,105]. The simulation results show that the regional quantity state of geothermal aquifers is not expected to deteriorate appreciably if thermal water production does not considerably increase. Indeed, the latter is not expected in the view of current economic situation, despite very ambitious national energy policy goals. It is recommended that good management practice in the transboundary area between Hungary and Slovenia is transferred to the entire region [20]. The regional groundwater drawdown method was developed for evaluating the quantity state of geothermal aquifers in the Pannonian basin with a threshold value of the regional drawdown set to be 30 m below the pre-exploitation groundwater levels in the representative observation wells. According to the performed simulation, the present regional drawdown has reached approximately half of the critical value, and therefore the current regional exploitation of the Upper Pannonian geothermal aquifers cannot be classified as unsustainable. Nevertheless, the simulated transboundary flow reversals, continuously decreasing groundwater levels in north–east Slovenia [60], hydrogeochemical changes in west Hungary [46], and temperature changes near Lake Hévíz (HU) [Tóth, G., 2014, oral communication], do indicate locally critical areas where unsustainable exploitation is occurring. There, the local status assessment should be further investigated in more detail. The same method recommends the critical point of regional production rate to be set at 3.5 times the present abstraction rate (70% of the maximum tested one in the flow model). However, the future geothermal development should not focus on increasing the production rates, but on two other things: on increasing the energy efficiency to reduce the need for thermal water, and on obligatory use of geothermal doublets when geothermal heat is being used. This is an example of good and sustainable management practice, and reinjection in sandstone aquifers is not only possible but also needed to conserve the regional groundwater balance world-wide [22,60,119,120]. Such practice is already implemented at Lendava (SI) and at Podhájska (SK, where karst

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aquifer is predominant) [43], but it should be more strongly supported by the management authorities in the four countries. 5.2. Benchmarking of geothermal management Several additional points are proposed to be checked when granting or controlling issued water permits and concessions on thermal water use in transboundary areas [62,121]. These vary from rather detailed information on energetic use to quite technical points of view, e.g. improved thermal efficiency and using the best available technology. When addressing the management of regional and transboundary groundwater resources, the so-called “benchmarking indicators” can be very useful. Evaluation of aquifers’ status based on information from the monitoring systems, interpretation of the regional water balance, and risk assessment of over-exploitation collectively provide a methodology by which the exchange of information on the sustainability of exploitation of hydrogeothermal resources between the sharing countries can be made transparent, fast and effective. The methodology was tested in the investigated area [121] indicating that the management differs widely between the four countries. Consequently, the implementation of a joint management approach is again justified. 5.3. Regular exchange of information There is no point in conducting a one-time research only of such vast and dynamic regional flow systems as presented in this paper. Additional steps should follow soon to supplement these results with reliable time-series data on hydrogeological parameters, and to allow for more detailed evaluation of the quantity and quality state of areas where significant transboundary flows have been identified. The following approach is recommended. First, transboundary areas within a distance of 20 km from the state border should be delineated in the neighboring countries and all activities which might affect the state of transboundary geothermal aquifers should have to be reported in advance, e.g. to the bi- or multilateral water management commissions. Examples of such interventions are: intended increase in production, start of drilling activities, activation of preserved wells, and opening new perforated sections in wells. Secondly, joint monitoring networks of representative observation wells have to be set up in these areas, with measurements being freely available in real time. The costs of their establishment can be reasonable when using existing wells (or springs, where relevant) from the national or other monitoring networks. Thirdly, the monitoring information should be interpreted and exchanged between the countries regularly, if possible annually. These activities should also have a strong focus on presenting an informative summary of findings to the interested public. Finally, the regional flow model should be updated with new monitoring data at least every six years, and according to the WFD procedures [34]. This should also be the frequency of re-evaluation and adjustments of the water rights granted in transboundary areas, because fixed-quantity water agreements have been found to be non-optimal [32]. Limited water rights should be compensated [122], and several benchmarking indicators should be tested when informing thermal water users with the largest impacts on the state of the aquifers [121].

6. Conclusions The numerical flow model of the transboundary Upper Pannonian geothermal aquifers, which extend over 22,128 km2 in the west Pannonian basin, provides a unique tool for quantification of cross-border flow rates between various aquifers. Geothermal

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aquifers in sedimentary basins and basement rocks were jointly simulated to properly account for the main physical processes which are caused by two mainly separate regional flow systems, and which result in unique hydrogeological and ecological features at their mixing points. Three cross-border areas with significant transboundary flow rates in a pre-exploitation state were identified for the first time, while one [20] was re-assessed. At the Austrian–Hungarian border, the freshwater balance in confined aquifers was found positive for Hungary, while the thermal water balance was positive for Austria, especially due to the cross-border flow in the northern part. Both, the freshwater and thermal water, flows at the Hungarian–Slovenian border showed positive water balance for Hungary. The highest cross-border flow rates were simulated across the Hungarian–Slovakian border, also due to its length. The freshwater balance was positive for Slovakia, while the thermal water balance for Hungary. Regional production of thermal waters causes noticeable transboundary effects, and depression cones may have penetrated several tens of kilometers into the neighboring countries. Simulated drawdowns at the state borders are in the range of 2–10 m, depending on local geological settings, density of geothermal wells, and their production rates. The adopted methodology implies that the average regional drawdown in geothermal aquifers is not yet critical and that the joint exploitation cannot be regarded as unsustainable. It was even estimated that a 3.5-fold increase in thermal water production can be applied in this transboundary region. Despite these findings, there are strong indications that the quantity status of geothermal aquifers is deteriorating at least locally. The present production has reversed the regional flow direction of thermal waters at the Hungarian–Slovakian border, which now flow towards Slovakia. The transboundary flow of thermal water between Austria and Hungary has been dramatically reduced, but the flow direction remained the same. In a case of increased production, a flow reversal could occur and a drawdown of approximately 15 m is expected. The fresh and thermal waters still flow from Slovenia to Hungary, but at a reduced rate. Their flow direction could be reversed following a five-fold increase in production, when a drawdown of 40 m could also be expected. The presented description of the transboundary Upper Pannonian thermal groundwater body provides an extremely important scientific support for the establishment of a joint management structure of transboundary hydrogeothermal resources in the wider Pannonian basin, and can serve as an example on a European scale and world-wide. A wider legal, political and financial consensus is now needed to implement the suggested transboundary management in practice. The future activities should focus on two issues, on enhanced research cooperation with Croatia to overcome the presently missing geological and production information, and on larger efforts towards integration of higher-level management authorities to actually start the implementation of the attained results in practice. Even if transboundary issues do not occur within large countries, such as China, France or USA, the approach is applicable there at national or regional levels, where many stakeholders are exploring, exploiting, and trying to manage joint resources. All these activities should be multinational, systematic, transparent, and what is the most important, regularly summarized in a clear illustrative way to all stakeholders in order for them to understand why harmonized management is needed to exploit joint natural resources sustainably for many years to come.

Acknowledgments Datasets and reports are available at http://transenergy-eu. geologie.ac.at/. Research within the TRANSENERGY project was supported by the EU CE Program 2007-2013, Contract no. 2CE124P3, and the SI ARRS Program P1-0020 Groundwaters and Geochemistry. Preparation of the article was supported by the HSB grant of the Balassi Institute to N. Rman. The research was also supported by the SI MIZŠ and the ESF OP Human Resources Development Program 2007–2013, PA 1, M 1.1, Contract no. 333014-509001. Authors also thank to S. Mozetič for preparation of graphical material and Prof. S. Pirc for constructive comments and language improvement.

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