Structural constraints of groundwater flow and heat transport in the Lake Balaton region

Abstract

Naturally discharging springs and water wells have been providing high-quality water as a local drinking water resource in the Bakony Mts.-Balaton Highland area (Central Hungary, Europe) for centuries. On the other hand, the Balaton Highland-Lake Balaton region is a popular tourist destination with an outstanding ecological value of the Lake Balaton and the surrounding wetlands. For the proper and sustainable water management, we need to reveal the groundwater flow systems of the Bakony Mts.-Balaton Highland-Lake Balaton region. It means the understanding of hydraulic connection between the subareas of it, quantity and areal distribution of groundwater recharge, subsurface flow paths through the mainly carbonate formations and also the exploitable amount of groundwater which, meets the human and ecological needs. Therefore, the main aim of the study was to disclose the natural hydrogeological processes in the Lake Balaton region, applying the modern theory of basin hydraulics, considering the special behaviour of carbonate systems and the structural constraints. Hydraulic and hydrochemical connection among Bakony Mts., Balaton Highland, Lake Balaton and Somogy Hills were examined. Water management-related issues were also discussed regarding drinking water resources, sustainable water use and geothermal potential. The study area consists of ~2-3 km thick carbonate formations with hydraulic conductivity of 10-6-10-5 m/s. Two main tectonic events have played important role in determining the hydrogeological conditions: the first has resulted syncline structures before the Late Cretaceous and the second one has produced ~200 km horizontal displacement along strike slip faults during the Miocene. The hydrostratigraphy and the basin geometry modify the flow pattern. The folded basement aquitard restricted the groundwater flow at the boundary of the Bakony Mts. (~5 km deep) and Balaton Highland (0.5-1-km-deep basin) and it caused intensified flow toward the area of the Lake Balaton. The low-permeability thrust-fault caused a hydraulic head drop of 10-30 m at the footwall. South of the Lake Balaton, the carbonates are covered by a <2-km-thick siliciclastic cover. The intensity of groundwater flow is low in this confining layer, and groundwater is directed toward the Lake Balaton because of the water table difference. This could result in a subsurface convergence zone of groundwater flowing from the Northern and Southern basin. Subsurface temperature field reflected the advective heat transport caused by the groundwater flow. The recharging cold water could infiltrate and move down to –3 000 m asl under the Bakony Mts.,
thermal water (>30 °C) could be found in deeper parts of the basin, except the two near-surface heat accumulations under the boundary of Bakony Mts. and Balaton Highland and under the basin of Lake Balaton. Slightly elevated water temperature (20–23°C) can be found in the region of the Lake Balaton.
The regional-scale numerical simulation and hydraulic data evaluation could disclose flow
components from the North and South; revealed an asymmetric flow pattern caused by different topographic settings in the Northern and Southern basins; the hydraulic regimes in the broader vicinity of the Lake Balaton; and the groundwater discharge through the lakebed of the Lake Balaton. The constraints of the groundwater flow and heat transport are provided by the distinctly different hydrostratigraphic configuration, basin depth and topographic settings determined by structural evolutionary events.
This research is part of a project that has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 810980.

Full text

Hévíz, Hungary, 15-19 October, 2019 page 184 of 197 International Lithosphere Program
Structural constraints of groundwater flow and heat transport in the Lake Balaton region
Ádám Tóth1, Judit Mádl-Szőnyi1
1 József & Erzsébet Tóth Endowed Hydrogeology Chair, Department of Geology,
ELTE Eötvös Loránd University
Corresponding author: adam.toth@geology.elte.hu
Naturally discharging springs and water wells have been providing high-quality water as a local
drinking water resource in the Bakony Mts. – Balaton Highland area (Central Hungary, Europe) for
centuries. On the other hand, the Balaton Highland – Lake Balaton region is a popular tourist
destination with an outstanding ecological value of the Lake Balaton and the surrounding
wetlands. For the proper and sustainable water management, we need to reveal the groundwater
flow systems of the Bakony Mts. – Balaton Highland – Lake Balaton region. It means the
understanding of hydraulic connection between the subareas of it, quantity and areal distribution
of groundwater recharge, subsurface flow paths through the mainly carbonate formations and also
the exploitable amount of groundwater which, meets the human and ecological needs.
Therefore, the main aim of the study was to disclose the natural hydrogeological processes in
the Lake Balaton region, applying the modern theory of basin hydraulics, considering the special
behaviour of carbonate systems and the structural constraints. Hydraulic and hydrochemical
connection among Bakony Mts., Balaton Highland, Lake Balaton and Somogy Hills were examined.
Water management-related issues were also discussed regarding drinking water resources,
sustainable water use and geothermal potential.
The study area consists of ~2-3 km thick carbonate formations with hydraulic conductivity of 10-
6–10-5 m/s. Two main tectonic events have played important role in determining the
hydrogeological conditions: the first has resulted syncline structures before the Late Cretaceous
and the second one has produced ~200 km horizontal displacement along strike slip faults during
the Miocene.
The hydrostratigraphy and the basin geometry modify the flow pattern. The folded basement
aquitard restricted the groundwater flow at the boundary of the Bakony Mts. (~5 km deep) and
Balaton Highland (0.5–1-km-deep basin) and it caused intensified flow toward the area of the Lake
Balaton. The low-permeability thrust-fault caused a hydraulic head drop of 10–30 m at the
footwall. South of the Lake Balaton, the carbonates are covered by a <2-km-thick siliciclastic cover.
The intensity of groundwater flow is low in this confining layer, and groundwater is directed toward
the Lake Balaton because of the water table difference. This could result in a subsurface

Hévíz, Hungary, 15-19 October, 2019 page 185 of 197 International Lithosphere Program
convergence zone of groundwater flowing from the Northern and Southern basin. Subsurface
temperature field reflected the advective heat transport caused by the groundwater flow. The
recharging cold water could infiltrate and move down to –3 000 m asl under the Bakony Mts.,
thermal water (>30 °C) could be found in deeper parts of the basin, except the two near-surface
heat accumulations under the boundary of Bakony Mts. and Balaton Highland and under the basin
of Lake Balaton. Slightly elevated water temperature (20–23°C) can be found in the region of the
Lake Balaton.
The regional-scale numerical simulation and hydraulic data evaluation could disclose flow
components from the North and South; revealed an asymmetric flow pattern caused by different
topographic settings in the Northern and Southern basins; the hydraulic regimes in the broader
vicinity of the Lake Balaton; and the groundwater discharge through the lakebed of the Lake
Balaton. The constraints of the groundwater flow and heat transport are provided by the distinctly
different hydrostratigraphic configuration, basin depth and topographic settings determined by
structural evolutionary events.
This research is part of a project that has received funding from the European Union’s Horizon
2020 research and innovation program under grant agreement No 810980.