Numerical investigation of the groundwater age and heat transport processes in asymmetric hydrogeological situations

Numerical simulations focusing on groundwater age have not yet been carried out in the Buda Thermal Karst system (BTK) (Hungary), although isotopic and chemical data from thermal springs [e.g. Fórizs et al., 2019] are available to compare calculated and measured results. The main objective of this study was to improve understanding of regional-scale heat transport processes and groundwater flow associated with ageing in complex hydrogeological system, such as the BTK including deep carbonate sequences and adjoining sedimentary basins (“DC&SB”).

A comprehensive sensitivity analysis was completed to validate the numerical method [Zimmermann, 2006] implemented with ‘age mass’ concept [Good, 1996], and to reveal the influence of crucial hydrogeological parameters on the groundwater age distribution in 2D and 3D synthetic models. It was established that the average (τ_av) and the maximum (τ_max) groundwater mean age correlate with the permeability anisotropy, heterogeneity (exponential permeability decrease with depth), the model depth, while the values anti-correlate with the amplitude of water table and the permeability.

For general investigation of the “DC&SB” type groundwater flow systems, two “half-basins” [Wang et al., 2017] were combined into a single asymmetric basin. The “DC&SB” model was characterised by (i) a water table configuration with higher amplitude and a homogeneous (unconfined) aquifer on the left-hand side; (ii) a lower water table amplitude and a three-layered (unconfined aquifer – aquitard – confined aquifer) domain on the right-hand side. As a result of the forced convection, decreased temperatures and reduced groundwater mean ages are noted in unconfined parts of the model, while heat accumulation and increased ages were calculated in the aquitard and the confined aquifer. Using experiences from synthetic tests, the groundwater age calculation was integrated into the preliminary 3D hydrogeological model of the BTK system. The results showed that the measured and the calculated mean groundwater age values sampled the different parts of the hierarchically nested flow system are of the same order of magnitude.

The results are very important to uncover the groundwater flow in complex hydrogeological systems which is unavoidable both in regional-scale (e.g. drinking water management, geothermal exploration and geothermal energy utilisation) and local-scale explorations (e.g. managed aquifer recharge, environmental remediation). The research was supported by the National Research, Development and Innovation Office in the framework of project No. PD 142660; and by the National Multidisciplinary Laboratory for Climate Change, RRF-2.3.1-21-2022-00014 project.

References

Fórizs, I., Szabó, V.R., Deák, J., Halas, S., Pelc, A., Trembaczowski, A., Lorberer, Á. (2019). The Origin of Dissolved Sulphate in the Thermal Waters of Budapest Inferred from Stable S and O Isotopes. Geosciences 9(10), 433, p. 13.

Good, D.J. (1996). Direct simulation of groundwater age. Water Resources Research 32, pp. 289-296.

Wang, J.Z., Jiang, X,W., Zhang, Z.Y., Wan, L., Wang, X.S., Li., H. (2017). An analytical study on three-dimensional versus two-dimensional water table-induced flow patterns in a Tóthian basin. Hydrological Processes 31, pp. 4006-4018.

Zimmermann, W.B.J. (2006). Multiphysics modeling with finite element methods. Singapore: World Scientific Publishing Company, p. 422.