The influence of deep groundwater flow systems on the Earth’s critical zone

The depth of the Earth’s critical zone can be questionable especially in thousands meter deep sedimentary basins. Therefore, extension of the critical zone’s usually studied 10s of meters depth considering groundwater flow systems has critical importance. Growing demand for groundwater resources (water, geothermal energy), economic services of the groundwater flow related surface and subsurface processes and phenomena (e.g., groundwater dependent ecosystems, surface salinization), and the potential role of groundwater in the adaptation to and mitigation of the effects of human activities and climate change represent the significance and functions of groundwater flow systems in the critical zone.

Regarding the complexity of these flow systems, the primary goal could be the determination of their relative significance in the shallower parts of the critical zone. To this end, the present study proposes a methodology based on the hydrodynamic analysis of measured data to separate flow systems with different driving forces (topography, vertical compaction) and pore pressure regimes (normal or  close to hydrostatic, overpressured, underpressured). These characteristics define the renewability of groundwater resources, the near-surface conditions (e.g., distribution of nutrients, salts and heat, type of vegetation and soils, slope stability, etc.), and the exposure of flow systems to the effects of global and climate change.

As a case study, groundwater flow systems of the Great Hungarian Plain (Pannonian Basin, Hungary) were evaluated and characterized by analyzing about 5,800 measured hydraulic data (pre-production static water levels and static formation pressures) in hydraulic head vs. elevation and pressure vs. elevation profiles, tomographic maps, and hydraulic cross sections in combination with the geologic build-up and some surface phenomena (distribution of saline soils and vegetation). As a result, spatial extension and distinct functions in the critical zone were defined for three flow regimes, namely i) the near-surface topography-driven groundwater flow systems, ii) an underlying overpressured regime, and iii) the transition zone of i) and ii). For instance, outstanding significance of the upward flows of saline water from the transition zone was revealed in the generation of saline soils and vegetation.

The research was funded by the National Multidisciplinary Laboratory for Climate Change, RRF-2.3.1-21-2022-00014 project.