Regional groundwater flow in hard rocks

Area of Science:

• Hydrogeology
• Structural Geology
• Geomorphology

Background:

• Continental Portugal's geology is shaped by the Hercynian orogeny, creating NW-SE ductile shear zones and NNE-SSW fragile faults.
• These tectonic structures are known conduits for mineral and thermal water circuits, with over a hundred documented spring locations.
• The integration of shallower, non-mineral groundwater into regional flow systems via these structures remains underexplored.

Purpose of the Study:

• To investigate the role of Hercynian tectonic structures in channeling shallow, non-mineral groundwater.
• To determine if these structures are integral components of a large-scale regional groundwater flow system.
• To classify watersheds based on groundwater dynamics and their relationship to tectonic features.

Main Methods:

• Application of an integrated water balance and recession flow modeling approach.
• Calculation of catchment turnover times using groundwater discharge rates and recession flow parameters.
• Classification of 46 watersheds into open and closed basins, and identification of source and sink basins within open systems.
• Utilized Geographic Information System (GIS) to analyze spatial associations between basin types and tectonic structures.

Main Results:

• Watershed classification revealed distinct open and closed basin types based on calculated turnover times and catchment area.
• A significant spatial correlation was established between open basins and the underlying Hercynian ductile and fragile tectonic structures.
• Open basins were predominantly identified as discharge cells within a regional flow system, particularly in the Douro River basin's mouth area.

Conclusions:

• Hercynian tectonic structures in Portugal actively influence and channel shallow groundwater flow, integrating it into regional systems.
• Open basins, spatially associated with these structures, function as critical discharge zones within the studied hydrogeological framework.
• Future research should extend to the Douro River basin's headwaters to identify the source basins feeding these discharge cells.