This study aims to improve the hydrogeological understanding of fractured aquifer systems in granitic environments, focusing on the Gardunha Mountain region in central Portugal. It addresses the limited knowledge regarding the interaction between precipitation and groundwater in crystalline terrains, particularly under the influence of climate variability. Three hydrogeological units were identified: (i) sloping deposits with high interstitial permeability; (ii) granitoids and hornfels with predominantly fissural permeability, locally enhanced in decomposed or fractured zones; and (iii) schistose rocks with very low fissural permeability. A comprehensive dataset was collected, including physical, chemical, and isotopic analyses of groundwater and precipitation samples. The groundwater is classified as fresh, soft, and slightly acidic (pH range from 5.34 to 6.66), with low mineralisation (Total Dissolved Solids (TDS) averaging 37.8 mg·L⁻¹). Chemically, it is of the sodium bicarbonate type, with a significant component in silica (SiO₂ representing 40% of TDS), and low concentrations of trace elements. Isotopic data (δ²H, δ¹⁸O) confirm a meteoric origin, with no evidence of high-temperature water–rock interaction. Tritium (³H) levels indicate a short residence time (< 5 years), consistent with recent recharge. Hydrological response to rainfall is limited, with no immediate increase in discharge following precipitation events. However, prolonged dry periods result in sustained declines in spring flow, suggesting that recharge occurs primarily through deep fracture systems rather than direct infiltration. Although this study does not include future climate change projections, its findings offer valuable insights into recharge mechanisms in fractured granitic aquifers and provide a solid scientific foundation for assessing their resilience to precipitation variability. The groundwater quality meets standards for multiple uses, including human consumption, agriculture, and the bottled water industry. Graphical Abstract Based on the graphical snapshot, this study investigates the hydrogeological behaviour of fractured granitic aquifers in central Portugal, using the Gardunha Mountain system as a case study. The research integrates physical, chemical, and isotopic analyses of groundwater and precipitation to assess recharge dynamics and potential climate change impacts. Groundwater in the region is characterized as hyposaline and siliceous, with a low average Total Dissolved Solids (TDS) concentration of 37.8 mg·L⁻¹. The dominant water type is Bicarbonate-Sodic, with a significant presence of non-ionised silica (SiO₂), accounting for 40% of TDS. The water is soft (hardness < 13 mg·L⁻¹) and slightly acidic, with pH values ranging from 5.34 to 6.66. Isotopic signatures confirm a meteoric origin, with no evidence of deep water–rock interaction. Tritium (³H) levels in groundwater closely match those in local precipitation (mean = 2.54 ± 0.20 TU), indicating a short residence time (< 5 years). Despite seasonal rainfall, groundwater discharge shows limited short-term response to individual precipitation events, suggesting a diffuse recharge mechanism through the fractured granite matrix. These findings enhance the understanding of fractured crystalline aquifers and provide a baseline for evaluating their vulnerability to climate variability

Groundwater Behaviour in a Granitic Mountain Aquifer of Central Portugal: A Study of Climate–Aquifer Interactions

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发表日期：

2025年10月6日

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Eric Mendes,
Luis Manuel Ferreira-Gomes

摘要

This study aims to improve the hydrogeological understanding of fractured aquifer systems in granitic environments, focusing on the Gardunha Mountain region in central Portugal. It addresses the limited knowledge regarding the interaction between precipitation and groundwater in crystalline terrains, particularly under the influence of climate variability. Three hydrogeological units were identified: (i) sloping deposits with high interstitial permeability; (ii) granitoids and hornfels with predominantly fissural permeability, locally enhanced in decomposed or fractured zones; and (iii) schistose rocks with very low fissural permeability. A comprehensive dataset was collected, including physical, chemical, and isotopic analyses of groundwater and precipitation samples. The groundwater is classified as fresh, soft, and slightly acidic (pH range from 5.34 to 6.66), with low mineralisation (Total Dissolved Solids (TDS) averaging 37.8 mg·L⁻¹). Chemically, it is of the sodium bicarbonate type, with a significant component in silica (SiO₂ representing 40% of TDS), and low concentrations of trace elements. Isotopic data (δ²H, δ¹⁸O) confirm a meteoric origin, with no evidence of high-temperature water–rock interaction. Tritium (³H) levels indicate a short residence time (< 5 years), consistent with recent recharge. Hydrological response to rainfall is limited, with no immediate increase in discharge following precipitation events. However, prolonged dry periods result in sustained declines in spring flow, suggesting that recharge occurs primarily through deep fracture systems rather than direct infiltration. Although this study does not include future climate change projections, its findings offer valuable insights into recharge mechanisms in fractured granitic aquifers and provide a solid scientific foundation for assessing their resilience to precipitation variability. The groundwater quality meets standards for multiple uses, including human consumption, agriculture, and the bottled water industry.

Graphical Abstract

Based on the graphical snapshot, this study investigates the hydrogeological behaviour of fractured granitic aquifers in central Portugal, using the Gardunha Mountain system as a case study. The research integrates physical, chemical, and isotopic analyses of groundwater and precipitation to assess recharge dynamics and potential climate change impacts.

Groundwater in the region is characterized as hyposaline and siliceous, with a low average Total Dissolved Solids (TDS) concentration of 37.8 mg·L⁻¹. The dominant water type is Bicarbonate-Sodic, with a significant presence of non-ionised silica (SiO₂), accounting for 40% of TDS. The water is soft (hardness < 13 mg·L⁻¹) and slightly acidic, with pH values ranging from 5.34 to 6.66.

Isotopic signatures confirm a meteoric origin, with no evidence of deep water–rock interaction. Tritium (³H) levels in groundwater closely match those in local precipitation (mean = 2.54 ± 0.20 TU), indicating a short residence time (< 5 years).

Despite seasonal rainfall, groundwater discharge shows limited short-term response to individual precipitation events, suggesting a diffuse recharge mechanism through the fractured granite matrix.

These findings enhance the understanding of fractured crystalline aquifers and provide a baseline for evaluating their vulnerability to climate variability