Five decades of groundwater change across a diverse Mediterranean climate region: Disentangling natural and human drivers of water quantity and quality

Groundwater systems in Mediterranean regions are under increasing pressure from climate variability and human activities, yet long-term, large-scale assessments of water quality dynamics remain scarce. This study compiles and analyses over 4.4 million observations collected from 2,501 monitoring stations across Portugal, covering an area of 89,015 km² over a 3.5-decade period (1970-2024), and encompassing 15 national and international river basins as well as four major hydrogeological units. Using custom data extraction workflows and spatial-statistical methods, we explored trends and patterns along climatic gradients, seasonal cycles, and land use types, focusing on 32 groundwater quality parameters spanning physicochemical, microbiological, and nutrient indicators. Results reveal consistent large-scale north-to-south groundwater quality degradation gradients. This pattern could be partially explained by the geology and effect of temperature on rock-water interactions, increasing mineral dissolution, particularly for calcium, sodium, and magnesium. In contrast, parameters more affected by large-scale land use patterns, such as nitrates, chlorides, and potassium, showed a weaker response to the climate gradient, and their concentrations were more closely correlated to anthropogenic activities, with higher levels typically associated with agricultural, pastoral, and urban land uses. These findings reveal that groundwater is already showing signs of degradation linked to agricultural activities. This underscores the need for integrated monitoring frameworks that consider both climatic and land use drivers, enabling more targeted protection and restoration efforts in Mediterranean groundwater systems.