Groundwater Quality Degradation and Nitrate Transport in a Mediterranean Aquifer System: Synergistic Effects of Extreme Drought and Anthropogenic Pressures (Barcelona Province, Spain)

This study examines groundwater quality evolution and nitrate transport across a heterogeneous aquifer system in Barcelona Province (NE Spain), assessing compound effects of the 2021–2023 drought (the most severe on record) and anthropogenic pressures over 2016–2024. A total of 3,543 samples from 578 natural springs acting as aquifer discharge points were analyzed for physicochemical and hydrochemical parameters.

Results reveal contrasting trends in nitrate contamination. Mean concentrations decreased 25% (from 39.82 to 29.84 mg/L), attributable to action programs in vulnerable zones under EU Directive 91/676/EEC. However, springs exceeding the 50 mg/L drinking water threshold remained stable at ~24%, indicating persistent structural contamination unresponsive to conventional management. Spatially, elevated nitrates cluster in central and southern sectors coinciding with intensive agricultural and livestock activities.

Hydrochemical characterization confirms carbonate aquifer dominance, with 61.2% of springs exhibiting calcium-bicarbonate facies. Facies evolution between 2023–2024 reveals diagnostic trajectories: transitions toward sulfate facies indicate evaporitic formations contact, while shifts toward chloride facies signal increasing anthropogenic pressure in coastal areas and alluvial plains, reflecting synergistic degradation from drought-induced concentration and diffuse contamination.

The system shows clear climate signals: 0.5°C temperature increase during 2016–2024 and rising electrical conductivity consistent with severe drought. Reduced recharge diminishes natural dilution while increasing contribution of deeper, mineralized flows. This synergy between water stress and pre-existing contamination amplifies degradation beyond what either stressor would produce independently.

These findings demonstrate spring monitoring networks' value as complements to official surveillance, providing higher spatial resolution for early detection of localized deterioration. Differential aquifer vulnerability, with porous alluvial systems showing highest sensitivity to drought and contamination, has direct implications for prioritizing protection in recharge areas.