Hydrogeological and hydrochemical processes affecting groundwater quality on volcanic islands: Insights from El Hierro (Canary Islands, Spain)

Groundwater resources on volcanic islands are vital for societal and economic development, especially due to their scarcity and reliance on agriculture and tourism. This study examines the hydrogeological and hydrochemical processes shaping groundwater quality on volcanic islands, focusing on El Hierro Island (Canary Islands, Spain). The findings reveal that volcanic dykes play a critical role in controlling groundwater flow, creating freshwater reservoirs, and influencing flow paths. Four primary processes affecting groundwater quality are identified: seawater intrusion, volcanic CO₂ emissions, nitrate contamination from fertilizers, and CO₂-driven water–rock interactions. A 3D groundwater flow model shows that the anisotropy in hydraulic conductivity induced by volcanic dykes reduces seawater intrusion in specific areas, thereby protecting groundwater quality. Volcanic CO₂ emissions are found to lower pH, increasing acidity and altering groundwater chemistry. CO₂-driven water–rock interactions result in the dissolution of basaltic minerals, raising concentrations of key rock-forming elements such as sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), and silica (SiO₂) in groundwater. Additionally, nitrate pollution is linked to fertilizer use, particularly in agricultural regions. These insights highlight the need for sustainable water management to address the challenges posed by salinization, pollution, and volcanic activity. This research not only advances understanding of El Hierro’s groundwater system but also offers a framework applicable to other volcanic islands with similar hydrogeological conditions, supporting improved management strategies for freshwater resources.