Chemical and microbiological analysis of urban and associated natural water systems of inhabited volcanic islands of the Galapagos (Ecuador)

The Galapagos Islands, known for their unique ecology, are confronted with significant challenges in water management and freshwater scarcity. These challenges are further exacerbated by a lack of comprehensive monitoring, assessment and understanding of the quality of water systems on inhabited islands. This study examines the urban and associated natural water systems of the remote volcanic islands of Floreana, Isabela and Santa Cruz in the Pacific Ocean through an unprecedented combination of comprehensive physicochemical analysis, including nutrient measurements, microbiological analyses and isotope-based source detection of nitrate (NO₃⁻) contamination. A systematic monitoring campaign across 66 sites revealed critical anthropogenic impacts on water quality. This includes contamination by sewage and the presence of coliforms and/or Escherichia coli in groundwater, reservoirs, tap water and the receiving water bodies of the three evaluated islands. Major impacts are present in the most densely populated island (Santa Cruz), where about 39 % of the monitoring sites are classified as polluted according to the Basic Prati Index, with the presence of E. coli in 57 % (4/7) of the evaluated sites and NO₃⁻ contribution from sewage accounting for up to 57 % of the NO₃⁻ in the receiving water body. In Isabela, treated wastewater is discharged about 300 meters from a recreational site, while it contains high ammonium concentrations (41.5 mg N L⁻¹), E. coli and very low oxygen levels (< 0.5 mg O₂ L⁻¹). In Floreana, the groundwater (aquifer) closest to the urban area shows a NO₃⁻ contribution of up to 94 % from sewage, while this figure is only 11 % in the other islands. On top of this, water analyses in the drinking water reservoirs and tap water in the three islands indicate quality issues in the urban water provision system that could lead to human health impacts. The role of tourism, agriculture, urbanization and population growth exacerbates these issues, presenting urgent public health concerns and highlight the need for sustainable practices by stopping and treating the critical sources of pollution and contamination. These findings on the Galapagos emphasize the need for better water treatment and distribution, as well as sewage and agricultural management in islands to improve and protect the islands' highly vulnerable ecosystems and related water provisioning services.