Tracing urban Drinking water sources using isotope techniques: insights and applications

Climate change, inter-annual precipitation variability, recurrent droughts, and flash flooding, combined with increasing water demands, are influencing the evolution of socioeconomic and cultural structures, water laws, and equitable access to drinking water worldwide. To address the need for strategies to ensure drinking water availability in urban areas, the Isotope Hydrology Section of the International Atomic Energy Agency (IAEA) conducted a comprehensive global assessment titled ‘Use of Isotope Techniques for the Evaluation of Water Sources for Domestic Supply in Urban Areas (2018–2023)’. This initiative aimed to evaluate water sources and the distribution of drinking water supply in urban centres using isotopic tools.

The project successfully covered (a) current research trends in studying urban drinking water systems over the past two decades and (b) the development, testing, and integration of new methodologies for better assessment, mapping, and management of water resources used for drinking water supply in urban settings. Examples of water isotope applications from countries such as Canada, USA, Costa Rica, Ecuador, Morocco, Botswana, Romania, Slovenia, India, and Nepal provide context to the insights and recommendations presented, demonstrating the versatility of water isotopes in capturing seasonal and temporal variations across different environmental and climate scenarios.

The study found that urban areas rely on a diverse range of water sources, including mountain recharge, extensive local groundwater extraction, and water transfer from nearby or distant river basins. This diversity is reflected in the spatial isotope snapshot variability. High-resolution monitoring (hourly and sub-hourly) revealed significant diurnal variations in the wet tropics (Costa Rica) (up to 1.5‰ in δ¹⁸O) and more uniform diurnal variations in urban centres supplied by groundwater sources (0.08‰ in δ¹⁸O) (Ljubljana, Slovenia). Additionally, while d-excess values were generally close to the global mean (+10‰) across all urban centres (10‰–15‰), reservoir-based drinking water systems showed lower values (up to ~ −20‰) (Arlington, TX, USA and Gaborone, Botswana) due to strong evapoconcentration processes. δ¹⁸O time series and depth-integrated sampling highlighted the influence of the catchment damping ratio on the final intake water composition.

By introducing new, traceable spatial and temporal tools that span from the water source to the end-user and are linked to the engineered and socioeconomic structure of the water distribution system, governmental, regional, or community-based water operators and practitioners can enhance drinking water treatment strategies (including more accurate surface water blending estimations) and improve urban water management and conservation plans in the context of global warming.