1. Introduction to urban isotope hydrologyThe urban population growth (e.g., a 2.5 billion increase projected over the next 30 years; He et al., 2021; Mahtta et al., 2022), escalating living standards, and fast-expanding metropolitan centers across inland and coastal regions collectively exert unprecedented pressure on the available drinking water sources and invoke the urgent need for innovative approaches to assessing urban water management (Daigger, 2009). Commonly, water scarcity issues are intensified in densely populated centers (Jenkins et al., 2003; Padowski & Gorelick, 2014), where large disproportionality exists between water demand and headwater supply systems (Jenerette & Larsen, 2006). These urban centers often depend on water recharge across mountain ranges, extensive groundwater or surface water extraction, and water transfer from nearby or distant river basins (Viviroli et al., 2007; de Andrade et al., 2011; de Graaf et al., 2019; Inmerzeel et al., 2020; Kellner & Brunner; 2021). In addition, opportunistic or programmed water allocation from multiple sources is altering environmental flows (in time and magnitude) and the overall ecological functioning of perennial and intermittent urban streams (e.g., urban stream syndrome; Walsh et al., 2005; Dickson and Dzombak, 2017: Wenger et al., 2009; Breyer et al., 2018; Marx et al., 2021). These impacts have notably disturbed the urban watershed continuum (e.g., natural and engineered hydrologic flow paths) by modifying water fluxes and solute transport (vertically and horizontally) (Kaushal & Belt, 2012; Kaushal et al., 2014).These challenges are exacerbated by the current inter-annual climate variability (Brown et al., 2019) and water-related issues such as direct and diffuse pollution, inadequate or inexistent sewage treatment systems (Sánchez-Gutiérrez et al., 2023), drinking water leakage (i.e., water losses within the pipe network) and groundwater depletion, which underscore the critical need to evaluate and monitor urban water storage, sources, use, and distribution to ensure long-term sustainability (Lee & Schwab, 2005; Foster et al., 2013; Marsalek, 2014; Chini & Stillwell, 2018; Olivares et al., 2019; Luo et al., 2019; Xu et al., 2019; Oswald et al., 2023). Water is at the core of sustainable development goals (SDG No.6; Sadoff et al., 2020), which address issues related to drinking water, sanitation, and hygiene and the quality and sustainability of water resources worldwide. However, despite the global awareness of urban drinking water vulnerability, the water sources and distribution networks of most cities are still poorly evaluated, hampering the current and future spatial and temporal assessment of urban drinking water stress under drought (i.e., water scarcity) and extreme precipitation conditions (i.e., the incursion of traditional and emergent contaminants into the distribution system).Environmental tracers constitute a well-established and reliable tool for urban hydrology, as isotopes can provide important information to water managers to assess sources and interactions between water bodies (Ehleringer et al., 2016). The provision of water for domestic supply in urban areas is complex. Usually, it involves many sources (i.e., precipitation harvesting, streams, canals, lakes and reservoirs, springs, groundwater, and inter-basin water transfer, among others) with different isotopic compositions (see Kuhlemann et al. 2020 for an example of the city Berlin, Germany). These unique compositions can define sources, pathways, transit times, and interactions of water bodies in urban environments. For instance, stable water isotopes can be used to understand links between water consumption and sources, which is key for developing strategies to ensure the long-term sustainability of domestic water supplies (Jameel et al., 2016). National-scale surveys of stable isotope ratios of drinking water have been reported to provide information on the stability of water supply and provide warning signals for impending water resource changes (Bowen et al., 2007; West et al., 2014; Zhao et al., 2017; Bhuiyan et al., 2023). On a local scale, the spatiotemporal distribution of stable water isotope ratios across a single metropolitan area can be used to understand and monitor the function of municipal water systems at finer scales, an important and common issue for water managers in urban systems (Jameel et al., 2018; Sánchez-Murillo et al., 2020; Nagode et al., 2021; Kebede et al., 2023; Sánchez-Murillo et al., 2022). Moreover, the past two decades have seen the rapid development of water isotope sampling and low-cost analytical capabilities (Wassenaar et al., 2018; Terzer-Wassmuth et al., 2020), which have led to novel data analysis approaches such as isoscapes (Bowen, 2010) that can provide useful information to water managers to understand and predict the availability and quality of freshwater resources.One of the main needs of urban water managers and practitioners is to track the water source and movement in the pipe network and monitor water quality modifications and blending ratios (mixing) between the points of treatment and use (Boryczko et al., 2014). Tracking drinking water sources is required as the associated quantity (e.g., water source depletion) and quality (e.g., source contamination) risks differ among sources. Water quality at the point of use may differ from the source because of chemical, physical, and biological modification processes within the distribution network (Liu et al., 2018; Mohammed et al., 2021). In cities with centralized water sourcing and engineered pipe networks, it may be feasible to determine the water source using pipe network maps (Okwori et al., 2021) and pressure distribution within the system. However, this task becomes more difficult in rapidly growing urban centers in developing countries, whereby multiple decentralized water sources (e.g., seasonal reservoirs, well fields, solitary wells, and springs) (Peter-Varbanets et al., 2009) contribute to the piped network. In this context, stable isotopes have the potential to backtrack the water source and determine the transit time of different water sources at the point of use.In addition, understanding the vulnerability of freshwater sources to pollution is key for water managers to improve water security. Pollution problems caused by point or non-point sources in urban areas can be readily identified by the use of a multi-stable isotope approach (e.g., oxygen, hydrogen, nitrogen, carbon, boron, and sulfur) combined with conventional hydrochemical data (e.g., major ions, trace elements, electrical conductivity) (Petrucci et al., 2014). Environmental isotopes provide information on water sourcing, identification of hydrological and biogeochemical processes, detection of leaky distribution systems for domestic supply, and defective sewers into urban aquifers. Determining the age of freshwater resources is also paramount as it provides information on circulation and renewability, which is critical for water resource planning. Environmental isotopes such as3H, 3He, 14C,222Rn industrial gases (e.g., CFCs, SF6), and noble gases are well-established dating tools and provide information on the transit time of water through the water cycle (Newman et al., 2010). Over the past decades, the application of dating tools in urban tracer hydrology has been facilitated by improving analytical and modeling techniques, easier sampling techniques, and lower analytical costs (Smith et al., 2023).
