Climate change, inter-annual precipitation variability, recurrent droughts, and flash flooding, coupled with increasing water needs, are shaping the co-evolution of socioeconomic and cultural assemblages, water laws and regulations, and equitable drinking water access and allocation worldwide. Recognizing the need for mitigation strategies for drinking water availability in urban areas, the Isotope Hydrology Section of the International Atomic Energy Agency (IAEA) coordinated a state-of-the-art global assessment to evaluate water sources and distribution of drinking water supply in urban centers, an initiative entitled “Use of Isotope Techniques for the Evaluation of Water Sources for Domestic Supply in Urban Areas (2018-2023)”. Here, we report on a) current research trends for studying urban drinking water systems during the last two decades and b) the development, testing, and integration of new methodologies, aiming for a better assessment, mapping, and management of water resources used for drinking water supply in urban settings. Selected examples of water isotope applications (Canada, USA, Costa Rica, Ecuador, Morocco, Botswana, Romania, Slovenia, India, and Nepal) provide context to the insights and recommendations reported and highlight the versatility of water isotopes to underpin seasonal and temporal variations across various environmental and climate scenarios. The study revealed that urban areas depend on a large spectrum of water recharge across mountain ranges, extensive local groundwater extraction, and water transfer from nearby or distant river basins. The latter is reflected in the spatial isotope snapshot variability. High-resolution monitoring (hourly and sub-hourly) isotope sampling revealed large diurnal variations in the wet tropics (Costa Rica) (up to 1.5‰ in δ 18O) and more uniform diurnal variations in urban centers fed by groundwater sources (0.08 ‰ in δ 18O) ([Ljubljana](https://www.google.com/search?client=firefox-b-1-d&sca_esv=f5a20a2e9138d638&sca_upv=1&sxsrf=ADLYWIKR6-DvBtjaWqFYRhn6VgnegOa8kg:1717189104058&q=Ljubljana&stick=H4sIAAAAAAAAAONgVuLQz9U3SMrNNXnEaMwt8PLHPWEprUlrTl5jVOHiCs7IL3fNK8ksqRQS42KDsnikuLjgmngWsXL6ZJUm5WQl5iUCAAFa64FOAAAA&sa=X&ved=2ahUKEwjMrrz047iGAxWyG9AFHSVwCBgQzIcDKAB6BAgTEAE), Slovenia). Similarly, while d-excess was fairly close to the global mean value (+10 ‰) across all urban centers (10-15‰), reservoir-based drinking water systems show significantly lower values (up to ~ -20 ‰) (Arlington, TX, USA and Gaborone, Botswana), as a result of strong evapoconcentration processes. δ 18O time series and depth-integrated sampling highlighted the influence of the catchment damping ratio in the ultimate 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 socio-economic structure of the water distribution system, governmental, regional, or community-based water operators and practitioners could enhance drinking water treatment strategies (including more accurate surface water blending estimations) and improve urban water management and conservation plans in the light of global warming.

Erica Almance

and 6 more

Extracting water from discrete xylem and soil samples and continuous (long-term) monitoring of water vapor across the soil-plant-atmosphere continuum remains challenging and under a large debate. Here, we present a detailed one-year study in the Fort Worth Botanic Garden (north-central Texas) to evaluate the analytical robustness of the centrifugation extraction method and understand water sourcing from three common urban tree species (Elderberry, Sambucus canadensis; Cherry Laurel, Prunus caroliniana; and Boxelder Maple, Acer negundo). Xylem (N=110) isotope ratios (δ 18O and δ 2H) are compared to local precipitation (N=498), throughfall (N=33), and soil water (N=105) at different depths (0-38 cm). Complementary soil water samples were obtained from cup suction lysimeters (N=42) (0-38 cm). Soil and xylem water extraction volumes ranged from 100 µL to 7.5 mL in plant samples and from 100 µL to 10.5 mL in soil samples. Extraction success rates were 68.8% and 75.2% for xylem and soil samples, respectively. The minimum sample total water content for effective extractions was determined as 10.6% (soil) and 17.8% (xylem). Xylem mean narrowband and broadband (proxy for organic contamination) were 0.23±0.40 (-) and 1.00±0.01 (-), respectively. These values agree with mean narrowband and broadband metrics from throughfall and soils, which highlight the non-invasive nature of centrifugated extractions. Annual mean soil δ 18O compositions (-3.6±1.7‰) corresponded with the throughfall input (-3.6±2.4‰). Xylem δ 18O compositions exhibited a strong temporal enrichment trend at the end of the winter, summer, and fall seasons. Mean spring xylem δ 18O (-2.85‰) was less variable and close to soil mean compositions (-2.82‰). For this season, Bayesian mixing analysis showed source water contributions from distinct soil depths: 0 cm to 12.7 cm for Boxelder Maple, 12.7 cm to 25.4 cm for Cherry Laurel, and 12.7 cm to 38.1 cm for Elderberry. Our results offer a standardized and effective protocol for centrifugation extractions and reveal plant water uptake preferences in a highly altered urban green space during an unprecedented warm year.
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).

Beyer Matthias

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The spatial variation of soil water isotopes (SWI) - representing the baseline for investigating root water uptake (RWU) depths with water stable isotope techniques - has rarely been investigated. Here, we use spatial SWI depth profile sampling in combination with unmanned aerial vehicle (UAV) based land surface temperature estimates and vegetation indices (VI) in order to improving process understanding of the relationships between soil water content and isotope patterns with canopy status. We carried out a spatial sampling of ten SWI depth profiles in a tropical dry forest. UAV data were collected and analyzed to obtain detailed characterization of soil temperature and canopy status. We then performed a statistical analysis between the VI and land surface temperatures with soil water content and SWI values at different spatial resolutions (3 cm to 5 m). Best relationships were used for generating soil water isoscapes for the entire study area. Results suggest that soil water content and SWI values are strongly mediated by canopy parameters (VI). Various VI correlate strongly with soil water content and SWI values across all depths. SWI at the surface depend on land surface temperature (R² of 0.65 for δ18O and 0.57 for δ2H). Strongest overall correlations were found at a spatial resolution of 0.5 m. We speculate that this might be the ideal resolution for spatially characterizing SWI patterns and investigate RWU. Supporting spatial analyses of SWI with UAV-based approaches might be a future avenue for improving the spatial representation and credibility of such studies.
The El Niño-Southern Oscillation (ENSO) phenomena, originating in the tropical Pacific region, is an interannual climate variability driven by sea surface temperature and atmospheric pressure changes that affect weather patterns globally. In Mesoamerica, ENSO can cause significant changes in rainfall patterns with major impacts on water resources. This commentary presents results from a nearly 10-yr hydrometric and tracer monitoring network across north-central Costa Rica, a region known as a headwater-dependent system. This monitoring system has recorded different El Niño and La Niña events, as well as the direct/indirect effects of several hurricane and tropical storm passages. Our results show that ENSO exerts a significant but predictable impact on rainfall anomalies, groundwater recharge, and spring discharge, as evidenced by second-order water isotope parameters (e.g., line conditioned-excess or LC-excess). The Oceanic Niño Index (ONI) is correlated with a reduction in mean annual and cold front rainfall across the headwaters of north-central Costa Rica. During El Niño conditions, rainfall is substantially reduced (by up to 69.2%) during the critical cold fronts period, subsequently limiting groundwater recharge and promoting an early onset of baseflow conditions. In contrast, La Niña is associated with increased rainfall and groundwater recharge (by up to 94.7% during active cold front periods). During La Niña, the long-term mean spring discharge (39 Ls -1) is exceeded 63-80% of the time, whereas, during El Niño, the exceedance time ranges between 26% and 44%. These stark shifts in regional hydroclimatic variability are imprinted on the hydrogen and oxygen isotopic compositions of meteoric waters. Drier conditions favored lower LC-excess in rainfall (-17.3‰) and spring water (-6.5‰), whereas wetter conditions resulted in greater values (rainfall=+17.5‰; spring water=+10.7‰). The lower and higher LC-excess values in rainfall corresponded to the very strong 2014-16 El Niño and 2018 La Niña, respectively. During the recent triple-dip 2021-23 La Niña, LC-excess exhibited a significant and consistently increasing trend. These findings highlight the importance of combining hydrometric, synoptic, and isotopic monitoring as ENSO sentinels to advance our current understanding of ENSO impacts on hydrological systems across the humid Tropics. Such information is critical to constraining 21 st century projections of future water stress across this fragile region.
Nitrate contamination is affecting groundwater across the tropics. This study describes isotopic and ionic spatial trends across a tropical and volcanic multi-aquifer system in central Costa Rica in relation to land use change over four decades. Springs and wells (from 800 to 2,400 m asl) were sampled for NO3- and Cl- concentrations, δ18Owater, δ15NNO3, and δ18ONO3. A Bayesian isotope mixing model was used to estimate source contributions to the nitrate legacy in groundwater. Land use change was evaluated using satellite imagery from 1979 and 2019. The lower nitrate concentrations (< 1 mg/L) were reported in headwater springs near protected forested areas, while greater concentrations (up to ~63 mg/L) were reported in wells (mid- and low-elevation sites in the unconfined unit) and low-elevation springs. High-elevation springs were characterized by low Cl- concentrations and moderate NO3-/Cl- ratios, indicating the potential influence of soil nitrogen inputs. Wells and low-elevation springs exhibited greater NO3-/Cl- ratios and Cl- concentrations above 100 mg/L. A decreasing trend in NO3-/Cl- ratios coupled with greater Cl- values was also detected. Bayesian calculations suggest a mixture of sewage (domestic septic tanks), soil nitrogen (forested recharge areas), and chemical fertilizers (coffee plantations), as a direct result of abrupt land use change in the last 40 years. Our results confirm the incipient trend in increasing groundwater nitrogen and highlight the urgent need for a multi-municipal plan to transition from domestic septic tanks to regional sewage treatment and sustainable agricultural practices to prevent future groundwater quality degradation effectively.

Sánchez-Murillo, R

and 7 more

Tracer-aided studies to understand source water partitioning in tropical ecosystems are limited. Here we report dry season source water partitioning in five unique ecosystems distributed across Costa Rica in altitudinal (<150-3,400 m asl) and latitudinal (Caribbean and Pacific slopes) gradients: evergreen and seasonal rainforests, cloud forest, Páramo, and dry forest. Soil and plant samples were collected during the dry season (2021). Plant and soil water extractions (triplicates) were conducted using controlled centrifugation. Stem water extraction efficiency and stem water content were calculated via gravimetric measurements. Water source contributions were estimated using a Bayesian mixing model. Isotope ratios in soil and stems exhibited a strong meteoric origin. Enrichment trends were detected mainly in stems and cactus samples within the dry forest ecosystem. Soil profiles revealed nearly uniform isotopic profiles; however, a depletion trend was observed in the Páramo ecosystem below 25 cm depth. More enriched compositions were reported in cactus samples for extracted water volumes above ~20% ( Adj. r2=0.34, p<0.01). The most prominent dry season water source in the evergreen rainforest (74.0%), seasonal rainforest (86.4%), and cloud forest (66.0%) corresponded with soil water. In the Páramo ecosystem, recent rainfall produced by trade wind incursions resulted in the most significant water source (61.9%), whereas in the dry forest, mean annual precipitation (38.6%) and baseflow (33.1%) were the dominant sources. The latter highlights the prevalence of distinct water uptake sources between recent cold front’s rainfall to more well-mixed soil moisture during the dry season.