Groundwater-surface water interactions are important in controlling lake water residence time, biogeochemistry, and water availability for downstream communities in tropical volcanic catchments. To better understand these complex seasonal interactions, a multi-tracer approach including water and inorganic carbon stable isotopes (δ2H, δ18O, δ13CDIC), hydrochemistry, and 222Rn was applied in Lake Hule, northern Costa Rica. Seasonal isotope mass balance calculations using lake, stream, precipitation, and groundwater isotope compositions were supplemented with local hydrometeorological information. Evaporation to inflow ratios (E/I) revealed a small variability between the dry (December-April) and wet seasons (May-November), with relatively low evaporation losses, 2.9±1.0 % and 3.2±1.8 %, respectively. Bayesian end-member analysis indicated that annual inputs from groundwater, precipitation, and runoff represented 61.3±8.1%, 24.4±8.4, and 14.3±5.9% of total inflow, respectively. Temporal variations of δ13CDIC also confirmed the key role carbonate buffering plays in this lake and indicated greater CO2 degassing from groundwater sources in the wet season. This first tracer-aided assessment in a volcanic lake maar of northern Costa Rica provides evidence of previously unknown groundwater-surface water interactions and poses a promising tool for estimating seasonal variability of groundwater discharge into natural lakes across the volcanic front of Central America.

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.

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.