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Isotope hydrological studies to understand groundwater‐surface water interactions in tropical, high‐elevation catchments are limited. These interactions are important in controlling lake water residence time, aqueous biogeochemistry, and water availability for downstream communities and ecosystems. To better comprehend the complexity of spatio‐temporal variations in the aquifer‐lake domain in tropical volcanic regions, 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 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 lake inflow, respectively. Temporal variations of δ13CDIC confirmed the key role volcanic carbonate buffering plays in this lake and indicated greater CO2 degassing from groundwater sources in the wet season. This tracer‐aided assessment in a volcanic lake maar of northern Costa Rica provides evidence of previously unknown groundwater‐surface water interactions and illustrates the application of isotopic tools for estimating water balances and seasonal variability of groundwater discharge into natural lakes across the volcanic front of Central America. Plain Language Summary: This study analyzed the interactions between groundwater and surface water at Lake Hule, a volcanic maar lake located in northern Costa Rica. The results showed that evaporation losses were relatively similar and low in both dry (December–April) and wet (May–November) seasons and that most of the total inflow to the lake was related to groundwater inputs. The study also revealed the key role of carbon dioxide (CO2) dissolution in the lake water chemistry, with greater volcanic CO2 inputs in the wet season. This study reveals previously unknown groundwater‐surface water interactions regarding groundwater discharge into natural lakes across the volcanic front of Central America. Key Points: Isotope‐based metrics evidenced low evaporation to inflow (E/I) ratios (<5%)The water balance revealed seasonal water residence times of 0.61 (wet) ‐ 1.08 (dry) years under steady‐state conditionsEstimates of groundwater input indicated an annual contribution of 61.3 ± 8.1% of total inflow [ABSTRACT FROM AUTHOR]

Subduction of the Cocos and Nazca oceanic plates beneath the Caribbean plate drives the upward movement of deep fluids enriched in carbon, nitrogen, sulfur, and iron along the Central American Volcanic Arc (CAVA). These compounds fuel diverse subsurface microbial communities that in turn alter the distribution, redox state, and isotopic composition of these compounds. Microbial community structure and functions vary according to deep fluid delivery across the arc, but less is known about how microbial communities differ along the axis of a convergent margin as geological features (e.g., extent of volcanism and subduction geometry) shift. Here, we investigate changes in bacterial 16S rRNA gene amplicons and geochemical analysis of deeply-sourced seeps along the southern CAVA, where subduction of the Cocos Ridge alters the geological setting. We find shifts in community composition along the convergent margin, with communities in similar geological settings clustering together independently of the proximity of sample sites. Microbial community composition correlates with geological variables such as host rock type, maturity of hydrothermal fluid and slab depth along different segments of the CAVA. This reveals tight coupling between deep Earth processes and subsurface microbial activity, controlling community distribution, structure and composition along a convergent margin. [ABSTRACT FROM AUTHOR]

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. Recognising 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 centres, 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 δ18O) and more uniform diurnal variations in urban centres fed by groundwater sources (0.08‰ in δ18O) (Ljubljana, Slovenia). Similarly, while d‐excess was fairly close to the global mean value (+10‰) across all urban centres (10‰–15‰), reservoir‐based drinking water systems show lower values (up to ~ −20‰) (Arlington, TX, USA and Gaborone, Botswana), as a result of strong evapoconcentration processes. δ18O 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. [ABSTRACT FROM AUTHOR]

Tropical peatlands are distributed mainly in coastal lowlands; however high elevation regions exhibit a large prevalence of small and fragmented peatlands that are mostly understudied. Artificial drainage of peatlands to expand the area of cattle farming, horticulture, and urbanization is increasing carbon losses to the atmosphere and streams worldwide. Here, we present an exploratory characterization of dissolved carbon optical properties in ombrotrophic peat bogs of the Talamanca range of Costa Rica, across an altitudinal gradient (2,400–3,100 m a.s.l.) during the rainy season. Dissolved organic matter (DOM) sources and decomposition processes were evaluated in the light of dissolved organic and inorganic carbon (DOC and DIC), optical properties, and major water chemistry. DOC concentrations ranged from 0.2 up to 47.0 mg/L. DIC concentrations were below 2 mg/L and δ13CDIC values indicated a mixture between soil organic matter, CO2 in soil water, and to a lesser degree DIC derived from bacterial CO2. Absolute fluorescence intensity of humic-like peaks was 6–7 times greater than fresh-like peaks across all sites. Fluorescence peak ratios coupled with the biological and humification indexes point to a greater relative contribution of recalcitrant soil-derived DOM. Excitation/Emission matrices denoted a high prevalence of humic and fulvic acids in the peat bogs, with moderate intensities in soluble microbial by-products-like and aromatic protein regions at three sites. Our data provides a baseline to underpin tropical carbon dynamics across high elevation peatlands.

Strong convective systems characterize the tropical central-southern region of Brazil. These systems provide abundant water supply for agroindustrial activities and pose flood risks to large cities. Here, we present high-frequency (2–10 min ; inter- and intra-event) rainfall isotopic compositions (n = 90 samples) to reveal regional and local atmospheric processes controlling the isotopic variability of convective systems between 2019–2021. Inter-event rainfall weighted-average (δwgd) values were low (δ18Owgd ≤ - 10.0 ‰) due to the higher rainfall along the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model trajectories from the Amazon forest during the summer. In contrast, during autumn and spring seasons HYSPLIT model trajectories from the Atlantic Ocean and southern Brazil exhibited lower rainfall amounts, resulting in high δ18Owgd ≥ - 4.2 ‰. This strong regional δwgd pattern often masks intra-event isotopic variability. Therefore, we analyzed the vertical structure of local rainfall using reflectivity (Z) from microradar data. Variations in Z indicate that microphysical processes as raindrops fall led to changes in δ18O and deuterium excess (d -excess). Our findings establish a novel framework for evaluating the meteorological controls on the isotopic variability of convective precipitation in tropical South America, fill the gap in high-frequency studies in this region, and generate an isotopic dataset for convective model evaluations. [ABSTRACT FROM AUTHOR]

Tracer‐aided studies to understand plant water uptake sources and dynamics in tropical ecosystems are limited. Here, we report the analysis of dry season source water uptake patterns of five unique ecosystems of Costa Rica across 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 in 2021. Plant and soil water extractions were conducted using centrifugation. Stem water extracted volume and stem total water content were calculated via gravimetric analysis. Water source contributions were estimated using a Bayesian mixing model. Isotope ratios in soil and stems exhibited a strong meteoric origin. Enrichment trends were only detected 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. 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 to well‐mixed 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 rainfall (near‐surface soil storage) to more well‐mixed soil moisture during the dry season, revealing ecohydrological processing previously unknown in this tropical region. [ABSTRACT FROM AUTHOR]

The tropical central-southern part of Brazil (CSB) is characterized by strong convective systems bringing generous water supply for agro-industrial activities but also pose flood risks for large cities. Here, we present high-frequency (5–10 minutes) rainfall isotopic compositions to better understand those systems, with a total of 90 intra-event samples collected during the period 2019–2021. Convective activity and moisture transport modulate the seasonal rainwater isotopic composition with low δ18O values during summer and high during autumn and spring. In summer, both regional and local factors contribute to the observed depletion in heavy isotope contents of rainfall, with strong, continuous rainout along the trajectories of moisture-laden air masses arriving at the rainfall collection site from the Amazon basin, and diurnal convective activity of the local atmosphere, respectively. This activity generates convective clouds with distinct features (cloud depth and cloud base height) and induces differences in atmospheric conditions below the cloud base level (relative humidity and rainfall rates) modifying isotopic characteristics of rainfall and revealing novel perspective on day-night contrast in δ18O and d-excess values. During daytime, enhanced sub-cloud effects lead to high δ18O and low d-excess while continuous regional rainout during night-time results in low δ18O and high d-excess values of local rainfall. Our results offer a new framework of key drivers controlling the isotopic variability of rainfall in tropical South America that must be considered in future studies of convective systems across the tropics. [ABSTRACT FROM AUTHOR]

Stable isotope ratios of nitrogen and oxygen (15N/14N and 18O/16O) of nitrate (NO3–) are excellent tracers for developing systematic understanding of sources, conversions, and deposition of reactive atmospheric nitrogen (Nr) in the environment. Despite recent analytical advances, standardized sampling of NO3–) isotopes in precipitation is still lacking. To advance atmospheric studies on Nr species, we propose best-practice guidelines for accurate and precise sampling and analysis of NO3– isotopes in precipitation based on the experience obtained from an international research project coordinated by the International Atomic Energy Agency (IAEA). The precipitation sampling and preservation strategies yielded a good agreement between the NO3– concentrations measured at the laboratories of 16 countries and at the IAEA. Compared to conventional methods (e.g., bacterial denitrification), we confirmed the accurate performance of the lower cost Ti(III) reduction method for isotope analyses (15N and 18O) of NO3– in precipitation samples. These isotopic data depict different origins and oxidation pathways of inorganic nitrogen. This work emphasized the capability of NO3– isotopes to assess the origin and atmospheric oxidation of Nr and outlined a pathway to improve laboratory capability and expertise at a global scale. The incorporation of other isotopes like 17O in Nr is recommended in future studies. [ABSTRACT FROM AUTHOR]