Stable isotopes of hydrogen and oxygen in precipitation provide insights into water cycle dynamics, yet characterizing the meteorological processes driving isotopic variability remains challenging. We introduce a method to rasterize air mass trajectory data from HYSPLIT for isotopic analysis and assess the impact of trajectory initiation height on HYSPLIT’s representation of local precipitation and temperature. Incorporating HYSPLIT rasters into spatial analysis improved deuterium composition isoscape accuracy for precipitation compared with a model relying solely on surface predictors (i.e., temperature, elevation). The resulting isoscape patterns reflected strong, temperature-dependent seasonality, aligning well with previous studies. Trajectory cluster analysis further elucidated the links between meteorological patterns and observed seasonality in isotope compositions. We also analyzed daily precipitation at two sites (n=204, n=138) over ~2 years using Random Forest and Multiple Linear Regression. Our analysis identified a significant amount effect and evidence for sub-cloud evaporation. However, additional factors-such as cloud-top temperature, reflectivity, and convective versus stratiform fractions-are likely needed to explain residual daily-scale variance. Finally, we determined that an initiation height of 1000 m was sufficient for our study domain. Using the local condensation level as the initiation height provided no substantial improvement in correlations between modeled and observed precipitation or temperature.

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.

A document by Taylor Joyal. Click on the document to view its contents.

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.

Groundwater recharge in highly-fractured volcanic aquifers remains poorly understood in the humid tropics, whereby rapid demographic growth and unregulated land use change are resulting in extensive surface water pollution and a large dependency on groundwater extraction. Here we present a multi-tracer approach including δ18O-δ2H, 3H/3He, and noble gases within the most prominent multi-aquifer system of central Costa Rica, with the objective to assess dominant groundwater recharge characteristics and age distributions. We sampled wells and large springs across an elevation gradient from 868 to 2,421 m asl. Our results suggest relatively young apparent ages ranging from 0.0±3.2 up to 76.6±9.9 years. Helium isotopes R/RA (0.99 to 5.4) indicate a dominant signal from the upper mantle across the aquifer. Potential recharge elevations ranged from ~1,400 to 2,650 m asl, with recharge temperatures varying from ~11°C to 19°C with a mean value of 14.5±1.9°C. Recharge estimates ranged from 129±78 to 1,605±196 mm/yr with a mean value of 642±117 mm/yr, representing 20.1±4.0% of the total mean annual rainfall as effective recharge. The shallow unconfined aquifer is characterised by young and rapidly infiltrating waters, whereas the deeper aquifer units have relatively older waters. These results are intended to guide the delineation and mapping of critical recharge areas in mountain headwaters to enhance water security and sustainability in the most important headwater dependent systems of Costa Rica.