| Sumario: | Understanding groundwater recharge mechanisms in karst zones is critical for preserving water supply, establishing water-use policies, and monitoring water pollution. In this study, we analyzed the isotope composition (δ2H and δ18O) of rainfall, surface water, and groundwater in the Talgua River basin in northeastern Honduras between August 2019 and December 2023. The main goal was to study the relationship between rainfall inputs and recharge mechanisms in a complex karstic environment, characterized by high rainfall seasonality and variable land use. The δ18O and δ2H relationship in rainfall resulted in a significant local meteoric water line of δ2H = 7.68 ± 0.08·δ18O + 9.47 ± 0.42 (N = 223, r2 = 0.97, p < 0.001). HYSPLIT air mass trajectory and cluster analysis using δ18O and deuterium excess (d-excess) revealed that moisture sources in the Talgua River basin originated mainly from the Caribbean Sea (89 %) with minimal contributions from the Pacific Ocean and the Gulf of Mexico (11 %). Recharge is mainly driven by heavy, isotopically depleted rainfall during the May–November (wet season), as indicated by the rainfall to groundwater (P/GW) 18O/16O ratios. d-excess and the line-conditioned excess (LC-excess) values indicate substantial meteoric inputs during the wet season, reflecting a recycled-moisture signal that contributes to recharge, with only minor evapoconcentration effects. This study systematically provides essential insights into rainfall, surface water, and groundwater interactions in the poorly understood karstic regions of Central America. It advances our understanding of tropical karst hydrological processes, such as moisture transport and recharge mechanisms, and provides valuable information for water resource evaluation and management.
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