Isotope hydrogeology of precipitation runoff and groundwater discharge as source waters for stream flow in the central Amazon

Study region: Central Amazonia, Brazil Study focus: We developed an a priori conceptual model of aquifer–stream connectivity across a forest–peri-urban–urban gradient (ZF2 = forest; Ducke Reserve = peri-urban forest; IFAM = urban forest). Piezometric heads, hydrochemical data, and stable water isotopes (δ¹⁸O, δ²H) from precipitation, groundwater, and streams were measured in wet and dry seasons. These data supported the conceptual model, while a Bayesian mixing model (SIMMr) provided a posteriori validation by quantifying the proportional contributions of precipitation and groundwater end-members to streamflow.New hydrological insights for the region: Groundwater dominated streamflow year-round (74–98 % dry; 64–90 % wet), with greatest seasonal variability at Ducke Reserve. Hydrochemical contrasts paralleled isotopic trends: dilute recharge in ZF2, elevated alkalinity and conductivity in Ducke, and nutrient enrichment in IFAM. Isotopic enrichment increased along the gradient, reflecting reduced infiltration and stronger evaporation. Groundwater heads consistently exceeded stream heads, with hydraulic gradients showing aquifer-to-stream flow across all basins. Together, isotopic, hydrochemical, and hydraulic evidence reinforce a groundwater-fed conceptual model relevant to tropical and monsoon headwaters under land-use change.