Little is currently known about the hydrochemistry of tropical glacierized mountain watersheds, which are among the most vulnerable systems in the world. Glacier retreat may impact their export of nutrients, with possible implications for downstream ecosystems. Solute export depends on dynamic and heterogeneous processes within the watershed, which calls for investigations of the different factors controlling hydrochemical variability. To examine these in a sub-humid glacierized watershed in Ecuador, we implemented a hydrological model that incorporates reactive transport, RT-Flux-PIHM. Our results demonstrate that calibrating the model to hydrochemical in addition to hydrological data is important for constraining groundwater fluxes, which we found to contribute 78% of stream discharge and to include 35% of the total glacial meltwater. Stream chemistry fluctuations are strongly controlled by varying contributions of groundwater, which contains high concentrations of reactive ions predominantly sourced from silicate mineral dissolution. The spatial variability in these concentrations, however, is driven more by heterogeneous evapotranspiration resulting from sharp montane vegetation gradients. With this concentrating effect, evapotranspiration also largely determines seasonal patterns in groundwater chemistry, with highest concentrations occurring in dry seasons, even when dissolution rates are low due to low soil moisture. While groundwater serves as a primary end-member source of streamwater, glacier melt-dominated surface runoff acts as a second source that imposes dilution events on an otherwise chemostatic concentration and discharge (C-Q) graph. Glacier melt overall decreases stream concentrations and increases discharge, with the latter effect dominating such that solute exports (C*Q) increase by 23% with melt.