Hydrogeochemical Characterisation of Headwater-Derived Solute and Metal Concentrations in Peri-Urban Páramo Rivers

High-Andean tropical páramo streams play a critical role in water provision to downstream populations. Since these peri-urban ecosystems are generally considered near-natural, few studies have quantified their natural background hydrochemistry. This study addresses two research questions: (1) How do solute concentrations vary spatially across páramo catchments? and (2) What are the dominant concentration–discharge (C–Q) relationships for the analysed solutes? Long-term water quality data collected between 2008 and 2021 from 28 sites in southern Ecuador were analysed for major solutes and heavy metals using multivariate statistics and C–Q analysis. Major ions showed a general downstream enrichment. The Zhurucay catchment (site Z₁₁) exhibited elevated concentrations of Al, As, Cd, Fe, Mn, Pb, and Zn, which exceeded national and international water quality standards. PCA and k-means clustering identified three hydrogeochemical stream types: (1) weakly mineralised headwaters; (2) Fe–Mn-rich wetland waters; and (3) mineralised downstream sites. Dilution behaviour was observed for Ca, Mg, Na, K, Sr, Ba, Rb, and electrical conductivity, whereas Al and Fe displayed mobilisation. In contrast, As, Pb, and Cd lacked clear C–Q patterns. Elevated concentrations are likely associated with the region's volcanic origin, including organic-rich soils and hydrothermally altered formations. These findings demonstrate that natural background concentrations can approach or exceed water quality standards before anthropogenic influence, limiting the assimilative capacity of these systems. Therefore, establishing baseline monitoring is crucial for water management.