Assessment of metal concentrations in High Andean Tropical rivers of southern Ecuador

Abstract

High Andean tropical streams play a critical role in water provision. Although these ecosystems are generally pristine, few studies have quantified their water quality. This study assessed spatial variability and concentration–discharge (c–Q) relationships of major and minor solutes, as well as heavy metals concentrations in streams of southern Ecuador, using data collected between 2008 and 2021 across 28 monitoring sites. Major ions revealed a general trend of downstream enrichment. The Zhurucay catchment exhibited the highest concentrations of several heavy metals at Alumbre stream, which exceeded both national and international standards for drinking water and aquatic life. Principal Component Analysis (PCA) and k-means clustering identified three hydrochemical water types: (1) weakly mineralized waters, primarily in headwater sites, reflecting cation-poor lithology (2) Fe–Mn-rich waters related to wetlands, and (3) mineralized waters, primarily associated with downstream sites, likely influenced by groundwater and cation-rich lithology. Concentration– discharge relationships exhibited dilution behavior for Ca, Mg, Na, K, Sr, Ba, Rb, and electrical conductivity, whereas Al and Fe displayed mobilization patterns. In contrast, As, Pb, and Cd did not show a clear c–Q pattern. Elevated concentrations are likely associated with the volcanic origin of the region, influenced by both recent and past volcanic activity, including the presence of organic-rich soils and hydrothermally altered geological formations. Although Páramo ecosystems are widely recognized for their high-quality water, this research demonstrates that natural geological anomalies can introduce elevated metal concentrations. Therefore, it is crucial to establish baseline concentrations and assess natural variability for water management.