Abstract

The Atacama Desert in northern Chile is characterized by its naturally high metal concentrations; however, human activities have significantly increased their availability and concentration in aquatic environments. In the Loa River basin, copper mining is the main economic activity, and the extremely arid conditions contribute to high levels of evaporation and salinity. This study evaluated the concentrations of As, Cd, and Cu in the 63 & micro;m sediment fraction from three areas, Lequena, La Finca, and Quillagua, during the years 2014, 2015, 2017, and 2023. Contamination levels were assessed using multiple approaches, including the Geoaccumulation Index (Igeo), the Enrichment Factor (EF), the Pollution Load Index (PLI), and the mean Probable Effect Concentration Ratio (m-PEC-Q). The results showed that Lequena (upper river zone) had no evidence of anthropogenic contamination over time; however, the ecological risk assessment highlighted the significant natural contribution of arsenic, representing a potential risk to the ecosystem. In contrast, La Finca (mid-river zone) and Quillagua (river mouth) showed significantly high levels of contamination. The Geoaccumulation Index consistently classified these sites as "moderately" to "heavily" contaminated or "heavily contaminated" for arsenic, while the Enrichment Factor indicated "very high enrichment" for arsenic, reflecting a strong anthropogenic influence. Ecological risk assessments indicated a persistent 76% probability of toxicity at La Finca throughout all sampling years, a level also observed at Quillagua in 2017 and 2023, with concentrations frequently exceeding international sediment quality guidelines. These patterns are attributed to the proximity of mining activities in the middle zone and the downstream transport of contaminated sediments to the river's mouth, resulting in persistently high ecological risks over time. This study provides important baseline information for pollution control and ecological safety in the Loa River basin.