Vol.:(0123456789) 1 3 Bulletin of Environmental Contamination and Toxicology https://doi.org/10.1007/s00128-019-02545-w Heavy Metal Enrichment Factors in Fluvial Sediments of an Amazonian Basin Impacted by Gold Mining Abrahan Mora 1,2  · Diana Jumbo‑Flores 2  · Max González‑Merizalde 2  · Santos A. Bermeo‑Flores 2  · Pablo Alvarez‑Figueroa 2  · Jürgen Mahlknecht 1  · Arturo Hernández‑Antonio 1 Received: 24 April 2018 / Accepted: 9 January 2019 © Springer Science+Business Media, LLC, part of Springer Nature 2019 Abstract Artisanal and small-scale gold mining (ASGM) has been performed in the southern Ecuadorian Amazon since the colonial period. However, its effects on fluvial systems have been poorly investigated. Thus, in order to calculate the normalized enrichment factors (NEF) of several heavy metals in fluvial sediments of the Zamora River basin (Ecuadorian Amazon), we analyzed bottom sediments along ASGM-affected and unaffected river sections. The results indicated that sediments of the Congüime River have NEF between 2.2 and 2.3 for Cu (moderate contamination) and higher than 3 for Mn, Zn, Pb, and Hg (severe contamination). Similarly, a severe contamination is also observed in the lower Nangaritza River, due to sediments of this sector have NEF > 3 for Zn, Pb, and Hg. Bottom sediments from the Nambija and Zamora rivers showed a severe contamination with Hg (NEF > 3), suggesting the existence of ASGM activities in the upper Zamora River basin. Keywords Amazon · Heavy metals · Contamination · Rivers · Sediments Artisanal and small-scale gold mining (ASGM) has been proven to cause heavy metal contamination in soils and flu- vial sediments of the upper Amazon basin (Diringer et al. 2015; Johnson et al. 2018). During the ASGM process, the ore associated with gold skarn deposits and epithermal gold deposits is extracted by hard-rock mining methods and then crushed and grinded to a fine powder. Subsequently, this powder is mixed with water and metallic Hg, which binds to the gold to form the Au–Hg amalgam. Then, the amalgam is gravity-concentrated and finally burnt to evaporate the Hg. However, during the finely ground ore washing process, both the excess of Hg added during the amalgamation step and other heavy metals present in the ore are released into min- ing effluents as mining tailings. Indeed, these effluents reach rivers and streams, resulting in the pollution of water bodies. It is well known that elevated concentrations of Hg in soils and biota of the lower Amazon basin are due to natural biogeochemical processes (Lechler et al. 2000; de Oliveira et al. 2001). The deforestation and land-cover change in the Amazon basin enhance the topsoil erosion and thereby the mobilization of Hg that naturally occurring in soils to water bodies (Oestreicher et al. 2017). Consequently, the high Hg accumulation in sediments of rivers and lakes of the lower Amazon basin is rather caused by deforestation than by the release of metallic Hg from ASGM (Moreno-Brush et al. 2016; Oestreicher et al. 2017). Thus, the environmen- tal impact caused by ASGM in the Amazon basin has been mainly focused on the distribution of Hg and other heavy metals in soils and river sediments of sites surrounding min- ing centers (Ramírez et al. 2003; Diringer et al. 2015), which may pose a high risk for populations living near those areas (González-Merizalde et al. 2016). The large gold skarn deposits occurring in the Zamora batholith, in the border area of southern Ecuador and north- ern Peru, has led to the development of mining districts and small towns around this area. The Nambija and Chinapintza mining districts, located in that zone, are important min- ing areas because they had one of the largest gold produc- tion in Ecuador in the last decade (Chiaradia et al. 2009; Sánchez-Vásquez et al. 2016). Indeed, the ASGM activities performed in both sectors have impacted soils and rivers, leading a socio-environmental conflict (Ramírez et al. 2003; * Abrahan Mora abrahanmora@itesm.mx 1 Escuela de Ingeniería y Ciencias. Tecnológico de Monterrey, Av. Eugenio Garza Sada Sur 2501, CP 64849 Monterrey, Nuevo León, Mexico 2 Universidad Nacional de Loja, Provincia de Loja, La Argelia, Loja, Ecuador