Partitioning of copper at the confluences of Andean rivers
Graphical abstract
Introduction
Metal contamination in rivers impairs water use and increases environmental and health risks (Abdullah and Royle, 1972). In many South American watersheds, metal mobilization from Andean geological repositories is enhanced by human activities such as mining (Cui et al., 2014; Kalbitz and Wennrich, 1998; Nriagu and Pacyna, 1988). Point and diffuse sources such as urban runoff deliver additional metal(loid) fluxes into basins (Beane et al., 2016; Hudson-Edwards, 2003), increasing the concentrations of arsenic (As), copper (Cu), lead (Pb) and zinc (Zn) in the freshwater systems of Peru, Bolivia, Colombia, Ecuador and Chile (Alonso et al., 2014; Appleton et al., 2001; Cooke and Abbott, 2008; Groesser et al., 1994; Martin and Calvert, 2003; Parra et al., 2011; Vezzoli et al., 2013).
In Chile, past and present mining has increased the concentration of As, Cu and sulfate (SO4) in freshwater systems (Pizarro et al., 2010; Vega et al., 2018). In these systems, the total and dissolved Cu concentrations ranged from 0.5 to 170 mg L−1 and from 0.03 to 6.0 mg L−1, respectively (Dittmar, 2004; Ginocchio et al., 2008; Higueras et al., 2004; Narváez et al., 2005; Parra et al., 2011), largely exceeding the threshold of the potential no effect concentration (PNEC) for aquatic organisms in freshwaters (8–22 μg L−1) (European Copper Institute, 2008), as well as the acute no observed adverse effect level (NOAEL) for Cu in drinking water (4 mg L−1) (Araya et al., 2001).
The toxicity and fate of Cu are determined by complex chemical processes between metals and particles. Once Cu reaches rivers, the abundance of the most mobile and toxic species, Cu2+ and CuOH+ (Cuppett et al., 2006; Miwa et al., 1989), depends on the environmental conditions (e.g,. pH, redox potential (ORP) and dissolved organic carbon (DOC) concentration) (Cuss et al., 2010; Flemming and Trevors, 1989; Hoppe et al., 2015). Moreover, mixing of metal contaminated streams with other freshwaters triggers the non-conservative behavior of metals due to the precipitation/dissolution of Al and/or Fe oxyhydroxides, and the sorption of Cu onto the formed particles (Boult, 1996; Perret et al., 2000; Ren et al., 2010; Schemel et al., 2007; Tonkin et al., 2002). In fact, several studies have addressed how changes in the pH, ORP and DOC concentration can impact the fate of Cu in rivers with low Cu/Al and Cu/Fe molar ratios (typically < 0.2) (Balistrieri et al., 2007; Boult, 1996; Carrero et al., 2015; Cidu et al., 2011; Dittmar, 2004; Lee et al., 2002; Parra et al., 2011; Sánchez-España et al., 2006; Schemel et al., 2007; Tonkin et al., 2002) (see Supplementary Material Section A). However, little is known about the partition and fate of Cu for freshwater systems critically impacted by Cu, where there is a comparatively low concentration of Al and Fe sorbents (i.e., Cu/Fe > 3; Cu/Al > 0.2).
In rivers impacted by acid mine drainage (AMD), confluences alter the particle size distribution (PSD) and metal sorption due to the multiple chemical equilibria promoted by partial mixing (Guerra et al., 2016). Such conditions promote the formation of larger particles downstream from confluences (Abarca et al., 2017), resulting in the attenuation of suspended metals by transfer from the water column to the riverbed (Kumanova et al., 2015; Ren et al., 2010). Nonetheless, the impact of confluences on the settling properties of the metal-enriched particles and subsequent physical and chemical fates of contaminants remain largely unknown in these reactive freshwater systems.
This work aimed to characterize the dynamics and partition of Cu in a highly impacted Andean watershed by acid rock drainage (ARD). We used the upper Mapocho watershed as a representative system, where the ARD-impacted headwater has Cu total molar concentrations of the same order of magnitude as those of Fe and Al. We assessed the role of confluences on Cu speciation, particularly the partition and changes toward nonlabile compounds, and on the physical properties of particles (i.e., size and settling velocity), resulting in the attenuation of Cu downstream from confluences.
Section snippets
Model site
The upper Mapocho watershed is located in the Metropolitan Region, Chile (Fig. 1a). With a length of ∼100 km, the Mapocho watershed is a critical water source for the population of Santiago. The hydrology of the upper Mapocho watershed presents strong seasonality. Rainfall occurs from May to September with a mean value of 445 mm/yr. During this period, precipitation falls mainly as snow, which persists until November when higher temperatures promote snow-melting (DGA, 2004; Di Castri and Hajek,
Hydrology of the upper Mapocho watershed
Although the mean discharge (n = 16) of the Molina River (2.7 ± 1.5 m3 s−1) was higher than values measured in the Yerba Loca River (1.2 ± 0.7 m3 s−1) and San Francisco River (0.3 ± 0.3 m3 s−1), significant seasonal variations were observed in the discharge contribution of tributaries (Fig. 1b). During the snowpack season (late April to October, n = 7), the mean discharge of the Molina River was approximately 4 times higher than mean discharge of the Yerba Loca River (Qmol = 3.0 ± 1.5 m3 s−1; Q
Conclusions
In rivers highly impacted by mining activities or natural weathering of Cu minerals, confluences enforce a key control on the partition of the metal along the watershed. Copper is transferred from the dissolved phase to the particulate fraction, as the metal impacted waters, characterized by a pH < 5 and high total molar ratios of Cu/Fe and Cu/Al, mix with rivers at circumneutral pH. This change is modulated by the capacity of freshwater influents to neutralize the ARD. At pH∼5, the sorption
CRediT authorship contribution statement
Mauricio Montecinos: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Visualization, Project administration. Marina Coquery: Conceptualization, Methodology, Validation, Resources, Data curation, Writing - original draft, Writing - review & editing, Supervision, Funding acquisition. Marco A. Alsina: Methodology, Software, Formal analysis, Writing - review & editing, Visualization. Marie Bretier:
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We thank José Díaz, Sofía Burford and Fernanda Carrasco for her support in field campaign and sample analysis. We thank Guillermo Arce for providing scripts for the modal analysis of PSD. This research was funded by projects from the National Agency for Research and Development (ANID) of Chile, ANID/FONDECYT/1161337, ANID/FONDAP/15110020, and ECOS-ANID/C15U03. Mauricio Montecinos acknowledges funding for his PhD from ANID/DOCTORADO NACIONAL/2015–21151580 scholarship. Portions of this work were
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