Natural attenuation process via microbial oxidation of arsenic in a high Andean watershed
Introduction
Arsenic (As) is a toxic element widely distributed in natural environments (Cullen and Reimer, 1989, Smedley and Kinniburgh, 2002). The contamination of surface and groundwater by As is a serious environmental concern because such contamination limits the use of the water and has adverse effects on human health (Berg et al., 2001, Bhattacharya et al., 2002, Smedley and Kinniburgh, 2002). Although the behavior of As in nature has been widely studied in recent years (Berg et al., 2001, Manning et al., 1998, Nickson et al., 1998, Nordstrom, 2002, Oremland and Stolz, 2003, Roussel et al., 2000, Stollenwerk et al., 2007), the biogeochemical controls governing the mobilization, stabilization, and release of As into river systems are still unclear.
The mechanisms of As attenuation in natural environments include a combination of physical, chemical, and microbial factors (Wang and Mulligan, 2006). The sorption onto iron (Fe) oxyhydroxides is the major natural attenuation process for the removal and sequestration (stabilization) of As species (Drahota et al., 2012, Leblanc et al., 1996). Co-precipitation of As and Fe also contribute to the stabilization of As in solid phase (Fuller et al., 1993). However, changes in the As speciation, particularly the oxidation from As(III) to As(V), have a crucial role in the release and mobilization of As because As species differ in their solubility, toxicity, transport, and bioavailability (Aposhian and Aposhian, 2006, Casiot et al., 2005, Drahota et al., 2009, Masscheleyn et al., 1991, Smedley and Kinniburgh, 2002). Therefore, microorganisms can oxidize As and therefore increase its sorption onto mineral phases because As(V) binds more strongly to Fe oxyhydroxides than As(III) (Oremland and Stolz, 2003). The impact of As oxidation reactions catalyzed by microorganisms on As attenuation may vary according to the climatic and geochemical conditions of a particular system (Lin and Puls, 2003, Smedley and Kinniburgh, 2002).
The occurrence of As in arid environments is widespread due to geogenic or anthropogenic sources (Baker et al., 1998, Del Razo et al., 1990, Guo et al., 2003, Smedley et al., 2002). Northern Chile is seriously affected by high concentrations (> 1 mg l− 1) of As in rivers (Alsina et al., in press, Caceres et al., 1992, Landrum et al., 2009, Pizarro et al., 2010, Queirolo et al., 2000, Romero et al., 2003, Sancha, 1999), which generates serious problems for the population reliant upon these waters in this arid region. The biogeochemical processes that control the fate of As in river systems are quite specific because the extreme conditions, which are high altitude (> 4000 masl) and high aridity (< 310 mm year− 1 of precipitation), impact the environment. In arid and semi-arid systems, metal enrichment processes may occur via capillary transport (Dold and Fontboté, 2001). The mobilized elements (e.g., As) are transported toward the surface of the soils and sediments where the presence of oxygen may facilitate oxidation (Bechtel et al., 2001, Dold and Fontboté, 2001, Smedley and Kinniburgh, 2002). Therefore, the As(III) oxidation and the As stabilization (especially as As(V) species) by adsorption processes occur predominantly under oxidizing conditions (Smedley and Kinniburgh, 2002). The role of biological reactions, particularly the reactions mediated by As-oxidizing microorganisms, in the biogeochemical cycling of As still is unknown for these types of systems.
The Chilean Altiplano is characterized by a scarcity of water and rich mining activity. Particularly, the Azufre River sub-basin contains a complex mixture of natural and anthropogenic contaminants from different sources, seriously affecting the quality of the surface water resources. We observed that in the upper section of this sub-basin, the As concentrations are high (1.0–3.5 mg l− 1); additionally, the hydrothermal waters and sediments exhibit naturally elevated concentrations of As (0.8 mg l− 1 and > 4 g kg− 1 respectively). Interestingly, the total dissolved As (AsD) concentrations are attenuated (Δ − 70% AsD), and a noticeable amount of As oxidation (Δ + 90% As(V)/AsD) occurs in the hydrothermal springs (Leiva et al., 2011). Effective risk management and remediation efforts for such As-enriched fluvial systems as Azufre River sub-basin requires the understanding and quantification of the changes in chemical speciation, as well as the biogeochemical control of the mobilization/stabilization of As in the solid phase. In this regard, the Altiplano in northern Chile is an ideal site to study the processes that control As mobilization under extreme conditions.
Section snippets
Climatic and environmental characteristics of the model site
The Altiplano is an area in the central Andes (15°–34°S) covering the western part of Bolivia, northern Chile, southern Peru, and northern Argentina, with an average altitude of 3600 masl (Allmendinger et al., 1997). The Chilean Altiplano is an elevated plateau 4000 masl covered by numerous andesitic stratovolcanoes that can reach up 6500 masl (Muñoz and Charrier, 1996, Stern et al., 2007). Particularly, the geological formations include conglomerates of the Upper Pliocene–Pleistocene, sandstones,
Experimental design in the study site, sampling and on site measurements
The hydrothermal site chosen for detailed study in this work is located on the west flank of the Tacora volcano (Fig. 1b). Two hot surface runoff channels that drain from the same hot spring were chosen for a transect sampling, each containing four sampling points for hydrogeochemical and microbiological analyses. The average flow velocities were 25.4 cm s− 1 in transect 1 and 12.9 cm s− 1 in transect 2, based on suspended particle travel times within a distance of 2 m in the surface water flow
Arsenic concentrations in the Azufre River
The AsT and AsD concentrations along the Azufre River were observed between 3.4 and 1.0 mg l− 1 (Fig. 2a), widely exceeding the maximum level for drinking water (10 μg l− 1) (WHO, 1993). The AsT and AsD were very high at the origin of the Azufre River (distance 0 m) (3.4 ± 0.3 mg l− 1 and 3.1 ± 0.3 mg l− 1, respectively), possibly due to hydrothermal discharges with high As concentrations. There were no significant differences between the AsT and AsD concentrations because the pH along the river was very low <
Arsenic natural attenuation occurs in the transects
The difference between the reduction of chloride concentrations (~ 60%) and the reduction in the As concentrations (~ 71%) along the Azufre River indicates than an active natural As attenuation process was active in this area. In particular, the reduction in the As concentrations in the surface runoff of hydrothermal waters reveals the presence of active As stabilization processes. Additionally, there were no significant changes in the ionic matrix of this runoff to suggest any local dilution
Conclusions
Our results indicate that there was a natural attenuation of As in the model system. The AsD concentrations decreased along the transects, while the conservative ion concentrations along the transects discount the possible effects of dilution or mixing of waters. The rapid oxidation of As(III), which occurred within a few meters of the hydrothermal source (As(V)/AsD increases of + Δ95% [transect 1] and + Δ100% [transect 2]), suggests the possible involvement of As oxidation in the natural
Acknowledgments
This research was supported by a FONDECYT 1100943/2010, a FONDECYT 1130936/2013, and a CONICYT grant 24121233/2012, as well as Fulbright Scholar Award 9561 for J.M.R. This study was also partially supported by a CORFO 09CN14-5709 grant and a CONICYT/FONDAP 15110020 grant. Thanks are extended to the reviewers for the corrections and suggestions, which significantly improved the manuscript.
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2022, Science of the Total EnvironmentCitation Excerpt :Arica and Parinacota (hereafter A&P) is the northernmost region of Chile, located 2000 km north of Santiago and encompassed within the Atacama Desert, the driest in the world (Clarke, 2006; Rech et al., 2010). The scarce freshwater sources from this region, including surface and groundwaters, are often characterized by slight to moderate salinity, and in many cases, by high concentrations of arsenic (As), boron (B), and other potentially toxic elements (Torres and Acevedo, 2008; Bundschuh et al., 2012; Leiva et al., 2014; Rámila et al., 2015; Guerra et al., 2016; Tapia et al., 2018, 2019; Morales-Simfors et al., 2020). The occurrence of elevated As, especially in water sources used for drinking, is a worldwide problem that potentially affects the health of millions of people chronically exposed to concentrations exceeding 10 μg/L (Bhattacharya et al., 2002; Smedley and Kinniburgh, 2002; Ravenscroft et al., 2009; Bundschuh et al., 2012; Polya and Lawson, 2015; Khan et al., 2020), which is the current provisional guideline by the World Health Organization (WHO, 2017) and the Chilean permissible limit (INN, 2006).