Mercury contamination level and speciation inventory in Lakes Titicaca & Uru-Uru (Bolivia): Current status and future trends☆
Graphical abstract
Box plot presentation with error bars for percentage methylmercury over total mercury for filtered and unfiltered surface waters along the upstream - downstream gradient from Lake Titicaca to Uru-Uru. Vertical bar chart show the median (upper full line) and standard deviation (SD) of each data set (N is the number of samples).
Introduction
The Bolivian Altiplano is one of the largest high plateaus in the world containing two large lakes, Lake Titicaca in the north part and Lakes Uru-Uru/Poopó in the central part of an endorheic system consisting in the Titicaca-Desaguadero-Poopó-Coipasa Salt Lake (Revollo, 2001, Delclaux et al., 2007). Lake Titicaca is the most important water resource of the Andean Altiplano, a major source of fish for ∼3 million people and the largest navigable water body in the world lying at an altitude of 3809 m above sea level (a.s.l.). Its ecological functioning and limnology have been widely investigated between 1980 and 2000 (Collot et al., 1983, Pourchet et al., 1994, Mourguiart et al., 1998, Dejoux, 1992). Nowadays, the ecological equilibrium of the region is disturbed by a recent but very intensive urban demography and the intensification of mining activities, fisheries and agriculture around the Lake. For example, Puno Bay, on the Peruvian side of Lake Titicaca, has been identified as a contaminated area by urban and industrial effluents since the 1980's (NorthCote et al., 1989). The most preoccupying issue on the Bolivian side of the Lake (Lago Menor – southern basin of the lake) is the extremely rapid development of El Alto city, which population increased from 95,000 inhabitants in 1976 to around 1.2 million according to the last census (Mazurek, 2012) with minimal land planning. Wastewater from El Alto city, its facilities, manufactures and small scale industries, are discharged in the Katari river, which flows to the Lago Menor with less than 50% of water treated (Chudnoff, 2009). Among the emitted pollutants (e.g., nutrients, traces metals and organic contaminants) (Duwig et al., 2014, Archundia et al., 2017), mercury (Hg) contamination is one of the preoccupying issues in this fragile ecosystem. So far, only three studies focused on Hg contamination in the coastal areas of the great Lake Titicaca (Gammons et al., 2006, Molina and Point, 2014, Monroy et al., 2014) revealing high Hg concentrations in all fish species collected in the northern part of the Lago Mayor, particularly at the Mouth of the Ramis River (up to 1840 ng g−1 THg, dw in Orestias agassii) where intense gold mining activities are documented. Hg levels in fish muscle from other coastal areas of the Lago Mayor where found below regulatory health guidelines. No data exists yet in Lago Menor which is considered as the most productive area and is also impacted by anthropogenic inputs.
Downstream Lake Titicaca, Lake Uru-Uru, a man-made reservoir, combines diverse metal and organic pollutions since it receives important discharges from urban and mining activities. Recently, several studies evaluating Hg contamination in Lake Uru-Uru have highlighted high concentrations and percentages of toxic MMHg in the water column and biota (Molina, 2015, Alanoca et al., 2016a, Alanoca et al., 2016b, Lanza et al., 2016).
In this paper, we report for the first time, a complete inventory of Hg levels and speciation, including inorganic Hg(II), elemental Hg (Hg0) and mono-methylmercury (MMHg), from samples collected between 2010 and 2016 in the different compartments of Lake Titicaca and Uru-Uru. Hg was measured in the atmosphere (total gaseous Hg - TGM), atmospheric fallouts (i.e., filtered and particulate Hg), water column (i.e., filtered, particulate and dissolved gaseous Hg), sediment (i.e., porewater and solid particles) and biota (i.e., plankton, fish and birds). Concentrations of Hg species are compared with previous studies performed in large or elevated lakes. Biogeochemical and anthropogenic factors influencing the sources, distribution and speciation of Hg in the different limnological compartments and selected sentinel organisms are discussed. Based on these discussions and the observed spatial gradients in the ecosystem, we finally propose scenarii of possible future trends in Hg species levels and their impacts on this ecosystem.
Section snippets
Lake Titicaca
Lake Titicaca (Fig. 1a) comprises of two nearly separate basins: the great lake named “Lago Mayor” or “Lago Chucuito” (7131 km2; mean depth = 100 m; max depth = 285 m) and the smaller lake named “Lago Menor” or “Lago Huiñaimarca” (1428 km2; mean depth = 9 m; max depth Chua trough = 40 m) (Dejoux, 1992). The two basins are connected by the Strait of Tiquina and a single outlet for the lake, the Río Desaguadero, which drains out the southern end of Lago Menor to the central Altiplano (i.e., to
Mercury speciation in air and atmospheric fallouts of Lake Titicaca
Total gaseous mercury (TGM) concentrations in air on the shore of Lake Menor were not significantly (p > 0.05) different between the end of the wet (1.11 ± 0.23, N = 1407) and of the dry season (0.82 ± 0.20 ng m−3, N = 1037). Measured TGM concentrations were in the lowest range of the background concentrations (Table 1) reported for the northern (1.5–2.5 ng m−3) and for the southern hemispheres (mean GEM: 0.84–1.09 ng m−3) (Meuleman et al., 1995, Lin and Pehkonen, 1999, Vette et al., 2002,
Acknowledgments
This work is a contribution to the LA PACHAMAMA project (ANR CESA program, No ANR-13-CESA-0015-01, PI: D. Amouroux: [email protected]), COMIBOL project (INSU CNRS/IRD EC2CO Program, PI: D. Point: [email protected]), EUTITICACA project (founded by the Impuestos Directos a los Hidrocarburos IDH administrated by the Universidad Mayor de San Andrés, PI: D. Achá: [email protected]) and TRACISOMER supported by a grant from Labex OSUG@2020 (PI: S. Guédron: [email protected]). S.
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This paper has been recommended for acceptance by Prof. W. Wen-Xiong.