Baseline investigation of (methyl)mercury in waters, soils, sediments and key foodstuffs in the Lower Mekong Basin: The rapidly developing city of Vientiane (Lao PDR)☆
Introduction
Mercury (Hg) contamination of tropical ecosystems has been studied in many countries around the world, in part because of the perceived threats to the local human population arising from the consumption of contaminated fish and rice (Brabo et al., 2000, Feng et al., 2008, Hall et al., 1997, Kuwabara et al., 2007). Among Hg species, monomethyl-mercury (MMHg) has been reported as the most dangerous species for local populations (Cheng et al., 2009, Fitzgerald and Lamborg, 2003, Zahir et al., 2005) in part due to its ability to biomagnify in the aquatic food web (Boudou et al., 2005, Durrieu et al., 2005). The main anthropogenic sources of Hg reported in southern tropical ecosystems are mining activities (i.e., gold and silver mining), industrial waste, fuel and coal combustion and the use of wastewaters for agricultural irrigation (Feigin et al., 1991, Swain et al., 2007).
Currently, little information is available regarding Hg contamination of the Lower Mekong Basin, and in particular in Laotian ecosystem where no published data on Hg are currently available. The rapid growth of the main cities located along the Mekong River during the last few decades, might be a potential source of Hg contamination for ecosystems through wastewater irrigation and waste, fuel or coal combustion (Feigin et al., 1991, Swain et al., 2007). Notably, Vientiane city (Lao PDR) has almost doubled its population during the past 15–20 years, now approaching 300,000 people (Rafiqui and Gentile, 2009). Despite this rapid expansion, Vientiane remains largely non-industrial with the urban fabric of the city being interspersed with paddy fields and orchards, rather than factories (Rafiqui and Gentile, 2009). Wastewater treatment systems are largely absent in the city and most untreated wastewaters from the urban area are discharged into the Houay Mak Hiao (H.M.H.) canal and thence to the large That Luang wetland, where the local population fish and use the water for the irrigation of rice paddies (Whelan et al., 2007).
In this context, we tried to assemble the first inventory of Hg distribution in the aquatic ecosystem of the emerging Vientiane city capital which combines both non-urbanized and urbanized areas. A special emphasis was placed on Hg speciation measurements (i.e., THg and MMHg) in the dissolved phase and THg was measured in the particulate phases of soils and sediments to evaluate the distribution of Hg in both urbanized parts of the city (i.e., the wastewater canal, agricultural areas) and “non-urbanized” areas such as wetlands and the Mekong River. We also measured Hg concentrations in a limited set of terrestrial (rice) and river (i.e., bottom feeder and carnivorous fish) key food stuffs collected in Vientiane in rice paddies and in the Mekong River, respectively, to evaluate potential health risks for the local population.
Section snippets
Geographic settings
Lao People's Democratic Republic (Lao PDR) is one of the poorest and least developed countries in South-East Asia (Kaufmann, 2003) and with the largest access to the Mekong River. Of the total population (~ 6.2 million inhabitants in 2008, World_Bank, 2010), 65% live along the Mekong and adjacent lowlands (Kaufmann, 2003). Agriculture remains the major sector of the economy providing more than 34% of the Gross Domestic Product (GDP) and employing over 80% of the labor force (NAFRI, 2008). Most
Baseline inventory of mercury in soil and sediment
Total mercury concentrations ([THg]) measured in soil and sediment of Vientiane area (Fig. 2) were low, being in the range of concentrations found in the Mekong River delta sediments (22–90 ng g− 1, Cenci and Martin, 2004) or in other non-urbanized environments (e.g., 30–100 ng g− 1, Heyes et al., 2000, Roulet and Lucotte, 1995) and far below the probable effect concentration (PEC) for Hg (1060 ng·g− 1); the concentration above in which harmful effects are likely to be observed in sediment-dwelling
Conclusion
Regarding Hg contamination, the ecosystem of Vientiane city district is in a pristine state according to low concentrations of THg (and MMHg) measured in water, soil and sediment of both non-urbanized and urbanized areas of the city and to its non-industrialized status. Although this set of data is not exhaustive, it presents the first baseline inventory of Hg and MMHg in Lao PDR which could be used as a baseline reference in the region for future work with regard to the extremely rapid
Acknowledgments
This study was funded by the European Union FP6 funded CALIBRE Asia-Link Programme (Contract No. KH/Asia-Link/04 142966). The authors acknowledge Associate Professor Lammai Phiphakhavong, Dr Bounthanh Bounvilay and students from the National University of Laos (Lao PDR) for their cordial welcome, help and advice in the field work, including sampling site selection. We also acknowledge the Mekong River Commission (MRC) based in Vientiane, who performed the sampling of biota along the Mekong
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Electronic Supplementary Information (ESI) available: Supplementary figures SI.1 and SI.2 show multi-resolution Electrical Resistivity images obtained at the non-urbanized wetland (WL) and rice paddy (RP) sites, Supplementary Table SI.3 shows water chemical parameters obtained in the Vientiane district area, Supplementary Table SI.4 shows the biota characteristics, Supplementary figure and Table SI.5 show the map of complementary biota sampling locations along the Mekong River (from the 2011 MRC campaign) with associated Hg concentrations and locations of known gold mining locations, and Supplementary Table SI.6 shows the biota key characteristics, including mercury concentrations.