Transport of Arsenic and Heavy Metals to Lake Poopó - Bolivia Natural Leakage and Anthropogenic Effects
Sammanfattning: Bolivia, a country located in the center of South America, has in the west part an Andean region, where the Poopó Lake is located. The area has many mineral resources, and thus since thousands of years,mining activity has had a large importance; however, mainly since colonial times, the extraction of these resources has been indiscriminate and the region of the Poopó Lake, has suffered the consequences of environmental contamination. The Poopó basin has been contaminated due to a natural leakage of heavy metals from minerals with toxic contents, high levels of salinity, and the contamination of organic sewage effluents from urban areas. Additionally, climate conditions aggravate the problems. The area is semi-arid and cold with a large change in temperatures between day and night. Furthermore the region has two distinct seasons: a dry season and a rainy season. In this study, arsenic and heavy metals have been analyzed (cadmium, zinc, lead, and iron) in surface waters, sediments, groundwater and biota from the rivers of the Poopo basin, the Poopó and Uru Uru Lakes and wells around the basin. The results indicate high natural contamination from weathering of minerals with high concentrations of lead and arsenic, generally dry conditions which results in high salinity in water and soil, and most importantly, anthropogenic contamination from the intensive mining and metallurgic activities. The metals are transported from the mines through the rivers to the Poopo Lake. This transport is influenced by season; high river flows create high sediment transport and thus high contaminant transport. In addition, some remediation possibilities were studied. The effectiveness of phytofiltration and biogenic sulfide precipitation to remove heavy metals from acid mine drainage were compared. The results show that the most effective sorption material in the phytoremediation is the macro-algae and thereafter the totora sulfide producing bacteria were the most effective method for removing heavy metals. Different time-dependent controls were identified as sources and sinks for dissolved species of arsenic and heavy metals. This behaviour cause element-specific seasonal changes in the chemical composition of the surface waters in Poopó basin, where natural and mining-related release of those elements contaminates the Basins Rivers and results in an accumulation of contaminants within Poopó Lake. The highest concentrations of Pb in surface waters occur in the rainy season and are probably caused by the dissolution of Pb containing sulfates and chlorides on the banks of the Poopó Lake (which were precipitated during the previous dry season), and increased rainfall induced erosion which causes the leaching of Pb which is then transported back into the rivers. In contrast, dissolved Cd is only found in the dry season indicating that during the rainy season a dilution effect occurs or a Cd-sink which removes practically all dissolved Cd from the surface waters exists. In the Poopó Lake, the highest As- concentrations correspond to the dry season whereas in most rivers in the basin maximum As-values are found in the rainy season, indicating for both areas different dominating As-mobility controls. In Poopó Lake dominance of control by evaporation, and a possible uptake by biological sinks explains the As- maximum in the dry season, whereas increased erosion and dissolution of weathered solids and transport into rivers explains the As maximum in the rivers in the rainy season. Dissolved Fe and Zn do not show significant seasonal variations.
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