CONTROL MECHANISMS OF SEASONAL VARIATION OF DISSOLVED ARSENIC AND HEAVY METAL CONCENTRATIONS IN SURFACE WATERS OF LAKE POOPO BASIN, BOLIVIA

In Poopó basin (Bolivia's Andean highland), surface water samples from rivers and Poopó lake have between 0.09 to 0.14 mg As/L in areas not affected by mining activities and up to 2.0 mg As/L in rivers influenced by mining activities. Arsenic concentrations in groundwater are in the range between 0.01 to 0.09 mg/L. Different time-dependent controls, were identified as sources and sinks for dissolved species of Pb, Cd, As, Fe, and Zn, to cause element-specific seasonal changes in the chemical composition of the surface waters in Poopó basin, where natural and mining-related release of Pb, Cd, As, Fe, and Zn contaminates the basins rivers and results in an accumulation within Poopó Lake, which is receiving these waters. Sediments from the rivers and lakes contain simi-lar concentrations of As, Pb, Cd, Fe, and Zn in both, the rainy and the dry season. High-est concentrations of Pb in surface waters occur in the rainy season (e.g., Poopó Lake: 0.5-0.53 mg/l versus 0.21-0.22 mg/l in the dry season). They are caused by dissolution of Pb-containing sulfates and chlorides on the banks of Poopó Lake (which precipitated during the previous dry season) and increased erosion by rainfalls leaching Pb and trans-porting them into the rivers. In contrast, dissolved Cd is only found in the dry season (Poopó Lake 2.7-3.0 mg/l), indicating for the rainy season the existence a Cd-sink, which removes practically all dissolved Cd from the surface waters (e.g., biological up-take by aquatic plants). In Poopó Lake, the highest As-concentrations correspond to the dry season (0.21-0.22 mg/l versus 0.02 mg/l in the rainy season), whereas in most rivers maximum As-values belong to the rainy season (0.01-0.38 mg/l versus 0.01-0.05 mg/l in the dry 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 dry season, whereas increased erosion and dissolu-tion 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.