Occurrence and distribution of heavy metals in three rivers on the Bolivian high plateau

The basin of the Poopó Lake, situated on the Bolivian high plateau, is severely contaminated by the mineral extraction and processing activities within the area. Many rivers in the area contain high concentrations of arsenic, lead and cadmium both in the water and the sediment phase. Moreover, huge amounts of mineral waste are situated in the area. In this study the environmental conditions of three specific rivers, with focus on heavy metals, were investigated. Water and sediment samples were taken along the rivers Huanuni, Poopó and Marquez during October and December 2005 with the ambition to quantify the concentrations of the metals and determine how much the mining activities account for, how the concentrations develops along the river and how the metals dissolved in the river and bound in the sediments interact. During the second sampling trip the rain period had not yet set in; hence the results are representative for the dry period. All rivers were found to contain high levels of dissolved arsenic and lead, exceeding the WHO-guideline values for drinking water. The lead concentrations were similar in all rivers and varied between 0.15 and 0.49 mg/l. In the Poopó River thermal waters causes very high concentrations of arsenic which are higher than previously reported values; as high levels as 12 mg/l were observed. The processing plant pollutes the river with cadmium and levels of 0.04 mg/l were encountered. It may also be responsible for the higher pH-values encountered in the downstream parts. Adsorption of metals on suspended particles and deposition of leftovers from the mining activities are thought to be responsible for significantly higher levels of arsenic, cadmium and zinc within the sediments at the most downstream parts of the river. The Huanuni River is totally deteriorated because of the mining activities. The mining centre is the major pollution source causing elevated levels of arsenic, cadmium, iron and sulphate and lowers the pH-value to about pH 3. However, the small scale mining along the river and leakage from the waste piles and the river's sediment also contributes with large amounts of arsenic and acidity. Arsenic, cadmium and iron levels of 10, 1.4 and 500 mg/l were observed and mass flows to the Poopó Lake as high as 300, 20 and 3000 kg/day were calculated. The mining activities also increase the concentrations of arsenic, cadmium and zinc in the sediments greatly. Deposition of mining material seems to be controlling the zinc and cadmium concentrations, whereas adsorption on suspended particles followed by sedimentation seems to control arsenic. The unpolluted Marquez River contains low levels of arsenic, cadmium, lead and zinc in the sediments, but the concentrations of arsenic and lead in solution were high. Further studies, measuring suspended particles and organic content, are recommended for the understanding of the transport mechanisms and the interactions between metals in sediment and water phase. For the understanding of seasonal variations studies should be conducted during the rain period as well as during the dry period