ARSENIC DISTRIBUTION IN SURFACE WATER AND SEDIMENTS IN THE BOLIVIAN CENTRAL HIGHLAND

The purpose of this study has been to determine the arsenic concentrations in surface waters and sediments to see the correlation with the heavy metals as lead, iron, cadmium and zinc in Poopó and Uru-Uru lakes located in the central Andean highland of Bolivia. In the study area mining activities and natural weathering of rocks has resulted in several sites containing large quantities of arsenic and heavy metals in surface waters and sediments of both rivers and lakes.

Surface Water and sediment samples were taken in Poopó and Uru Uru lakes and in the adjacent rivers. Water samples were collected and for physical-chemical analysis, including anions and cations and for specific arsenic and heavy metal analysis, during each sampling.

The rivers in the North and East, strongly affected by mining activities show low pH (2.6 ≤ pH ≤ 4.3), and high pH in the case of Poopó river (8.1 ≤ pH ≤ 9.4) which receives additionally inflow from a geothermal spring, where also have the highest arsenic concentrations in surface water 2.07 mg As/L and in sediments (2400 mg/Kg). The other elements have concentrations as 0.6 mg Cd/l, 0.27 mg Pb/l, 4.8 mg Fe/l, and 7.5 mg Zn/l. The highest sediment concentrations are in Cd (120 mg/Kg) and lead 2800 mg/Kg).

The rivers in the South, and West, show arsenic concentrations, which are due to natural weathering of rocks containing As-bearing polimetalic minerals, ranging from 0.34 to 0.97 mg As/l, the concentrations of the other elements are lower than the rivers describer before (0.4 mg Cd/l, 0.07 mg Pb/l, 0.3 mg Fe/l, and 0.3 mg Zn/l). In sediments the highest concentrations correspond to Zn (230 mg/Kg) and Fe (17800 mg/Kg).

The Poopó lake is alkaline (8.3 ≤ pH ≤ 8.6) and it has high concentrations of dissolved compounds: (SO₄^2-: 3000-6000, Cl^-: 11800-12900 mg/l; Cd: ≤ 3.0, Pb: 0.21-0.53, As: 0.02-0.22, Fe: 0.1, Zn: 0.1 mg/l).

The metal concentration exceeds the health limits according to the Bolivian environmental regulations. The surface water is not suitable for drinking water. Most of the rivers receive natural contamination from the Andean Cordillera. This is due to the geology which affects the entire Poopó basin; however most of the rivers are higher in arsenic and heavy metals, The anthropogenic contamination from the mining and metallurgic activities here is notorious