NATURALLY OCCURRING ARSENIC IN DRINKING WATER FROM GROUNDWATER AND SURFACE WATER SOURCES IN THE SOUTHERN PART OF THE POOPÓ LAKE BASIN, BOLIVIAN ALTIPLANO

The presence of natural arsenic (As) from the perspectives of drinking water quality was studied in a rural area in the Bolivian Altiplano where groundwater from dug wells and surface water sources are used for drinking purpose. Water samples from forty-one wells and four rivers were collected at seven different sites. pH, electrical conductivity (EC), temperature, redox potential and alkalinity were determined in the field. Major anions were analyzed using ion chromatograph, while major cations and trace elements including arsenic concentrations were analyzed by ICP-OES. The pH ranged from 6.1 to 9.6 with an average of 8.3. Samples showed high EC in a range from 316 to 19670 µS/cm, with an average of 1646 µS/cm. Redox potential in groundwater shows a relatively oxidized environment. Principal components in the samples were sodium, chloride and bicarbonate indicating a Na-Cl-HCO₃ and Na-Cl water types. Total dissolved As concentration in drinking water ranges from 3.5 to 623 µg/L with an average 118 µg/L (n=48). Ninety-five percent of the drinking water samples exceed the WHO guideline (10 µg/L). Water samples collected from wells located in lower terrains contain highest levels of total dissolved As. Arsenic speciation indicates that the predominant specie is As(V). Zinc concentrations range from 15.6 to 4034 µg/L (average: 136 µg/L), Al concentration range from 3.4 to 44 µg/L (average 9 µg/L), Li between 52.6 to 8864 µg/L (average 1148 µg/L) and V ranging from 0.9 to 39 µg/L with an average of 10 µg/L. Among redox sensitive elements, Fe and Mn show wide variability in the ranges of 8.3–994 µg/L (average 124 µg/L) and 0.9–2787 µg/L (average 134 µg/L), respectively. The water resources in the area are severely impacted by the presence of high As concentrations and high salinity levels that make the water unsuitable for human consumption. Water in contact with alluvial sediments in wells located in lower terrains mobilize As more easily than that in the elevated terrains with predominance of volcanic and sedimentary rocks. The presence of As in drinking water is also attributed to the oxidation of sulfide minerals such as arsenopyrite in the mineralized hard rocks surrounding the basin. Further studies in the area will be carried out to develop a conceptual model of the genesis, mobilization and transport of As in the region.