ARSENIC OCCURRENCE IN SHALLOW GROUNDWATER IN THE CENTRAL BOLIVIAN ALTIPLANO

The study is to evaluate the extent of natural and anthropogenic groundwater contamination with arsenic and other toxic elements in the Poopó Lake basin located in the Central Bolivian Altiplano. Samples were taken from shallow wells placed at depths between two to nine metres. Groundwater from most wells is used for consumption as drinking and irrigation water. The north eastern side of the Poopó Lake is characterized by a semiarid climate and intense mining activities. The south-western side of the lake is an arid area where agricultural and cattle activities are occurring. Thirty-two well-water samples were collected following standards protocols for water sampling. The pH, electrical conductivity, temperature, redox potential and alkalinity were determined in the field. Major anions, Cl^-, NO₃^- and SO₄^2- were analyzed using an ionic – chromatograph instrument. Major cations, Na⁺, K⁺, Ca²⁺ and Mg²⁺ were analyzed using a flame atomic absorption spectrometer and trace elements and arsenic concentrations were analyzed by ICP-OES.

The pH values were circum-neutral ranged from 3.10 to 7.9 with an average of 6.83. The redox potential ranges from +141 to +417 mV (average: +199 mV). Dissolved As concentration in the groundwater ranges from below detection limit (5 µg L^-1) to 242 µg L^-1. The south west region presents the highest levels of natural dissolved arsenic, ranging from 116.8 µg L^-1 in Pampahullagas (south) to 242 µg L^-1 in Toledo (west). More than 75% of the wells exceeded the WHO guideline for arsenic (10 µg L^-1). Speciation modelling using the geochemical program PHREEQC indicates that the predominant species are HAsO₄^2- (80%) and H₂AsO₄^- (17%). Zinc, iron and manganese are found in well samples affected by mining activities with ranges of 0.01-205.3 mg L^-1, 0.01-35.2 mg L^-1 and 0.001-19.6 mg L^-1, respectively. These samples also showed elevated concentrations of Al (0.01-62.2 mg L^-1) and Si (7.3 to 26.1 mg L^-1) and among trace elements, boron shows high concentration (0.3 to 6.1 mg L^-1). The distribution of dissolved metals in some wells shows the strong impact of the local mining activities. Geographic distribution of dissolved arsenic in groundwater shows a wide variation in concentration in Poopó basin. Natural dissolved arsenic is present in the entire basin but the main impacted area is located to the south.