SEQUENTIAL EXTRACTION OF ARSENIC AND OTHER TRACE ELEMENTS IN SOILS AND ITS IMPLICATIONS ON BIOAVAILABILITY TO CROPS IN MINING AREAS OF BOLIVIA

Mineral resources in the Oruro district of Bolivia has been exploited since colonial times to present. There are very few comprehensive studies so far on arsenic (As) and other trace element (TE) contamination of water resources from geogenic sources or anthropogenic activities and their health impact on the population. The object of this study is to: i) evaluate As and TE in soils and cultivated crops (beans, potato, barley and alfalfa) and ii) determine the distribution of As in different soils fractions using sequential extraction procedures (SEP) as a base of risk assessment. The study areas are located in three semi-arid mining sub-basins in the eastern range of the Andes.

The acid extraction of soil and crop samples indicated higher total TE concentrations. Crops contain relatively higher TE concentrations as compared to reference values in the literature. From the principal component analysis (PCA) we can observe that the soils and crops indicate that metallic TEs from the mine sources (Cd, Pb, Zn, As, Cu) plot distinctly from the other element group (Si, Al, B, Sr). The plots are in accordance with the anthropogenic and geogenic sources of the contaminants in this area. PCA analysis shows that the TE concentration in the crop do not relate to the TE contents of the soils.

Therefore, sequential extraction shows that the easily exchangeable As is 0.1mg/kg (median value, n=32) and ranged (0.1 – 0.8 mg/kg). The results in the three areas indicate that the bioavailable As content is a small portion of the mobile As fraction.

The As specifically sorbed to inner-sphere surface depicted low values (0.9 mg/kg median values, n=32) and ranged 0.4 – 4.3 mg/kg, with the maximum value in the downward site in the sub-basin. The As concentration in the fraction bound to poorly and well crystalline Fe- oxyhydroxide showed 6.5 mg/kg median value (ranged 2.4 – 12.3 mg/kg, n=32) and­­­ 5.9 mg/kg median value (ranged 2.3 – 17.5 mg/kg) respectively, with As concentration higher are in the downward sites. The last three fractions may provide information on potential lability of As from different solid phases as a result of crops are grown under rainfed conditions and environmental factor (e.g. pH, redox). The residual As concentration is bound to sulfides, silicate and oxide and is not bioavailable (ranged 1.7 – 28.3 mg/kg).