HYDROCHEMICAL ASSESSMENT AND SPATIAL VARIABILITY OF TRACE ELEMENTS IN GROUNDWATER IN THE LOWER KATARI BASIN, BOLIVIAN ALTIPLANO

Lake Titicaca is being polluted by heavy metals, nutrients and microbial contaminants. The pollutants are transported by the rivers Seque and Seco (both located at upstream of the basin) and the Pallina river, connected to the Cohana Bay through the Katari River. Hydrochemical investigations were carried out in order to better understand the spatial distribution of trace elements (TEs) such as As, Sb, B, Al, Mn and F.

Groundwater is slightly alkaline (mean pH 7.8), with Eh values (mean 189.3 mV), indicating a moderately oxidizing environment in the aquifers and characterized by high salinity ( EC up to 7984 μS/cm). The groundwater are generaly Na-SO₄-Cl type, Na-Ca-Mg-HCO₃ type and Ca-Na-HCO₃-Cl type. 22% of the samples exceeded the WHO guideline for NO^-₃ (50 mg/L). 87.5 % of the groundwater samples exceed the Bolivian regulation (NB-512) for Sb (5 μg/L), 56.2 % for B (300 μg/L) and 50 % for As considering the WHO guideline values (10 μg/L). Speciation of As indicates that the predominant oxidation state is As (V). The geochemical modelling with the calculation of the mineral saturation indices indicate that As could be associated with iron oxides and hydroxides which are probably the most important mineral phases for the As adsorption.

The spatial distribution of As shows a positive spatial autocorrelation (SA) to the northwest (Cohana Bay). The interaction between surface water and shallow groundwater within the volcanic formations and their dissolution could be the natural source of As. Boron indicate a positive SA close to the confluence of the Pallina and Katari rivers in the aquifers within the alluvial, colluvio - fluvial deposits due to anthropogenic contamination due to the agricultural activity. Antimony shows high spatial autocorrelation northeast of the study area result mainly from weathering of the carbonate rocks in the Bolivian antimony belt of the Eastern Cordillera. The study indicates that 64% of the wells sampled close to the Katari River, are impacted by the presence of high As, NO^-₃, and high salinity levels. The occurrences of Al, B, As, F and Sb indicate significant spatial relationships. More than 50% of the groundwater samples exceed the NB-512 and WHO guidelines for Sb, B and As and raise significant concern about drinking water quality.