HYDROCHEMISTRY AND SPATIAL VARIATION OF ARSENIC, BORON AND OTHER TRACE ELEMENTS IN WATER BODIES IN VOLCANIC ZONE OF THE SUD LIPEZ PROVINCE, BOLIVIAN ALTIPLANO

The Sud Lipez province is located southwestern part of the department of Potosí. Several sub-basins such as the Laguna Colorada, Pastos Grandes and Kapina are located within the Altiplano Puna Volcanic Complex (APVC), between 4200 and 4500 m a.s.l. The region is characterized by its aridity, where precipitation is less than 150 mm/year, high evapotranspiration and minimal water infiltration. The objective was to understand the processes and mechanisms that govern the hydrochemistry and spatial variation of arsenic, boron and other trace elements, as well as the influences exerted by different water bodies in this volcanic area. Twenty water samples were taken from various sources; including springs, hot springs, streams, and groundwater. Field measurements include determinations of temperature (T), pH, Ox-Red potential (ORP), electrical conductivity (EC) and total dissolved solids (TDS). Water samples were collected for analysis of anions, cations, and trace elements.

The bodies of water that contribute to the Laguna Colorada, Pastos Garndes and Kapina show variable temperatures (3.3 - 32.4°C) with a slightly alkaline pH (6.4 - 8.5). The salinity varies from 146 to 3105 uS/cm, with oxidizing conditions. The predominant water type in the Laguna Colorada is Na-HCO₃-Cl, Pastos Grandes is Na-Cl and Kapina is Na-Ca-HCO₃. More than 73% and 33% of the samples exceed the permissible limits established for human, animal and irrigation consumption, for arsenic (As) (5.6 - 2210 μg/L) and boron (B) (0.05 - 29.1 mg/L) in the Laguna Colorada and Pastos Grandes respectively. The 33% of the samples for iron (Fe) (6.4 - 468 μg/L) and 100% for As (10 - 51.6 μg/L) exceed the permissible limits for human consumption. The predominance of the type of water is associated with the dissolution of Na- and Ca-silicate minerals, as well as the dissolution of dolomite, calcite and the presence of ulexite. However, a significant variation observed in the hydrochemical characteristics of the three sub-basin, is mainly due to the alteration of volcanic rocks by the interaction with water, which dissolves various minerals, generating solutes that contain As and B. The water bodies that contribute to the Laguna Kapina appears to be the most suitable for irrigation and animal consumption, since it presents more favorable properties in terms of water quality.