DETERMINATION OF TRACE ELEMENT PROVENANCE IN THE RIO LOA BASIN, CHILE

The Atacama Desert, located between the Pacific Ocean and the Andes in northern Chile and southern Peru, is one of the driest regions on the planet. In spite of the extreme aridity, the Atacama is traversed by the Rio Loa, a perennial river which owes its continuous flow to precipitation and runoff at high elevations (>4000 m) along the western Andes, and the emergence of groundwater from thick alluvial aquifers. Although water within the Rio Loa is an extremely important resource, its water and sediment are contaminated, exhibiting levels of trace metals and metalloids that exceed drinking water standards (e.g., for As) and/or threshold effect guidelines for aquatic biota (e.g., for Cu, Cd, Sb, and Zn). Previous studies, combined with data collected in 2009, suggest that trace metals/metalloids are derived from multiple sources, including El Tatio (a large geyser basin) and three large Cu mines. Determination of the relative contribution of contaminants to the river from the geyser basin and the mines has proven problematic using spatial patterns in trace metal/metalloid concentrations. To address the problem, selected isotopic tracers, including ¹²¹Sb and ¹²³Sb, are being used to assess contaminant provenance. Sb isotopes have rarely been used as geochemical tracers primarily because their analysis is more difficult than that of many other trace metal isotopes and concentrations of Sb are typically low. Our preliminary results, however, are consistent with other studies that show Hydride Generation/ICP-MS systems can be used to effectively determine the Sb isotopic composition of sediment. Moreover, our initial data suggest that Sb isotopes may serve as an effective sediment tracer in the Rio Loa basin because (1) water discharged at El Tatio contains some of the highest reported surface water concentrations of Sb in the world, (2) sequential extraction experiments conducted on sediments from El Tatio and downstream drainages show that Sb is essentially immobile after its incorporation into the sediment, and (3) concentrations of Sb are relatively low in mine-contaminated sediment.