Determination of trace element provenance, Rio Loa basin, northern Chile.

摘要

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. Water within the Rio Loa is an extremely important resource, but its water and sediment are contaminated, exhibiting levels of trace metals and metalloids that exceed drinking water standards (e.g., for arsenic) and threshold effect guidelines for aquatic biota (e.g., for copper, cadmium, antimony, and zinc). 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 copper 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 arsenic concentrations. This study utilizes both total elemental concentrations (arsenic, antimony, copper, and lead) along with isotopes of antimony and lead to distinguish contaminant sources in the Rio Loa. Additionally, a sequential extraction procedure provided additional geochemical understanding of the elemental dispersal pathways via sediment binding.;Isotopes of antimony and lead did not provide enough information to distinguish contaminant sources. Total concentrations of arsenic, antimony, and copper proved to be more informative, the largest source of copper contamination was determined to be the copper mining operations of El Abra, Radiomiro Tomic, and Chuquicamata. But mining did not input significant quantities of arsenic or antimony, both of which were found in very high concentrations within the Rio Loa. Results showed that El Tatio Geyser Basin input most of the arsenic and antimony contamination. For this reason, a ratio of antimony/copper proved to be a good contaminant tracer to distinguish and quantify contaminated sediment from El Tatio and mining operations. This ratio was applied to floodplain cores and older terrace deposits to determine how sediments have been distributed within the basin over time.;The El Tatio Geyser Basin proved to be the largest contaminator within the Rio Loa basin, mostly due to older terrace deposits, which are high in contamination from El Tatio, continually being eroded and re-worked into the floodplains and channel bed deposits of the lower reaches of the Rio Loa. Contaminated sediment from copper mining operations was not as relevant of a concern because copper transport downstream is inhibited by its adsorption onto Fe-Mn oxide rich particles, which are quickly deposited and/or diluted within the Rio San Salvador (a tributary) before reaching the Rio Loa.