Sources, enrichment, and redistribution of As, Cd, Cu, Li, Mo, and Sb in the Northern Atacama Region, Chile: Implications for arid watersheds affected by mining

Long-established and widespread mining activities in the Northern Atacama Region of Chile have historically impacted the environment. Most notably, the Potrerillos and El Salvador mines, until 1976, were responsible for dumping over 150 ∙ 106 tons of tailings into the El Salado River, discharging directly into the bay of Chañaral on the coast. Water resources in the Northern Atacama Region are scarce; the few include the El Salado River and the Pedernales, Maricunga, and Laguna Verde basins. This region also contains two highly sensitive national parks: the Pan de Azúcar on the coast and the Nevado de Tres Cruces in the Andes. Protecting available water resources in this inherently dry region is critical and environmental degradation that has occurred has not been reported in terms of the most important superficial pollutants. In order to specifically evaluate the metals and metalloids polluting superficial water and fluvial sediments, a 3 year-long survey was carried out in the basins of the Northern Atacama Region. Additionally, impacts of the El Salado River flood in March 2015 were evaluated. When compared to the average concentrations of dissolved elements in river water worldwide, the most enriched elements of the Northern Atacama Region are, in decreasing order: Li, As, Mo, ± Cd, Sb, and Cu. In the case of fluvial sediments, compared to the composition of the upper continental crust, samples are enriched in the following elements (in decreasing order): As, Cu, Mo, Li, ± Cd and Sb. In surface waters, dissolved As, Li, Mo and Cd are naturally enriched, concentrations of Cu and Sb are inferred to be related to mining activities. In fluvial sediments, concentrations of As, Li and Cd are of natural origin while Cu, Mo and Sb are related to the exploitation and mineral treatment of porphyry copper deposits. During the intense March 2015 flood event, contaminant elements were remobilized in the Andes Mountains and El Salado Alto Basin, and concentrations increased in the El Salado Bajo Basin predominantly due to the creation of a hydrologic connection between adjacent basins. Despite the presence of world-class porphyry Cu-Mo and iron oxide copper‑gold deposits in the region, some of which have been mined since the end of the 19th century, concentrations of dissolved Cu are lower than previously reported. This is likely related to circumneutral pH and the complexation of Cu as a cation in contrast to As and Mo which might be stable as HAsO42 − and MoO42 −, respectively, in solution over long distances