Arsenic capture in dams: the Elqui River case, Chile

The case of El Indio mining district in the Andean headwaters of the Elqui River basin shows the indirect effect of an irrigation reservoir on the mitigation of As and Cu content in water. El Indio (29 degrees 45'/69 degrees 58'), a mining district rich in Au, Cu, and As, is located in a wide advanced hydrothermal alteration zone and is a natural source of acid drainage. It was intensively mined between 1975 and 1997, then went through a pre-closure stage until 2002 and to a closure phase, already completed. It has been difficult to decrease the acidity and Cu content of the Malo River, that drains the district area, and of its direct receptor, the Toro river. However, better results in decreasing the As water content have been obtained by sedimentation at hydraulic facilities in the district area as part of the mine closure plan. These efforts have received an independent and not foreseen complement in the effect of the Puclaro irrigation dam, in the middle course of the Elqui River. This reservoir, operative since 2000, has shown a remarkable efficacy in decreasing As, Cu, and Fe content in the water of this important river.