Strong arsenic enrichment in sediments from the Elqui watershed, Northern Chile:: industrial (gold mining at El Indio-Tambo district) vs. geologic processes

Sediments from the Elqui watershed river system (9800 km(2); northern Chile) are highly enriched in arsenic. The river system initiates in the high altitude domain of the Andes (3500-4000 m) and drains important hydrothermal alteration zones and epithermal deposits, including the copper-, and arsenic-rich gold veins of the well-known El Indio-Tambo district. In order to study the extent, source, and age of contamination, we carried out a geochemical survey of stream sediments and older, Early Holocene lacustrine sediments. The results reveal very high contents of arsenic in both, the stream (55-485 ppm As) and lacustrine sediments (119-2344 ppm As). As shown by the ESEM-EDX study, arsenic is associated with the iron oxide mineral phase. Given that arsenic is enriched in both the stream sediments and the Early Holocene sequence, we suggest that contamination is not only industrial derived (mining operations), but also a major geological process, related to long-lived erosion of the As-rich epithermal ores and alteration zones. Erosion in this realm is particularly important during El Nino years, when increased activity of the Westerly winds brings intense rains, with catastrophic consequences in the Elqui watershed, including the massive removal of sediments in the form of large debris flows. (C) 2004 Elsevier B.V. All rights reserved.