AN UNINTENDED CONSEQUENCE OF THE JUXTAPOSITION OF HYDROELECTRICITY AND MINING IN THE PERUVIAN ANDES: THE HEAVY METAL CONTAMINATION OF THE LAKE JUNIN NATIONAL RESERVE
Lake Junín (11.0°S, 76.2°W) covers ~300 km2 and is located at 4000 m asl in an intermontane basin in the central Peruvian Andes. Lake Junín was designated a National Reserve in 1974; it is situated between the mining districts of La Oroya (50 km south) and Cerro de Pasco (30 km north). Lake Junín drains northward to the Río San Juan, which joins the Río Montaro within several kilometers of the Lake. The construction of the Upamayo Dam on the Río Montaro in 1930 caused the Río San Juan to back up and drain directly into Lake Junín. The Río San Juan is the principal river draining Cerro de Pasco, a large sulfide mining district, and an unintended consequence of damming the Río Montaro was the contamination of Lake Junín with heavy metals. This study documents the heavy metal contamination of Lake Junín, and quantifies the contribution of heavy metals delivered via surficial runoff by comparing concentrations in Lake Junín sediments with those in nearby lakes that do not receive mine runoff. We acquired six sediment cores 0.6-1.3 m in length from locations across Lake Junín. Cores were sampled in the field every 0.5-1.0 cm. Subsamples were treated with dilute 1.0 mL of HNO3 and the resultant supernatant was analyzed on an ICP-MS for Ba, Co, Cu, Fe, Mn, Pb, Sr, and Zn. Results reveal increases in most metals at depths that range from 10-50 cm below the sediment surface. Zinc has the most dramatic increase from ~300 ppm to ~50,000 ppm. Other metals show substantial increases including Cu (28 to 6400 ppm), Pb (25 to 2400 ppm), and Fe (4706 to 600,000 ppm). The sediment record from Lake Junín indicates that metal laden sediment transported into Lake Junín from the Río San Juan has formed a deposit with metal concentrations up to several orders of magnitude greater than lakes that received only atmospherically deposited metals.