HEAVY METAL DEPOSITION IN LAKE JUNIN PERU

Lake Junin (11.0°S, 76.2°W) covers approximately 300 km² and is located at 4000 m asl in an intermontane basin between the eastern and western cordillera of the central Peruvian Andes. Lake Junin was designated a National Reserve in 1974; it is situated between the mining districts of La Oroya (50 km south of Lake Junin) and Cerro de Pasco (30 km north of Lake Junin). Lake Junin drains northward to the Rio San Juan, which joins the Rio Montaro within several kilometers of the Lake. Dam construction on the Rio Montaro in 1930 caused the Rio San Juan to back up and drain directly into Lake Junin. The Rio San Juan is the principal river draining Cerro de Pasco, a large sulfide mining district, and an unintended consequence of damming the Rio Montaro was the contamination of Lake Junin with significant quantities of heavy metals. The objectives of this study are to document the heavy metal contamination of Lake Junin sediments, and to quantify the contribution of heavy metals delivered via surficial runoff by comparing metal concentrations in Lake Junin sediments with that in other lakes in the region that do not directly receive run off from mining districts. We acquired seven sediment cores 0.5-1.5 meters in length from locations across Lake Junin. Cores were sampled in the field every 0.5-1.0 cm. Sub-samples were treated with dilute HNO₃ and the resultant supernatant was analyzed on an ICP-MS for Ba, Co, Cu, Fe, Mn, Pb, Sr, and Zn. Results reveal dramatic 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 have substantial increases including: Cu (28 to 6400 ppm), Pb (25-2400 ppm), and Fe (4706 to 600,000 ppm). A CHiRP seismic survey (4-24 kHz) of Lake Junin reveals the contaminated sediment is present in an acoustically distinct layer, between ~20 and ~60 cm thickness, that is consistently present across the lake below ~3.5 m depth (probably the base of wind driven circulation). Comparison of the sediment record from Lake Junin with other lakes in the region indicates that metal laden sediment transported into Lake Junin from the Rio San Juan has formed a deposit with metal concentrations at least one order of magnitude greater than lakes that received only atmospherically transported metals.