A RECORD OF HEAVY METAL CONTAMINATION AT MOUNTAIN LAKE IN THE PRESIDIO NATIONAL PARK, SAN FRANCISCO, CALIFORNIA, USA

Sediment cores from Mountain Lake, a natural lake in the Presidio National Park, San Francisco, provide a record of unusually high heavy metal contamination with lead and zinc concentrations peaking ca. 1975 at 8528 ppm and 1327 ppm respectively. Independent dating of the cores by exotic pollen and ²¹⁰Pb indicates the high concentrations post date the construction of a highway next to the lake in 1940. Downcore variations in lead concentrations closely reflect the history of land use around the lake. Background concentrations are low and vary little during the pre-European period (before 1776). Slight increases after 1776 reflect increased erosion in the watershed of the lake associated with the introduction of cattle and the beginning of Spanish settlement. Significant increases in lead in the modern period are coincident with the construction of US Highway 1 alongside the lake in 1940. Spatial analysis of lake surface samples point to runoff from the roadway as the source of lead contamination. Lead levels peak in 1975 and decline towards the surface, reflecting the history of leaded gasoline use in California. Zinc derived mainly form automobile tires, follows a similar pattern to lead but continues to increase in recently deposited sediments. The lead contamination is unprecedented for a natural lake in the United States and is of particularly concern for the National Park Service. Little is known about the biological impacts of the pollution although waterfowl mortality at the lake has been attributed to lead poisoning.