The team analyzed lead concentration and isotopic composition of recent and archived samples of the lace lichen (Ramalina menziesii). The contamination extends back to the oldest sample from 1892, when lead levels in lichen from the northern reach of the San Francisco Bay estuary were 9-12 µg/g and their isotopic composition corresponded to those of high lead emissions from the Selby smelter (e.g., 206Pb/207Pb = 1.165) that were killing horses in adjacent fields at that time, the authors said.

By the mid-1950s lead isotopic compositions of lichens shifted to the more radiogenic leaded gasoline emissions (e.g., 206Pb/207Pb = 1.18-1.22). Lead concentrations in the lichen peaked at 880 µg/g in 1976, corresponding with the maximum of leaded gasoline emissions in California in the 1970s. After that, lead concentrations in lichen declined to current levels, ranging from 0.2 to 4.7 µg/g. However, isotopic compositions of contemporary samples still correspond to those of previous leaded gasoline emissions in California.

—Flegal et al.

The team suggests essentially all of the atmospheric lead now being deposited in lichen within the San Francisco Bay Area is from historic emissions of leaded gasoline that were terminated in the state nearly two decades ago and still continue from that legacy contamination.

Resources

• A. Russell Flegal, Cline Gallon, Sharon Hibdon, Zeka E. Kuspa and Lo F. Laporte (2010) Declining—but Persistent—Atmospheric Contamination in Central California from the Resuspension of Historic Leaded Gasoline Emissions As Recorded in the Lace Lichen (Ramalina menziesii Taylor) from 1892 to 2006. Environ. Sci. Technol., Article ASAP doi: 10.1021/es100246e