Case Study: Source of Lead in Soil

The Golden Gate Bridge was constructed in the 1930’s. It connects San Francisco and Marin County. Originally the Golden Gate Bridge was painted with a red lead primer and coated with a lead-based top coat. Because of the possible health hazards of lead, a project to remove the lead-based paint and replace it with zinc-based paint started in 1965 and lasted 30 years.

THE PROBLEM:

Soil samples that were taken from the area under San Francisco’s famed Golden Gate Bridge showed high lead content. Leaded gasoline and rust-proofing paint from the bridge were suspected as possible sources of contamination.

THE ANALYSES:

During a thorough examination of the soil using a light microscope, our experts discovered numerous paint chips, which appeared as rust-colored specks among grains of sand and particles of rounded quartz.

Soil debris showing red-orange paint chips 

Several of these specks were carefully isolated and cross-sections were prepared for further analysis. They were then analyzed by scanning electron microscopy (SEM) coupled with energy dispersive x-ray spectrometry (EDS). The analysis by SEM-EDS showed that the specks were layered paint chips and that all of the layers contained lead. Many also contained lead chromate.

THE OUTCOME:

Microscopic comparison of the sequence of paint layers in the paint chips with paint chips isolated directly from the bridge showed the same sequence of colors, strongly implicating the bridge as the source of the lead in this soil.