17-year study confirms that lead in the soil descends slowly

In a 17-year experiment on Vermont’s Camel’s Hump, three Dartmouth researchers find that lead moves very slowly though the soil. Using the highly accurate technique of isotopic analysis for the first time at this field site, the researchers traced several varieties of lead with different atomic weights.

Their study was published online on July 12 on the Environmental Science & Technology Web site, a journal of the American Chemical Society.

“This definitively supports a few earlier studies,” says Friedland, “that show that lead in forests in the Northeast moves very, very slowly. The lead that was emitted from gasoline and settled into the soil over about 30 or 40 years is not going to end up in our drinking water anytime soon.”

This doesn’t mean we should be complacent, say the researchers. The Dartmouth team and others are working on mountains worldwide to discover how soil retains pollutants such as lead and why the lead moves so slowly through the soil.

According to the researchers, lead is one of the most widely dispersed natural contaminants in the world. At elevated levels, it can cause nervous system disorders. In children it has been linked to learning disabilities and other behavioral and developmental problems. Throughout most of the 20th century, people added lead to the atmosphere primarily by burning leaded gasoline, which eventually settled to the earth. High elevation forests, such as the one at Camel’s Hump, are good environmental indicators because they are very sensitive to atmospheric and climate conditions, and they effectively collect lead. Lead pollution is easily intercepted by the leaves on mountain trees, and rain washes it into the soil.

One piece of this study began in 1984 as part of Andrew Friedland’s dissertation research. Friedland, now a Professor and Chair of the Environmental Studies Program at Dartmouth, applied a trace amount of lead over a one-square-meter area in a mountain forest in Vermont. This lead, which was enriched with a stable isotopic signature of 207, is not toxic in small concentrations, and its atomic signature makes it easy to find, even when it descends into the soil.

In 2001, Friedland, James Kaste, a post-doctoral researcher in the Earth Sciences Department and with the Environmental Studies Program, and Stefan Sturup, Director of Dartmouth’s Trace Metal Analysis Core Facility, returned to the exact plot where the lead 207 was applied on Camel’s Hump, a heavily forested, undeveloped mountain near the village of Huntington, Vt. They took soil samples at the site, which is about 200 hundred yards off of a popular hiking trail at an elevation of about 3,300 feet, and brought them to the lab at Dartmouth for analysis.

“We found that the lead 207 applied in 1984 had only moved down into the soil about seven centimeters,” says Kaste, the lead author on the paper. “And it will probably move slower in the future because the soil becomes denser. It’s pretty rare to have a long-term study in this field, and here’s a 17-year experiment that we were able to conduct.”

Kaste also followed lead 210, which is a natural lead isotope that falls out of the atmosphere. He traced it to learn how long the forest floor, which is the top 10 centimeters of organic material at the top of the soil, retains it. He found that atmospherically deposited lead, like lead 210, will remain in the forest floor between 60 and 150 years, depending on the vegetation.

“The next step is to identify how the lead binds to the soil,” he says. “We want to learn if it binds to organic matter, for example, or if it precipitates out.”

The researchers explain that their findings are representative of deciduous and coniferous forests throughout much of the Northeastern U.S. and in some areas in Europe and Scandinavia.

“Since the forest floor retains lead for decades and decades,” says Kaste, “it could build up if we keep depositing it in levels that would be problematic, so it’s definitely good that we stopped adding lead to our gasoline.”

Adds Friedland, “No matter what you do, the natural environment records your history. So we’re leaving a legacy of this spike of lead. It will probably still be there in 500 years.”