Unleaded Fuel Credited for Lead-Level Drop in Soil

The switch to unleaded gasoline may have saved large tracts of trees in Northeastern forests.

Scientists a decade ago feared that the buildup in soil of lead, which is toxic, would damage and possibly kill trees. But 15 years after the Clean Air Act forced auto makers to market cars that burn unleaded gasoline, scientists say lead concentrations in the forest floor already are falling.

“It’s beautiful, the curve showing the decline of lead,” said Thomas G. Siccama, a forest ecologist at the Yale University School of Forestry and Environmental Studies. “It has left the forest floor faster than the general community of ecologists thought.”

In 1980, Siccama and two other scientists, William H. Smith of Yale and Donald L. Mader of the University of Massachusetts, documented that from 1962 to 1978, lead levels in surface soils in Massachusetts had jumped from 682 milligrams per square meter to 1,160 milligrams.

Siccama, Smith and others, including Andrew J. Friedland of Dartmouth College and Arthur H. Johnson of the University of Pennsylvania, began looking more closely at the surface soil of forests, taking samples from Pennsylvania to New Hampshire.

Over the last year, they and other researchers have begun to review the extensive data collected.

At one test site, the Cathedral Pines forest in Cornwall, Penn., Julie Tritschler, a Yale graduate student, found lead has fallen since 1980 from 1,275 milligrams per square meter to 940 milligrams.

At Hubbard Brook in West Thornton, N.H., lead levels in 1975 approached 1,100 milligrams per square meter. By 1988, they had fallen to less than 800 milligrams, Siccama said.

What is happening, Siccama believes, is that over time the lead is being diluted in the underlying soil.

Michael J. Kellett, Northeast regional director for the Wilderness Society, said the data is encouraging. But he cautioned that it is important to learn where the lead is going because it might be ending up in underground water supplies.

Siccama and others say the decline in lead levels in soil is almost certainly due to the mandated use of unleaded gasoline in new cars beginning in 1975.

“That was the whole point of the law,” Siccama said. “It is a one-on-one connection. We know it has gone down in the rain, and we’ve followed it going down in the soil where we knew it was accumulating.”