Lead that for decades accumulated in the duff layer of the forest floor in the Northeast is finally disappearing deep into the soil in low-elevation forests, though the process is taking a bit longer at higher elevations. This is according to a study conducted in the Green Mountains by Dartmouth College researcher Andrew Friedland and colleagues from the Northeastern States Research Cooperative.
Most of the lead that found its way to the forest floor came from automobile emissions from leaded gasoline, mostly in the 1960s and 70s, when lead was intentionally added to gas to improve engine performance. Much smaller quantities continue to arrive in the pollutants from coal- and oil-burning power plants. Naturally occurring lead found in many rocks contributes about 3 to 4 parts per million to the soil, but at the peak of lead concentrations in the 1980s, soils in upper elevation forests contained 150 to 250 parts per million of lead.
Friedland said that the lead can remain on the forest floor for 50 years or more.
“As we phased out lead in gasoline beginning in 1975, the amount of lead in precipitation decreased,” he said. “As trees are growing new material and dropping litter to the ground, this fresh material builds up on top of the lead that is bound to the humus. Over time, that lead slowly moves downward.”
Why is there more lead in upper elevation forest soils than in lower elevation soils? Because decomposition rates differ, according to Friedland. At lower elevations, warmer temperatures, a longer growing season, and easy-to decompose leaves speed up the process of decomposition and the movement of lead down into the soil. At higher elevations, where conifers often predominate, the cooler and wetter temperatures inhibit decomposition. Greater quantities of lead accumulated at higher elevations, too. “As you go up in elevation, you get more rainfall, and that rain would bring down more lead from the atmosphere,” Friedland explained. “There is also more cloud cover in the upper elevations, so there are more days when clouds interact with the vegetation, providing an additional input of lead.”
The researchers found lead amounts at lower elevation sites had decreased to between 0.0 and 1.0 gram per square meter of soil, down from 1.0 to 2.0 grams per square meter in 1980. Lead levels at high-elevation sites (greater than 800 meters, approximately 2,600 feet) remained about the same during the period, ranging from 1.1 to 2.9 grams per square meter in 1980 and 1.3 to 2.5 grams per square meter in the 2000s.
“The good news is that, in a way, the lead is moving from an area of the greatest biological activity – the surface layer – down into the mineral soil that contains fewer roots and insects and microbes and other biological activity,” Friedland said. “The upper surface of the forest floor accumulated the metals, but it is slowly moving down deeper. We still have this legacy of lead in the soils of the Northeast, but it’s now in a better location.”