Gasoline-derived lead in forest soils of the northeastern United States
Beginning in the 1920s, lead was added to gasoline to improve vehicle performance. As automobile technology improved and the environmental and human health hazards of lead became apparent, leaded gasoline began to be phased out in the 1970s. Unfortunately, the lead emitted from cars decades ago has stuck around in the environment.
Researchers at Dartmouth College are studying lead in one of the places it has settled – the forests of the northeastern United States. Their paper, recently published in the Journal of Environmental Quality, describes where the lead was deposited and why patterns in forest soils may be different in various areas.
One of the places that gasoline-derived lead can be found is in the organic soils of forests, also called the forest floor. The high concentrations of organic matter in forest floors provide ideal places for lead to bind.
“Organic soils, or the forest floor, acts like a sponge,” explains Justin Richardson, lead author of the study. “Lead is a large molecule that likes to stick to surfaces, and the forest floors have a lot of charged surfaces that can strongly bind lead.”
Richardson’s graduate advisor and co-author, Andrew Friedland, had previously looked at how lead from gasoline combustion was accumulating in rural areas, such as northeastern forests. Richardson wanted to expand on that work to look deeper into the forest environments.
“My project is to build on his previous work and take it a step further,” says Richardson. “I wanted to see where the [accumulated] lead has gone – is it staying in the organic horizons of the forest floor, or has it begun to migrate into the underlying mineral soils?”
To answer these questions, Richardson and his colleagues studied 16 forest sites that had been tested for lead every 10 years for the past three decades. They found that around 60% of the lead in the western sites in Pennsylvania and New York had been lost from the forest floor since 1980. The northern sites in New Hampshire and Vermont had retained more lead in the organic matter of the forest floor compared to western sites.
One explanation for the variation in lead retention is differences in forest floor thickness – the sites in Pennsylvania and New York have thinner forest floors compared to northern sites. Richardson explains that because western sites are warmer throughout the year, more decomposition of forest floor materials can occur there. In the cooler northern sites, less decomposition means organic matter persists longer, retaining lead for additional time.
If the lead is moving out of the forest floor in western sites, where is it going? The researchers addressed that question for the first time in the current study by looking at the lead present in the mineral soils underneath the forest floor. They measured how much was there and also determined if the lead was from gasoline or naturally found in the soil (geologic). Concentrations of lead in the mineral soils of the western sites were higher, and that lead was predominantly gasoline-derived. However, in the mineral soils of the northern sites, lead concentrations were lower, and more of that lead was geologic.
“We have this gradient in which western sites are moving lead through the forest floor and mineral soil much faster because those sites have shorter retention times,” explains Richardson. “The soils in Vermont and New Hampshire have been strongly holding on to the lead, and even 30 years later it’s still lingering in the forest floor.”
From these findings, the researchers hope to dig even deeper into the forest environment. With more time, lead could move through and out of the forest soil by different pathways. Animals, such as earthworms, could disturb the forest floor. Forest fires could disrupt the organic matter.
The lead could also go deeper and deeper into the forest soils until it ends up in groundwater or surface waters. Many of the sites being studied are at high elevations and are headwaters for important bodies of water. So far, lead has not been found in the water in appreciable concentrations, but it’s a possibility that is being monitored as gasoline-derived lead remains in the environment.
“Although we removed lead from gasoline, the problem is not over. The lead is eventually going to go somewhere,” says Richardson. “The problem is being addressed, but our understanding of the situation is not completely resolved.”
View the abstract at: https://www.soils.org/publications/jeq/abstracts/43/3/926.