Is the soil of my yard safe? Tackling lead in urban soil

On July 4, 1866 in Portland, ME, an errant firework sparked a fire in an empty boat house. That fire spread, and when the night was over, more than 1,500 buildings in the harbor city had burnt to the ground. It was the greatest fire ever seen in an American city at the time, and much of Portland was reduced to a layer of ash. After the fire, debris was cleared from the streets and used as fill to increase valuable land area in the city. Vacant lots were spread with ash. The Portland peninsula was extended into the Atlantic. The city began to rebuild on top of its former self, leaving the Great Fire of 1866 buried underneath.

But today, more than a century and a half later, a scientist is digging up the past and looking for answers hidden in the city’s soil. Samantha Langley-Turnbaugh is a soil scientist at the University of Southern Maine (USM) in Portland. She joined the university in 1996 to help establish its Department of Environmental Science. Before arriving at USM, Langley-Turnbaugh spent most of her career focusing on forest soils.

Child playing in soil

“When I got to southern Maine, I found out people here don’t care much about forest soils since it’s the most metropolitan area in the state,” she remembers. “I had to find a way to retool myself in a way that made sense to the community.”

Her search brought her to Portland’s city arborist, who shared that the health of many trees in the city’s parks had been declining in recent years. They began working on a project together studying the public soils throughout much of Portland. Soon, Langley-Turnbaugh became known in the community as “that lady who looks at dirt.” As she settled into her new home in southern Maine, she began to realize that she really was far from the forest soils she’d studied in the past. The people of Portland were more concerned about contaminants in their urban soil, specifically lead.

As hundreds of buildings burned in the Great Fire of 1866, the lead paint that covered them turned into the ash that was spread across the city. Combine that event with the port town’s historical ties to lead-producing industries in the 1800s, and you begin to see why Portland’s residents might be concerned. But lead pollution is not just a problem in Portland; in cities across all of the United States, lead paint and leaded gasoline are the primary sources of soil lead contamination. As urban gardening becomes an increasingly popular trend in America, city residents want to know if the soil in their backyard is safe to plant in.

Methods of Exposure

Lead is a serious health concern, particularly to children, who are more efficient at absorbing it into their bloodstream than adults. Too much exposure to the element and a child can experience reduced brain function and developmental delays. Kids are likely to ingest lead by touching their mouths and faces after coming in contact with contaminated soils, but lead in soil can also be inhaled by breathing in high-lead dust.

Woman placing a plant in the ground

While the health effects of soil lead poisoning are well documented, the severity of certain methods of exposure is less understood. Direct ingestion of soil lead is the most dangerous method for exposure, especially in children, according to Sally Brown, a research associate professor at the University of Washington. It’s as simple as kids playing in the dirt and then touching their mouth with their hands. “Getting kids to wash their hands before eating is the most effective tool for reducing their blood lead levels,” Brown says. “And preventing direct exposure to the contaminated soil is the most important thing.”

That doesn’t mean parents need to stop their kids from playing outside. Grass cover offers a natural and effective barrier between soil lead and children and can also prevent lead-contaminated dust from blowing in the air, a method of exposure that can be harmful for adults. “For most adults, our exposure to lead isn’t from ingestion. It’s from inhalation,” Langley-Turnbaugh says. “People think, ‘I’m just growing flowers, and this isn’t an issue.’ But it’s that inhalation of soil that’s more of an issue for adults.”

Urban Gardening

Ingestion and inhalation are fairly straight-forward methods of exposure, but the risks involved with consuming produce grown in lead-contaminated soils get more complicated. The EPA lists soil-lead levels greater than 2,000 mg/kg as “very high,” stating that all types of gardening are unsafe in soils containing these levels of contamination.

In a 2007 study, Langley-Turnbaugh and co-author Travis Wagner collected soil from 182 residences in four Portland neighborhoods, finding that the mean soil lead concentrations among the residences tested was 1,362 mg/kg. While that concentration falls below the EPA’s “very high” classification, each neighborhood had at least one yard that had lead values exceeding 20,000 mg/kg.

“If it’s me, in a city like Portland where you know those high levels of lead have been reported,” says Langley-Turnbaugh, “I would not plant in that soil.”

Because of its past, Portland is an extreme case of soil lead contamination. But Brown says that in general, people considering urban gardening shouldn’t be too concerned. “Plant uptake of lead is very, very minimal.” She also points out that it’s important to consider how people are consuming the produce they grow in their gardens. In the case of Maine, cold winters mean that home-grown produce won’t be a big part of the locals’ diets throughout the year. “Eating 70% of your diet from lead- contaminated soil for 30 years is one thing, but eating three heads of lettuce per year is a completely different story,” Brown says.

People gardening in a raised garden bed

Langley-Turnbaugh urges people who are concerned about the lead content of their soil to test it so they can resolve the problem. Lead doesn’t move around much in soil, and its distribution in soil can be extremely variable, so several tests are recommended to accurately map soil lead levels in an area. Samples collected can be given to local Cooperative Extension agencies in your area to be tested.

“Soil testing isn’t a bad thing at all,” Brown adds, “but if you’re going to be worried about the results and still want to have a garden, don’t let this stop you. Raised beds are such an easy solution, and they just make sense.” Raised gardening beds are an affordable answer to high soil lead levels in places like Portland and allow gardeners to bring in fresh soil free of lead contaminants. This gardening method will likely become more common in urban areas as more people learn about soil contaminants.

Looking Towards the Future

Although the dangers of eating produce grown in soils with high lead content remains a bit unclear, direct ingestion of lead is accepted as the most dangerous method of exposure. It’s also the easiest type of exposure to protect against by doing things like improving ground cover where children are likely to play and washing your hands after handling soil. Washing produce well that was grown in lead-contaminated soil is another easy precaution to take. And while Portland’s past has resulted in high and even extreme levels of soil lead contamination, it’s important to note that cases like that aren’t very common around the country.

As researchers like Langley-Turnbaugh and Brown continue to study lead’s role in urban soils and learn more about harmful soil contaminants, their goal will remain the same: educate the public and help ensure their safety from soil pollutants. While the methods of lead exposure differ among the opinions of scientists, the effects of high soil lead levels on human health are consistent and significant enough to warrant continued action and ongoing research.

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