Leaden Gardens

Soils in many cities of the United States carry a poisonous legacy: heavy concentrations of lead. The metal was deposited for years as fallout from flaking leaded house paint and the emissions of cars burning leaded gasoline. Recognizing the threat posed by tainted soil, environmental scientists have warned that growing edible plants in soils near streets or within several feet of homes and other painted structures risks extracting lead from the soil and bringing it to the dinner table.

Doing so could have serious health implications, the experts warn, because even low concentrations of ingested lead have been associated in children with impaired brain development, balance problems, heightened risk of tooth decay, hearing loss, and shortened stature. In adults, such lead can boost the risk of high blood pressure.

The good news, engineers at Northwestern University now report, is that even in heavily lead-tainted dirt, most of the metal usually becomes trapped in the parts of plants that few people eat, such as roots and stems. Indeed, the new data indicate that intentionally using inedible plants to sop up lead could prove a low-cost strategy for detoxifying backyards and school grounds, making them safer for children.

Root is the problem

For their new 2-year pilot study, Kimberly A. Gray and her colleagues sampled edible plants that home gardeners had been growing in two densely populated neighborhoods in Chicago. One community was dominated by brownstone apartment buildings; the other, by painted frame homes.

Gray says that few measurements of plants’ uptake of lead from soil had ever been reported before this study. Her team collected whole plants, so the researchers could compare lead deposition in roots, stems, leaves, and fruit. Sampled plants included apples, cantaloupe, corn, cucumbers, strawberries, tomatoes, zucchinis, cilantro, mint, peppers, rhubarb, carrots, radishes, and basil. The researchers also tested small quantities of the soil in which each plant was grown.

Since cooks wash garden produce before eating it, Gray’s team measured the metal’s concentrations only after a plant had been washed–either in a water rinse or a mild detergent solution. The idea was to remove any lead-tainted dirt that had clung to the plant.

Even after washing, many of the plants exhibited notable concentrations of lead–evidence that they had accumulated the toxic metal in their tissues. “From a scientific standpoint, that’s interesting, because lead is molecularly sticky,” Gray observes. “That is, it’s not very mobile. So, to get it from the soil into a plant isn’t easy.” Yet, once it did enter a plant, her data indicate, it could be transported upward into the shoots, leaves, and, occasionally, the fruit.

Invariably, roots showed the highest lead concentrations, followed by successively lower accumulations in stems, leaves, and the typically edible portions. Root concentrations varied widely, even within a given species. For instance, her team reports that one sample of soil with a lead concentration of 1,600 parts per million (ppm) produced no measurable lead anywhere in a grape plant. In another case, the roots of grapes grown in soil with 944 ppm lead showed lead contamination of 480 micrograms per gram (g/g)–though there was no measurable accumulation higher in the plant.

Most sampled plants accumulated at least some lead in their roots. The good news: In only a few species did the lead move higher into the plant. In almost every case, these were leafy crops, such as cilantro, collard greens, lemon balm, chard, and mint, or root crops such as carrot, onion, and radish. Overall, lead concentrations in the edible produce ranged from 12 to 60 g/g of tissue. Gray and her colleagues report the findings in an upcoming issue of Science of the Total Environment.

Invisible threat

The Environmental Protection Agency considers 400 ppm lead as the upper concentration that might be safe for dirt in which children play. The Northwestern engineers found that more than 75 percent of the garden soils they sampled had contamination exceeding this concentration. In many of the tested gardens, lead tainting ranged from 1,000 to 4,500 ppm.

“You’d have thought that lead levels that are so toxic to humans would stunt the growth of plants,” Gray says. “But they don’t.” Indeed, in many of the most contaminated gardens, plant growth was lush. What this means, she says, is that there’s no way a homeowner will know whether the heavy metal is present–short of sending the soil out for testing.

Moreover, the researchers had suspected that soil adjacent to painted, wood-exterior homes would have higher lead concentrations than soils around brick apartment buildings do. In fact, Gray’s team found no difference. She now suspects that this is because even brick buildings have substantial painted trim and their yards are subject to lead fallout from fossil fuel combustion.

Children are put at greater risk by lead ingestion than adults are, the Northwestern researchers note. First, lead is neurotoxic and the neural development of children continues for many years after birth. Second, children absorb between 30 and 75 percent of the lead in what they eat, whereas adults typically absorb only about 11 percent.

Gray’s team calculated that eating as little as 1 tablespoon (1.75 gram) of cilantro tainted with the lead concentration found in the new study–49 g/g–would result in an intake of 85.75 g of lead. Ingesting that amount daily, a child could build up a blood-lead concentration of roughly 10 g per deciliter of blood, which other studies have linked to several neurological deficits, including reduced IQ.

Therefore, the researchers argue, “it is imperative that people be equipped with the information and knowledge necessary to reduce or eliminate potential risks associated with urban gardening.” Toward that end, the team recommends that city gardeners plant their crops in containers or raised beds, using new topsoil. The researchers also recommend that gardeners line the bottom of the new soil with a semipermeable barrier that permits water drainage but prevents root crops from penetrating to potentially tainted soil.

Any crops brought in from the garden should be washed carefully. For leafy produce, outer leaves should be sacrificed, and root vegetables peeled.

However, Gray maintains, the best move would be to clean lead from urban soils, perhaps by trapping it in ornamental grasses or other inedible plants with substantial roots. Then the tissue could be harvested and removed for hazardous-waste treatment as part of a municipal lead-remediation program.

Currently, Gray’s group is evaluating garden species for the best candidates for such a lead-removal strategy.