Study: NYC’s urban gardens have a serious lead problem

Reporting by Erica Berry

Source: Soil Science 180, Special Issue. April-May 2015
Source: Soil Science 180, Special Issue. April-May 2015

Lead contamination, often acute, is common in New York City’s urban garden soil, according to a new study from the City University of New York. Published in the journal Soil Science last week, the study evaluated 1,652 soil samples, volunteered from 904 home and community gardens, for contamination by a number of trace metals. It says the city’s soil contains higher average concentrations of lead than a number of other metropolitan areas, including Hong Kong, Beijing, London, Bangkok, Berlin and Baltimore.

Fewer than 3-percent of the sites tested would qualify as safe for “unrestricted” use, based on the Soil Cleanup Objective (SCO) criteria set by the state Department of Environmental Conservation. An “Unrestricted” level of contamination is one in which no further soil remediation is necessary to protect groundwater or public health. The remaining 97 percent of the samples fall into the “restricted” use category and, depending on the level of contamination, would need some amount of chemical, biological or engineered remediation. (The paper also notes that though researchers applied SCO standards to assess contaminant levels, these criteria were developed for New York’s rural soil, so they may not be entirely appropriate for urban soils, which tend to be much more widely contaminated. There are currently no specific guidelines to regulate soil contaminant levels of the city’s gardens.)

Brooklyn, the city’s hub for urban farming and all things artisanal, fared the worst of the five boroughs. Researchers received the highest number of soil samples from Brooklyn, and they also found the highest levels of contaminated soil there—lead levels were twice as high as in Bronx soils, for example. This patchwork of contamination mirrors the unequal distribution found in other cities, where pollution tends to be concentrated in old industrial centers. The Gowanus Canal, for instance, which cuts through the heart of brownstone Brooklyn, is lousy with everything from PCBs to heavy metals to “volatile organics.” In 2010 the Environmental Protection Agency added it to the list of Superfund sites.

The study also found a large discrepancy between contamination in community and backyard gardens: 71 percent of home-garden samples exceeded the respective SCO limits for arsenic and lead, while only 21 percent of community gardens did. Public gardens tend to be more actively cultivated than backyard ones, and additions of new compost or topsoil can dilute contaminants.

The good news? Assuming you wash it first, produce grown in these gardens is probably okay to eat. There is little relationship between the contaminant levels of rinsed kitchen vegetables and the soil they are grown in. But soil is now one of the leading causes of exposure to lead, and this study suggests high potential risk for those exposed to soil particulates. “As long as [the soil is] not super contaminated, and if you follow all the steps to limit exposure and health risks, I think you should be fine [to eat the harvest],” said Joshua Cheng, who led the study, though he stressed that there is a wide range of opinions on what—and how much—produce grown in contaminated soil is safe to consume.

“The largest health risk is from working in the garden, or playing in the garden,” Cheng added, noting that children are particularly vulnerable to lead poisoning. Though adults typically retain less than 10 percent of the lead they ingest, children can absorb around 50 percent, which can lead to irreversible developmental problems that affect learning, hearing and behavior.

It’s important to understand that not all lead is “bioavailable” to humans. This means that, depending on what the metal is bound to, it may not enter your bloodstream even if it is absorbed by your body, and therefore would not be as harmful. For example, phosphates in the soil can render lead relatively harmless for human consumption. Nonetheless, Cheng strongly urged a precautionary approach until more research on lead’s bioavailability is done.

“There is no safe level of lead for children, the lower the better,” said Hannah Shayler, a Cornell University extension associate who spoke on behalf of the city’s Healthy Soils, Healthy Communities project, a research and education partnership that helps gardeners understand contamination and implement best practices.

Cheng did not know how many of the study’s soil samples were taken from raised beds holding new soil, but he said that this is still the best way to mitigate the risk of contamination, as long as gardeners are sure that imported dirt or compost is safe to begin with. Other steps—like mulching, planting grass to cover exposed soil, keeping soil out of the house, and washing hands and vegetables thoroughly after leaving the garden—can also help minimize human exposure. And even though heavy metals do not readily accumulate in the edible parts of produce, fruiting crops are still less able to pull in contaminants than vegetables like lettuce or root crops.

Getting the dirt on what’s in your backyard is easier said than done. Contaminants are not equally distributed in the soil, and tests can run $40-$60 per sample. Instead of paying for tests, Shayler urged urban gardeners to invest in precautionary improvements. “We are encouraging people to be proactive and to use healthy gardening practices to minimize their contact with contaminants that might be there,” said Shayler.

Cheng agreed that reducing risk is relatively simple once people are aware that widespread contamination exists. “We don’t want to discourage people from gardening,” he said.