Buried Treasure in the City’s Soil? It’s the Soil Itself

NEW YORK — Dan Walsh is New York’s unofficial czar of soil. On a Tuesday morning in April, the geochemist, who directs New York City's Office of Environmental Remediation, gestured proudly toward a mound of straw-colored dirt and said, “That pile is as clean as any soil in the Northeast.”

The sediment that Walsh and a dozen volunteers were admiring had been transferred a few days earlier from a construction site in the Jamaica neighborhood of Queens, in New York, to the Hall of Science in Flushing Meadows Park, where it is being used this summer to conduct an agricultural experiment. The transfer was part of the NYC Clean Soil Bank, a soil exchange that pairs local builders with environmental restoration projects that need fill materials.

While air pollution and spoiled waterways are the most visibly threatened environmental resources, the soils that lie beneath our feet have lately been receiving some long overdue attention as well — especially in the New York metropolitan area, which scientists say sits on top of some of the best soil on the continent.

Dirt, suddenly, is somewhat glamorous. New York City has been leading this reassessment. An Urban Soils Symposium was held at the New York Botanical Garden in 2017, attracting scientists from around the world, who spoke about how city soils can be used to grow more food, improve storm drainage and counter global warming by taking excess carbon out of the atmosphere. Another conference, slated for December, will bring gardeners and researchers together to discuss how to regenerate soils that have been degraded by urbanization.

Degraded soils are a big concern in New York, where lead contamination levels can be high. For much of the 20th century, soil excavated at construction sites was regarded as toxic waste and sent for disposal outside the city. Every year, between 2 million and 3 million tons are carted off to dumps upstate, in Long Island and in New Jersey.

That’s a shame, Walsh said, because heavy metal contamination is generally limited to soil surface. While this toxic surface material does need to be safely disposed of, New York’s deeper sediments are pollution-free, he said. And we are throwing away tons of valuable soils and other fill materials that can be used to address New York’s environmental problems.

Moreover, the demand for soil within the five boroughs has never been greater. As sea levels rise, New York needs landfill to build levees to protect neighborhoods that are susceptible to flooding during tidal surges like the one that inundated low-lying areas during Hurricane Sandy. Soil is also needed to create new coastal wetlands that can help buffer the impact of future storms.

This month, the PUREsoil NYC program was launched, which in addition to pursuing environmental goals intends to focus on cleaning contaminated community gardens.

In the past, material for projects like these was trucked into the city from quarries outside its borders. (A ton of fill costs between $30 and $60, and many projects require hundreds of tons.) But the NYC Clean Soil Bank is working to replace these expensive imports with the city’s native soils, cutting the miles it gets transported by 80 percent and reducing greenhouse gas emissions.

It’s an unexpected boon to builders. No longer will they need to pay to send their excavated materials out of town, while worthy environmental projects within the city will be delivered usable soil and sediments to their work sites for free. While the idea might seem obvious, Walsh maintains that this is the first soil exchange anywhere in the world that is run by a city government. It is being watched by officials from New Orleans and Los Angeles as well as municipalities in Germany, China and Australia, which are considering implementing similar programs.

Recipients of city soil have included a Superfund site in Sunset Park where PCBs (polychlorinated biphenyls) were removed and replaced with clean soil, and new wetlands that are being created in the boroughs of Queens and Brooklyn. In the past five years, half a million tons of excavated material (enough to fill the ball field at Yankee Stadium to a depth of 90 feet) have been delivered to a variety of projects, saving an estimated $30 million for both the construction companies that generate the soil and the recipients that use it.

“We’re essentially matchmakers,” Walsh said. “We don’t stockpile the soil, so both a donor and a recipient have to be ready at the same time. Our job is to coordinate the transfer.”

At the demonstration project at the Hall of Science, Walsh joined others who were shoveling the pile of sediment into a cluster of polygon-shaped wooden beds. Several museum volunteers enthusiastically raked the sandy loam together with bags of compost supplied by the Sanitation Department.

And Brooke Singer, the designer in residence at the Hall of Science, was mixing it with her bare hands. “Smell this,” she said, holding her palm out to another volunteer. “Makes you happy, huh? It’s the soil bacteria.” Singer is the driving force behind Carbon Sponge, a project that is exploring New York’s capacity to sequester carbon in soil by putting in test plots around the city.

“We’re planting eight different cover-crop types that were developed by a sunflower farmer in Kansas,” Singer said. In addition to testing the capacity of urban soils to suck carbon dioxide and nitrogen from the atmosphere, they want to find out what precise mix of compost, sediment and cover crops can best transform the sterile sediments that abound deep below the city’s neighborhoods into productive soils.

That New York sits atop a trove of potential agricultural materials might surprise anyone who has dug in a backyard or community garden. Sink a shovel into the ground and you will encounter brick fragments, ceramic pipes, glass shards and other industrial debris. But according to Joshua Cheng, a geologist at Brooklyn College, concealed under several feet of surface rubble are sediments that were laid down by glaciers during the Ice Age.

Cheng explained that 20,000 years ago, the last glacier had advanced as far as what is now New York City. The deposits of sand, silt and rounded pebbles dumped by the retreating glacier are 300 to 400 feet deep in parts of Brooklyn and Queens.

But the rich and clean glacial soil is well below the surface of the city. A study conducted by Cheng in 2015 found that 97 percent of the community gardens and backyards that they tested had elevated levels of lead and arsenic. While the main sources of these pollutants, leaded gasoline and lead-based paint, are now strictly regulated, high levels persist in topsoil and get blown into the air as dust, potentially putting gardeners and children who play in the most contaminated gardens at risk.

Cheng advises New Yorkers to avoid eating city-grown root vegetables and to thoroughly wash greens that lead particles can adhere to such as cabbage, lettuce and collard greens. The most effective way to eliminate the danger from heavy metals, however, is to cap them with clean materials, like those supplied by the NYC Soil Bank. A pilot study published this month showed that clean soil from construction sites can lower exposure to certain common pollutants by 98 percent. Tatiana Morin, the director of the NYC Urban Soils Institute, said she hopes that legitimate concerns about lead won’t blind people to the essential services that New York’s soils provide. “Soil scrubs our air, filters our water, grows food and moderates the heat island effect that makes cities warmer than their surroundings,” Morin said. “Yet many of us are scarcely aware that it is sitting right here below our feet.”

This is an oversight that the institute hopes to correct in the fall, when a permanent soil museum will open in Green-Wood Cemetery in Brooklyn. The museum will be partly tunneled into a cemetery hillside and will feature art fashioned from New York dirt, as well a working soil lab.

Morin applauds the fact that the city is using soil generated at construction sites to regenerate its environment. She said that New York is also busy expanding tree pits, putting bioswales on traffic islands, and removing concrete from some schoolyards and playgrounds to create gardens of native plants.

“New Yorkers need to uncover their soil and make better use of it,” Morin urged. “If we keep on beating soil up, we won’t be able to sustain our cities in the future.”