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How Safe Is Your Soil?

Urban farming has become hugely popular in the East Bay, but lead and other heavy metals in the soil pose potential health risks. Meanwhile, there's little consensus on what to do about it.



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Resident and garden planner Pamela Campbell was disappointed to see three years of work dissolve into thin air on account of lead levels that are representative of what exists just about everywhere else in the area — especially since her group planned to cover the existing soil and grow in clean dirt. "In my opinion, it's the same thing that you're dealing with in all of West Oakland," she said. "It's total red tape. It wasn't about food safety."

Had the city been clear and upfront about its lead-contamination policy, thousands of dollars and hundreds of hours of work could have been saved. Likewise, if it had been willing to accept the widely acknowledged remediation strategy proposed by the neighbors, the lot would now be providing its West Oakland neighborhood with fresh produce instead of sitting fallow.

These are the dilemmas that cities and urban gardeners now face. Yet organizations like City Slicker Farms are working hard to develop safe, practical solutions to soil contamination. Since 2005 the organization has set up 170 backyard gardens, including about 140 in West Oakland, all at no cost to the recipients. The initial step is always a soil test. While few lots exhibit truly dangerous levels, most are elevated and require some form of remediation — typically, covering the soil with mulch and growing vegetables in raised beds.

"So many backyards have lead paint in it, because lead paint was prevalent everywhere," said executive director Barbara Finnin. "So we're figuring out instead of digging out all the soil, how can we do that safely here?" In a two-foot-tall raised bed over an impenetrable root barrier, the risk of plant contamination is virtually eliminated, she said. Mulch as thick as six inches elsewhere in the yard helps protect children and animals from direct soil contact.

It's a simple solution supported by urban farming celebrity Novella Carpenter, who runs Ghost Town Farms on a vacant lot in West Oakland. In her backyard, which is adjacent to a gas station, she tested and found high levels of lead. So she topped the soil with landscape fabric and mulch to protect her animals, which can bio-accumulate lead just like humans and pass them along through their meat, milk, and eggs. Yet she won't grow any produce there. The majority of her front-yard beds are built directly atop a concrete patio that's covered with a few inches of dirt. "It's actually a nice thing to build on concrete," she said — the danger of contamination is always nil.

Still, raised beds aren't always practical, and they can be cost-prohibitive for some backyard and community gardeners to build and fill with clean soil. When the native topsoil tests at an elevated but acceptable level, the nuances become a bit more difficult to navigate.

As with lead safety levels and assessment methods, the science on plant uptake of lead from soil can be frustratingly vague. There's no easy answer: Based on different environmental and soil conditions, different plants uptake lead at vastly different rates.

In an attempt to shed some light on the subject, Ph.D student McClintock conducted a series of tests on leafy greens growing in different environments. Chard growing in West Oakland soil with 2,500 parts per million of lead showed levels as high as 30 ppm in its leaves, while collard greens in the same soil averaged only 5 ppm. Other tests found that chard growing in beds with 320 parts lead per million took up 3 ppm, and that collards in 215 ppm soil took up almost nothing. And a test in clayey North Oakland soil with lead levels of 700 ppm revealed lead in mustard greens ranging from 1 to 5 ppm. These figures may appear low, but consumption is dangerous in aggregate — particularly for children and when combined with accidental ingestion of contaminated dust during or after gardening.

While there's no clear rule for determining lead uptake, as a general guideline the further you are from the roots, the less lead will be present. A root crop poses the highest risk, a short leafy green is apt to absorb a moderate amount of lead, and a piece of fruit growing from the branch of a tree (or tomato plant) is unlikely to uptake much lead at all. Peeling or scrubbing produce can help reduce the risk. For the curious, plant-tissue testing is available to the public.

With the right precautions and the right soil, it can be completely safe to grow vegetables directly in urban East Bay dirt. When Esperanza Pallana decided she wanted to garden in her yard near Lake Merritt, which abuts a gas station, she started with a soil test. The result was a surprisingly low 96 ppm, small by any standard. So she opted to grow directly in the ground, and says she's now more concerned with particulate matter from nearby freeways.

As a food educator and consultant for many urban gardening projects, and a member of the East Bay Urban Agriculture Alliance, Pallana also advocates for various forms of bioremediation — the easiest and most popular of which is mixing compost with native soil. Not only does this improve soil conditions for growing vegetables, but it also dilutes lead content while binding it in the soil and making it less available to plants and humans. Yet this takes time and energy, and is no magic bullet for urban gardeners eager to plant their first row of lettuce.