How to Get the Lead Out Of Your Soil... Maybe

Urban Farming Nerd Alert!

Today's topic is phytoremediation!  

Since we live in Brooklyn, and Brooklyn has an industrial past, I read that our soil was likely contaminated with lead.  As I understand it, the lead gets into the soil by settling out of the air (in the form of pollution) and flaking off of buildings (in the form of paint).  

I had our soil tested by Brooklyn College in Autumn 2014.  The middle area of the yard tested at 332 parts per million (ppm).  The flower beds around the edge of the yard tested at 413 ppm.  The front yard tested at 453 ppm.  The green roof soil and the potting soil in all the containers on the roof were lead-free.

Once I got our soil test results, I had to figure out what to do about it.  

Brooklyn College's recommendation is: 

• for lead levels below 100 ppm, no precautions are necessary (the green roof and all the potted plants are safe).
• for lead levels between 100 and 400 ppm, follow best‐management practices for garden soils, i.e. don’t grow green leafy vegetables or root crops, children should not play in areas of bare soil. Other suggestions would be to further investigate actual lead distribution in the area and to test the blood lead levels of children. (this applies to the middle section of our back yard)
• for lead levels above 400 ppm, the soil should not be used for growing food plants, and remedial actions should be taken for residential use. (this applies to our front yard and the garden beds around the edges of the back yard)

So, for the middle area of the back yard - where the lawn has been - the recommendation is "best-management"...  The lead levels are below 400, so it's okay that we've got an apple tree planted in that section, but the soil is still pretty contaminated, and I'd rather it wasn't.

For the surrounding garden beds, the lead level is over 400, which means we're not supposed to be growing any food plants in that area, and the soil should be cleaned up.  Unfortunately, that's where I've got black raspberry vines, may apples (a native species of plant that produces small fruits in July, not May), and ramps.  I have two theories as to why the edge beds tested higher than the central section of the yard.  First, I've been putting a lot of compost in the central section to try to raise its level up to match the brick walkway.  If I understand my research properly, compost can make heavy metals less accessible to plants.  Maybe the compost also makes heavy metals less detectable to soil tests?  Second theory: the back bed runs along the rear side of our neighbor's garage, and the paint has been peeling off that thing for a long time.  I took soil samples from all around the edge beds, including the back, so maybe the soil from the back skewed the results.

Regardless, I don't want to live with lead in our backyard soil.  I want to clean it up.

Back when I lived in Boston (in the 1990's), I worked with a community action group to get an area along the Chelsea Creek cleaned up.  The soil there was throughly contaminated, and we were told that the only way the land could be made safe enough to become a park was to actually scrape the top 12" of the soil off the land, send it to a land fill, cap the remaining earth with plastic, add new topsoil over the plastic, and, voila: Safe park!  By the looks of it, that's exactly what was done to create "The Condor Street Urban Wild":

It's a wonderful triumph that the site was cleaned and made into a public park.  However, before the soil was scraped off, capped and/or replaced, the land had varied contours.  It had trees.  It was interesting.  I like interesting.

Digging our own backyard down 12", throwing all the soil away, buying 12" of clean replacement soil, and carting everything through the house both ways (we don't have an alley or any other way of accessing the backyard) didn't sound like a viable option.  It sounded wasteful in every way, and it would likely remove any character that's there.

But then I remembered I had once heard that sunflower plants pull toxins out of soil.  I did a lot of internet research and found that sunflowers aren't the only plants with the ability to "clean" soil.  In fact, other species of plants draw lead up into their roots, stems, and leaves even better than sunflowers!

According to my research, indian mustard (Brassica juncea) was one of the best plants for bio-accumulating lead and phytoremediating soil of all those I read about.  Here's how it is supposed to work:  

You seed the contaminated soil with indian mustard, grow the plants to maturity, pull them out (roots and all), and dispose of them.  While I could find no record of any land that had been 100% cleaned by this process (perhaps because no one has tried for long enough), The Boston Health Department conducted a study for 2 years in Dorchester, MA in 1997-98.  During those two growing seasons, they managed to reduce the lead in the soil by 63%.

So I thought this year I'd give it a shot!  I tilled the middle part of the back yard with a pitch fork and sowed the whole thing with indian mustard seeds.  I left the side beds un-touched for a control to my experiment.  In the Fall, I'll pull up all the indian mustard plants and retest both the central area and the side beds to see if there has been any change in the lead content.  Of course, I'm hoping we see a noticeable improvement in our lead levels.

Half-way through tilling the back yard with our new tiny pitchfork.

Mustard Greens (about 3 weeks old)

Below is a wide shot of the back yard today.  You might notice that I've taken down the back fence, scraped and painted the back wall of the neighbor's garage, and thereby hopefully stopped the flow of lead into the soil from that source.  

In my next garden post, I'll explain the cube-shaped contraption in the back.  In the meanwhile, baby mustard greens look nice as a lawn!

If you'd like more information on phytoremediation and creating a lead-safe garden, have a look at the Lead-Safe Yard Manual and this paper called Phytoremediation for Lead-Safe Yards.