We just tested the garden soil of the Brooklyn Green Show house and the results were a staggering 2000ppm of lead. Considering anything above 400ppm is rated as unhealthy we have a bad situation.
High levels of lead in Brooklyn is a pretty common problem. The back walls and fire escapes were commonly painted in lead paint. Then over time they were stripped of the paint to put a new coat. All that paint went flying into the garden.
If you do that for a couple decades you have a drastic build up of heavy metals in the soil.
Eco Brooklyn provides soil remediation services for Brooklyn gardens. Soil remediation is not hard, but it does involve a lot of hard work. You have do to a lot of digging, lugging and more digging.
The best way we have found is to remove the top foot of soil and get rid of it in a safe way. We then mix the remaining soil in the garden with lots of lime to raise the PH. A university study on the topic of soil remediation found that adding lime to the soil in a soil to lime ratio of 21:1 reduced the danger of lead by 88%.
Along with lime we mix in organic matter like leaves, manure and sod, all of which help bind the lead particles and render them less harmful.
Having significantly reduced the threat of the heavy metals we then add more clean top soil over the old lime/soil mixture. This provides a healthy exposed soil for kids to play in and plants to grow from without ingesting the large amounts of heavy metals that were there previously.
Another way is to plant lots of greens like kale and spinach. They suck up heavy metals very effectively, but alas not so much lead. One study found that 30% of the heavy metals were pulled out of the soil with one season of spinach crops. You then cut the spinach and dispose of the toxic plants. Unfortunately takes time since it could be several years before the soil levels reach safe levels again.