Sunday, November 6, 2011

Cleansing The Soil

Like every big city New York went through an industrial phase which left its mark on the place. Shadows of that era can be glimpsed in the names of neighborhoods such as the Meatpacking District, historic buildings such as the Domino Sugar Factory, and the still-active industrial waterfront in places such as the Gowanus Canal.

Parts of Greenpoint feel mellow, maybe even suburban, and personally that is one of the things I love about it. But this is something of an illusion. Greenpoint is part of the big city and shares its industrial history. Greenpoint still has an industrial zone which lines the waterfront along the East River and Newtown Creek. A street called "Monitor" runs past a statue representing the USS Monitor, the famous ironclad ship built on the Greenpoint waterfront which helped the Union win the war. On a less uplifting note the Greenpoint Oil Spill is still around. It is the largest of several chemical spills in the area.

The Java St. garden site is not on top of any particular chemical spill. The fact that some plants are growing there now suggests that the soil is not irredeemably toxic. Good! But this does not mean that the soil is clean or particularly safe for planting. Surely we would not want to plant carrots or potatoes in soil with a high lead content even if they were capable of growing there!

In general we would like to have as few toxins as possible in the soil, but in practice we have to work with specifics. We need to know exactly what kinds and quantities of toxins we are dealing with and we need to have specific targets for the reduction of these toxins.

One of the first things we did when this project started was to take a soil sample from the plot and sent it to Brooklyn College for analysis. This week we received the results of that analysis. We now have a good idea of the concentration in parts per million (ppm) of toxic heavy metals (e.g. arsenic) and metallic nutrients (e.g. calcium) in the soil.

This is not helping
Our goal will be to meet the NYS Department of Environmental Conservation (NYSDEC) Soil Cleanup Objectives (SCOs) for residential use. These are target levels in ppm for contaminants such as chromium, arsenic and lead.

It appears that 59 Java contains acceptable levels of several heavy metals but has a bit more chromium (38ppm) than the objective (36ppm). While the lead levels (299ppm) on the plot are below the objective for residential use (400ppm), lead levels below 100ppm are recommended for growing fruits and vegetables. We should probably aim for the latter even though we might well decide to grow food crops in raised beds.

So now what can we do about this? We will not remove and replace the topsoil. That would be expensive, not to mention wasteful. The best answer for a small community garden like 59 Java would appear to be phytoremediation. This refers to planting species which can absorb toxins from the soil and render them harmless in various ways. Some plants (or their associated microbes/fungi) can convert certain toxins to less harmful forms. Some can absorb and release volatile contaminants (e.g. benzene) as gases. What we want are plants which will store toxic heavy metals within them, rendering the toxins harmless. Depending on the plant these may be removed so as to remove heavy metals from the site, or left in place to simply lock the heavy metals away.

For now our plan is to plant poplar trees and sunflowers. Both have been shown to take up toxic heavy metals from soil. The former will likely remain a permanent fixture of the garden. Poplar trees grow quickly and will pull a lot of those heavy metals out of the soil. Sunflowers also absorb heavy metals but can easily be removed. Perhaps visitors will be allowed to take a cut sunflower home? Both the poplar and sunflower will need to be pruned and will drop leaves and petals. All of these clippings can be removed.

The downside of this approach is that nutrients are also removed from the site when plant matter is removed. However it is relatively easy to add these back in. We can apply fertilizer to the garden or add potting soil. Perhaps we can add compost? We are currently looking into the possibility of either composting at the site or taking in compost from elsewhere.

The best thing about phytoremediation is that it does not make the place look like a construction project. Those sunflowers and poplar trees will contribute to the beauty of the garden. We can clean the soil and start growing the garden at the same time! We just have to be a little bit clever about it.

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