Lead, Mercury, Cadmium, Zinc, and Arsenic...Oh My!

By jess

Here is more great work from our summer intern Talia!

We're dedicating this blog post to a discussion about potential soil contamination in urban areas. It's an issue that should NOT stop you from gardening in the city, but is good to be aware of. Here is the problem. Previous building practices and historic chemical additions to gasoline have left some soils contaminated with toxic chemicals such as lead, mercury, cadmium, zinc, arsenic, and petrochemicals. Lead gets the most attention, and that’s what this page will focus on, but best to be aware of the other chemicals as well. You can get your soil tested for some of the more common contaminants by sending soil samples to your local agricultural extension. For Bostonians, that is UMass Amherst. Simple instructions are here. Keep in mind that while soil tests look for many contaminants, they do not work for contaminants such as petrochemicals. We suggest investigating your soil history to find out what used to be built on your land.

Now, it’s time to focus on Lead…

Why should you care?
Lead is a terrible problem in the urban environment, as 82% of urban soils have elevated levels. It poses the highest risk to children, because kids tend put dirty hands in their mouths increasing exposure and lead impacts mental development.

But wait! You can still garden!
Gardening in the city is still a great idea, but we want you to do so in an informed manner. The following list is especially designed for dealing with a lead problem, but these suggestions will help with other contaminants too:

1. Garden in Raise Your Beds
Many benefits come with using raised beds. The best is peace of mind that you brought in soil from a safe source. Remember that plant roots reach pretty deep, so line the bottom of your bed with landscaper’s fabric. To construct the raised bed, use untreated wood. Cedar is great because it naturally repels termites, albeit it's pricy.

2. Wash Your Vegetables
Plant root cells are way better at filtering out heavy metals than our own bodies. Minute amounts of the lead present in the soil solution will find its way to the fruit of the plant. The real danger comes from dust that coats the outside of your vegetables. It’s tempting to eat vegetables or fruit right off the plant after a quick wipe on your shirt, but you should actually wash them. In fact, don’t just wash the vegetables from your garden; always wash your fresh produce, even if it’s organic.

3. Use Ground Cover on High Lead Areas
If you know certain areas of your yard contain contaminated soil, make sure you plant grown cover plants. Plant roots hold on to soil, keeping dust from blowing into the air, your raised beds and your lungs. This article is a great resource for native Massachusetts groundcovers.

A Note on Phytoremediation and its Effectiveness:
In phytoremediation, plants extract contaminants from the soil, and then the gardener simply gets rid of the contaminated plants. Sounds easy, right? Not quite. Phytoremediation is an innovative idea, but it needs work. As mentioned previously, plant roots are good at filtering out contaminants when they absorb nutrients from the soil solution. Furthermore, certain heavy metals like lead tend to bind to soil particles and do not easily dissolve into the soil solution. Only a few plants, called “hyperaccumulators,” will actually translocate the contaminants from the soil solution into their leaves, but these plants tend to not produce much biomass. Scientists are currently researching ways to trick non-hyperaccumulator plants, which do not naturally absorb contaminants but do produce a lot of biomass, to start accumulating. These tricks include adding inducers like chelates, which attach to chemicals and make them soluble, so plants will absorb the contaminants. In general, lowering pH increases the amount of Lead in the soil solution, so citric acid works pretty well too.
If you want to try out phytoremediation, then go for it. However, many gardeners mistakenly believe that after one season of growing geraniums, mustard, or collard, their soil will be good to go. Not true! Without any inducing agents (chemicals that make contaminants soluble) phytoremediation happens way too slowly. As a home gardener, phytoremediation is not the solution for a lead problem.