What are lead contaminants?
The most common type of contaminant in an urban soil is lead. Elevated lead in urban soil comes from the historic use of leaded gasoline and lead paint.
Your soil is most likely to be contaminated with lead if you live next to a very busy, high traffic road that has existed for more than 40 years. Lead in exhaust from cars when leaded gasoline was still in use will have contaminated the soil.
Your soil is also more likely to be contaminated if you live in an older home (50+ years) that is painted. Lead paint may have chipped off your home and landed in the soil directly next to the house.
In other words, if you live in a brick house or in a newer house on a quiet street it’s highly unlikely that you have elevated lead in your soil. But if you live in an older home or near a busy street, your soil may have high lead.
Can plants in lead-contaminated soil hurt you?
It’s hard to be exposed to lead by eating vegetables or fruits. Plants do not take up lead on purpose, because lead is not a plant nutrient. Plants may contain measurable amounts of lead, but this isn’t because plants are actively taking up lead from soil, but because we’re able to measure very low concentrations of lead in environmental samples.
Plant concentrations of lead are generally very low—in the range of parts per billion.
- 1 part per million is the same as one penny in $10,000
- 1 part per billion is the same as one penny in $10,000,000
Not only do plants take up minimal amounts of lead, but it’s much harder for the body to absorb lead—even from food that contains it—on a full stomach in comparison to an empty stomach.
This is because an empty stomach is very acidic, which makes the lead in soil or food more soluble and more easily absorbed. A full stomach, in contrast, is not acidic and so the lead will be much less soluble. In addition, there are many other elements that the body needs, including iron, zinc, and calcium, that will be absorbed instead of the lead.
Click here for information about what to do about lead in your soil.
Testing for lead in your soil
If you’re concerned that your soil has high lead, you can collect a soil sample and send it to a laboratory to be tested.
You’ll need to specify what you want the soil tested for. Total soil lead concentration is the first test to get if you’re concerned. This information is included when you have your soil tested for total metal concentrations. Testing for total metals is done by dissolving your soil in concentrated acid. A standard method for this is the U.S. EPA 3050 extraction. But many labs will use a standard soil fertility test in lieu of measuring total metal concentrations.
Here are some labs where you can send your soil to be tested for total lead concentration:
- Penn State University: http://www.aasl.psu.edu/EnvirSoilTests.HTM
- Colorado State University: http://www.soiltestinglab.colostate.edu/
- Cornell University (choose “Environmental Analysis following US EPA SW-846”): http://cnal.cals.cornell.edu/analyses/index.html
- Purdue University: http://www3.ag.purdue.edu/counties/marion/Pages/SoilSamplingTesting.aspx
What do the results mean?
The total soil lead concentration that is cause for concern varies by whom you ask. The U.S. EPA says that you should start thinking about lead if the total concentration is above 400 parts per million in a child’s play area or 1200 parts per million in the rest of the yard. These limits are based on the potential for children to eat soil, rather than on the soil’s likelihood of contaminating homegrown produce.
Soil lead concentrations limits have not been established to protect people from eating plants grown in lead-contaminated soil, because plants generally take up almost no lead. The primary way you can be exposed to lead from plants grown on lead-contaminated soil is by eating plants that have not been fully washed.
In other words, washing vegetables before you eat them is the best way to reduce any chance of lead exposure. Washing your hands and your kid’s hands before meals is another easy and effective way to the reduce chances for lead exposure.
You should be aware, however, that total soil lead concentrations are not always a good measure of the portion of lead that can cause harm. It’s very hard to show that lead in soil is the reason children have elevated blood lead. Other factors, such as lead paint in older homes and household dust, can have much bigger impacts.
How important the soil is for lead poisoning depends on how high the lead concentration is in the soil, how much bare soil is around, how much time your children spend outside playing in the soil, and so on. 400 parts per million soil lead in Louisiana will be more dangerous than the same concentration in North Dakota, because North Dakota soils are snow-covered and frozen for much of the year and kids have no access to them.
Actions if there is lead in your soil
If you have high-lead soils or are worried about lead in your soil there are a number of things you can do to feel safe about growing food in your yard.
First steps
Lead stays where it lands. So, if you have an older home with a high potential for lead paint, plant your vegetable garden away from your home drip line. The lead contamination is almost certain to be highly localized—right under your house. Move away from the drip line and you move away from the high-lead area.
If you live near a busy street, the easiest thing to do is to plant your garden a few feet away from the curb. The further you are from the street, the lower the soil lead concentration will be.
Till your soil. Because lead typically lands on the top of the soil, it’s likely that high concentrations will be limited to the top inch or two of your soil. Mixing the soil with a hand tiller will reduce the lead concentration by mixing the contaminated soil on top with the lower-lead soil on the bottom.
More aggressive options
Add fertilizers to your soil. Remember that urban soils are often neglected soils. Adding fertilizers, especially phosphorus fertilizers, will help your plants grow. Phosphorus will also bind the lead and make it much less dangerous over time. The EPA has tested adding phosphorus to Superfund sites as a way to take away the hazard while still leaving the lead in place.
Add composts to your soil. Most cities produce yard-waste compost. Many also produce compost made from the biosolids from wastewater treatment. These composts are regulated and can be a great source of both fertility and improved tilth for your soils. Some of these composts will also directly bind lead and make it less available. All will dilute the lead concentration in your soil.
Cover the bare soil around your garden with mulch. Remember: Bare soil is a much greater hazard than soil covered by plants or bark.
Most aggressive option
Build a raised bed and fill it with commercial topsoil (metals concentrations in these products aren't regulated, so be sure you aren't making matters worse here) or with compost, or a mixture of the two. You can also use landscape fabric to cover your existing soil as an extra safety precaution if you are really concerned. Raised beds are expensive to build and require a lot of soil or compost to fill them. But if this is what you need to feel comfortable, go for it.