Lead in Residential Soils
Lead naturally occurs in soils at 10 to 50 parts lead to one million parts of soil (ppm). Lead becomes a concern when its soil levels reach 400ppm or greater. Additionally, lead does not break down or biodegrade, so it can contaminate soil for many thousands of years, if not remediated!
Soil lead becomes a problem when it is taken into the body by ingestion or inhalation. This is a concern for gardeners, as contaminated soil attached to the roots or leaves of vegetables may be consumed. In fact, Dr. Richard Stehouwer, associate professor and environmental soil scientist from Penn State University, states that “the greatest exposure risk comes from soil or soil dust that clings to edible plant parts.” Lead can also be taken into plants in small amounts, accumulating lead in their tissue.
Children under 6 are at greatest risk of lead poisoning due to their play habits. Many small children will eat soil, put dirty hands in their mouths and be more susceptible to inhaling dust as they play in the dirt! Additionally, the developing systems of young children are sensitive to the effects of lead.
Lead poisoning affects the neurosystem and can cause brain damage, lowered IQ and attention deficiency in children. In adults, lead poisoning can cause anemia, gastrointestinal and kidney disorders as well as nerve disorders and reproductive problems. Once lead enters the body it is nearly impossible to remove, thus prevention is essential to avoid these serious health consequences.
Lead in the soil is a concern for both urban and suburban residents, yet instances of lead poisoning are high among city kids. Additionally lead is a concern in older cities, with a higher proportion of buildings containing lead paint and plumbing. Lead paint from buildings built prior to 1978, before lead paint was banned, is a primary source of lead in the soil. The demolition, weathering and chipping of materials covered with lead-based paint deposits lead in the soil.
Leaded gasoline, common before 1989, is another substantial contributor of lead. Car exhaust from this time period put lead into the air which settled in yards all over the city. Industrial emissions deposit lead to the soil as well.
It is important to have your soil tested for lead before you begin an edible garden (or if you have children who play in the yard), especially if your home is located within an older development. Soil test results will include information and suggestions on how to treat the soil in order to reduce your exposure to lead. Please look here for information on soil tests.
Good Gardening Practices
There are many practices that you can take to reduce your risk of lead poisoning.
- Cultural practices which can reduce the risk of inhaling soil dust:
- Keep soil moist. Wetting the soil before you till or work in the soil will reduce the movement of lead attached to soil dust.
- Keep the soil covered with mulch.
- Cover highly contaminated soil with clean soil and plant grass or other perennial ground cover. Covering the soil reduces both the risk of dust getting into the air and children interacting with contaminated soil.
- Cultural practices which can reduce the risk of ingesting lead:
- Wash all produce and peel root vegetables.
- Wash your hands before you eat and prepare vegetables.
- Teach children to wash their hands after playing outside and not to put their hands in their mouth!
- Prepare the soil with lime in order to reach a neutral pH of 6.5; this greatly reduces the availability of lead in the soil.
- Incorporate organic matter into the soil, which reduces the uptake of lead by plants.
- Place vegetable gardens away from areas most likely to have high concentrations of lead, such as, roads, driveways and old painted structures.
- Avoid planting and consuming leafy greens such as spinach and lettuce which will accumulate lead in their tissue and tend to hold soil onto their textured surface.
- Avoid planting and consuming root vegetables like carrots, potatoes, and radishes which hold onto soil and are difficult to wash free of all soil.
- Plant fruiting crops such as tomatoes, eggplant, peas, beans, sunflowers, squash and corn, as lead does not readily accumulate in the fruit of plants. Fruit is also fairly easy to wash and the risk of lead attached to the outer skin of fruit is minimal.
Depending on your soil’s lead levels, it may be necessary for you to remediate the soil. The EPA recommends remediation of soils that contain lead levels of 400ppm or greater, especially if it will be used as a play area or to grow vegetables. There are four main ways to remediate a site: raised bed construction, excavation, bioremediation, and phytoremediation are all options for lead remediation.
1. Raised bed construction
The quickest, most cost effective way to garden with high lead levels in the soil is to construct a raised bed. In this way you can grow your vegetables in clean, healthy uncontaminated soil.
Contaminated soil should be covered with landscape fabric or 4-6 millimeter thick plastic. Most hardware stores or nurseries will sell landscape fabric in a roll that can be cut to fit your garden bed. This fabric or plastic will provide a physical barrier between your plants and the contaminated soil. After you have a barrier covering your soil you can build a framed garden bed and fill in with healthy soil. For more information on constructing a raised bed click here.
This strategy is used here in Pittsburgh by the Larimer Green Team, as advised by Joe McCarthy, Urban Greening Coordinator of the local Penn State Extension office.
Another option is to remove the top three inches of contaminated soil. Most contamination will be located in the top three inches of soil, as lead does not readily move through the soil. The top layer of soil could be completely removed and replaced with clean soil. This method is labor intensive and may be cost prohibitive. Retest the soil to the expected root depth before planting.
Bioremediation is the use of compost to remediate the soil. Compost neutralizes the pH of the soil, making lead in the soil less bioavailable to plants. In addition, lead binds tightly to organic matter in the compost. Thus lead will remain in the soil, yet in a less harmful form if inhaled or consumed. If enough compost is used, about 2ft mixed with the top three inches of existing topsoil, plants can grow in healthy soil. Lead will still show up on a soil test, yet plant tissue can be tested for lead to see if the lead is present in the plant material. If lead does not show up in the plant tissue, then it is safe to consume the vegetables grown in your garden. This method is still being explored as a viable option for remediation and is not widely utilized by home gardeners.
Phytoremediation is the use of plants to remove lead from the soil. Some plants take up lead more readily than others. Sunflowers and plants from the Brassica family, such as mustard greens, are referred to as accumulator plants as they readily pull lead out of the soil and into the plant itself. In order to remediate a site, these plants are grown and then disposed of.
Locally, GTECH Strategies is exploring the use of phytoremediation to reduce the amount of lead in the soil. Through a partnership with the University of Pittsburgh, they are researching the effectiveness of planting sunflowers on vacant lots to remediate the soil. Troy Hottle of GTECH says that, “there is need for further study involving various remediation techniques in urban settings”.
Phytoremediation of a site is completed over many years and therefore is not a viable strategy for most homeowners ready to begin a vegetable garden in their backyard, though it has value in remediating areas of your yard that aren’t to be planted with vegetables right away. Sunflower plants can be used for remediation as well as beautification.