Lead is the most common metal contaminant in urban areas. Children younger than 6 years living in older houses are at greatest risk of exposure to chips and dust from lead-based paints. Other common sources of lead exposure are from drinking water contaminated by lead pipes or lead solder that joins the pipes, or from contaminated soil or dust. Eating soil or eating unwashed produce are the most common exposure routes from soil. This publication is designed to help homeowners evaluate and manage lead hazard in their landscape or garden. We explain:
1. How soils become contaminated with lead
2. How people are exposed to lead
3. How to get your soil tested for lead
4. How to interpret soil lead test results
5. How to reduce exposure to soil lead
6. What to do if your soil is contaminated
7. Which garden produce is easier to wash
This publication does not address indoor lead hazards or provide a comprehensive lead risk assessment. See “Common questions about lead” and the resources listed under “More information” for additional information on lead.
Lead exists naturally in soils at concentrations of 10 to 50 parts per million (ppm). This range is the background level. Higher concentrations may indicate lead contamination. The longer humans have occupied a site, the more likely it is contaminated with lead and other metals. Urban and residential soils often have higher lead concentrations because paints contained lead until 1978 and gasoline contained lead until 1996.
Areas near existing or former smelters, mine tailings, coal-fired power plants, and cement factories often have elevated soil lead concentrations.
Lead-arsenate sprays were used for pest control in fruit and nut orchards from about 1910 to the 1950s. If your home is located on an old orchard site, soil lead and arsenic concentrations may be elevated.
In contaminated soils, lead accumulates in the top 1 to 2 inches unless soil is disturbed by digging or tilling. Lead is held tightly on the surfaces of very fine clay and organic matter particles, which may stick to skin and clothing. Almost all of the lead in soil is in a solid form; it does not dissolve in water. Plant roots can only absorb what is dissolved in water. The function of plant roots is to take in nutrients and to exclude non-nutrients, such as lead. Read more at “Plants take up less lead when soils are well managed.”
The best way to learn whether soil lead concentrations are high enough to pose a risk is to have the soil tested for lead. Accurate results come from submitting a good sample.
To take a soil sample, collect 15 to 20 subsamples from the area of concern. For garden soils, sample from the surface to a depth of 6 inches. For play areas, sample the top 2 inches of soil. See OSU Extension publication Soil Sampling for Home Gardens and Small Acreages (EC 628) for detailed sampling instructions. Mix the subsamples thoroughly in a plastic container, place about 1 cup of the mixed soil in a clean, sealable plastic bag, and submit it to a laboratory. Laboratory soil lead concentrations (ppm) are expressed on a dry weight basis. This is the same as the EPA guidance.
Lead concentrations in soil are expressed as parts per million. This means parts of lead per million parts of dry soil. State and U.S. Environmental Protection Agency (EPA) regulations limit lead concentrations in soils for play areas for children, high-contact areas for children, and bare-soil areas in the landscape. The United States does not have specific guidelines for garden soils or for lead concentrations in foods. The guidelines shown in Table 1 are based on state and federal rules and apply only to soils. They do not apply to plant tissues or to water.
The EPA has established the following maximum lead concentrations in bare soil at residences:
Oregon Administrative Rules (OAR) Chapter 333 regulates the certification of individuals and firms engaged in lead abatement activities. This state regulation is slightly more restrictive than the federal standards set by the EPA. Under OAR 333-069-0015(68), soil lead hazard is defined as “bare soil on residential real property or on the property of a child-occupied facility that contains total lead equal to or exceeding 400 parts per million in a play area or average of 1,200 parts per million of bare soil in the rest of the yard based on soil samples.”
Note that the concentrations in Table 1 are total soil lead. If your soil is tested by dissolving the soil sample in strong acid, you can interpret your test results according to Table 1. This test measures all the lead in the soil, including lead that is very tightly bound to other minerals.
It is not appropriate to interpret results from tests measuring available or dissolvable lead using Table 1. Table 1 cannot be used to interpret results from drinking water or plant lead testing. See the section “Soil tests for lead” and the sidebar “Which soil test? It makes a difference” for details on choosing a soil test. See section “Soil test interpretation for garden and bare soils” to understand the results of a soil test.
If you don’t know the history of your garden site, have the soil tested before you establish a vegetable garden. If lead or other metals are a concern, grow root crops and leafy greens in raised beds or containers filled with an uncontaminated planting mix (Figure 1). Cover bare soil with mulch; mulch garden paths, perennial beds, and other bare soil areas in the landscape (Figure 2).
Mulching with organic matter increases the soil’s capacity to store water and reduces dust formation. It also provides a barrier between the gardener and any lead-contaminated dust that may have been deposited.
For intensive gardening with bare soil and frequent tillage on sites with high soil lead concentrations, reduce lead hazard by:
Wear work gloves and shoes, don’t track soil into the house, wash your hands, and change clothes if they are dusty or dirty (Figure 3). Monitor children while they are in the garden. Supervise hand washing after they play outdoors or work in the garden.
Lime and phosphorus change the soil environment so that lead and other metals such as zinc, copper, cadmium, and nickel are less soluble. Plants can only take up minerals that are dissolved in soil water. When soil lead is greater than 400 ppm, maintain soil pH near 7.0 by applying lime. Before adding amendments, including lime, have the soil tested for the basics: phosphorus (P), potassium (K), pH, and lime requirement. Adding lime and phosphorus to soil will not reduce total lead, but may reduce the bioavailabity of lead in soil. Applying compost or other organic matter will reduce exposure to lead-contaminated dust. Meeting basic plant needs will ensure robust growth and reduce the concentration of any lead that might be taken up by plants. See “Soil tests for lead” for more information.
Add lime and phosphorus when a soil test indicates a need for them.
See Oregon State University Extension publication Improving Garden Soils with Organic Matter (EC 1561) for more on mulching and organic matter. See Washington State University (WSU) Extension publication Using Biosolids in Gardens and Landscapes for more on using biosolids in home landscapes.
Plants grown in well-managed soils do not absorb much lead through their roots. Dust on the outside of the edible portion of vegetables is the main route of exposure to lead. In soils that are high in lead, crops that are difficult to wash present the highest lead hazard to humans. Table 2 lists plants that will supply more lead to the diet, because they are more difficult to wash.
Eat only well-washed vegetables and fruits; discourage eating produce straight from the garden. Lead-contaminated dust on any unwashed vegetable is a concern.
In addition to washing garden produce, these techniques will limit exposure to lead-contaminated dust:
Cover bare ground. Plant perennials or shrubs in areas with greatest metals contamination, and mulch beneath them. Cover bare soil under perennial flowers, fruits, vegetables, and ornamentals with a perennial groundcover, dense grass, or heavy organic mulch (Figure 2 and Figure 4). Vegetable gardens are annual plantings that require routine digging or tilling. Minimize bare soil there by planting transplants and mulching immediately afterward.
Avoid digging or tilling to control weeds. Forking is less likely to raise dust than rototilling. Consider using an appropriate herbicide (glyphosate) for weed control. Always read and follow label instructions when using any herbicide or pesticide.
Garden location. Place annual flower and vegetable gardens as far as possible from busy traffic ways and older structures.
Add compost to vegetable and flower gardens to reduce the likelihood of dust. The individual particles of soils amended with compost are held together more strongly. Compost-amended soils also store more water and promote more robust plant growth (Figure 5).
Reduce soil splash. Overhead watering can cause soil to splash onto edible plant parts. If possible, use drip irrigation. When watering with a hose or watering can, hold the water source close to the soil surface to reduce splashing (Figure 5).
Lead is a metal found in minerals, rocks, and soil. It is used in paints, lead-acid batteries, solder, pewter, bullets, and fusible alloys.
Lead poisoning usually causes long-term effects rather than acute toxicity in humans. For this reason, poisoning may occur over a long period of exposure without obvious symptoms.
Nerve and brain damage are possible, as well as harm to the heart, kidneys, blood, reproductive system, and nervous system. Children from 6 months to 6 years are more often exposed in the home than outdoors. Adult lead poisoning normally occurs in the workplace or during hobbies or recreational activities. Lead dust may be brought into the home on clothing, shoes, and the skin.
If you have reason to suspect lead-related health problems, contact your physician. Your local health department and the Oregon Health Authority can also assist you in evaluating lead hazards and remedying them.
Lead-contaminated soil does not look or smell different than other soils. Lead does not break down in the soil, so the soil must be made “lead safe.”
Although lead does not dissolve readily in water, water that has a low pH can dissolve lead from pipes, solder, or fixtures. “Hard” water has a high mineral content. If mineral buildup on the inside of pipes prevents contact between the water and the pipes or solder, it may offer some protection by reducing contact between water and the lead.
The EPA has established an “action concentration” of 15 parts per billion for lead in tap water.
The amount of lead added to soil through irrigation usually is quite small compared to the amount of lead present in soils. Remember that the background concentration of lead in non-contaminated soil is usually 10 to 50 parts per million. The action concentration for lead in tap water is 1,000 times lower than the concentration of lead in soil. If you are concerned about lead in water, use drip irrigation to keep water off edible plant parts.
Fertilizers sold in Oregon must meet standards specified by OAR 603-059. This requirement is enforced by the Oregon Department of Agriculture. Oregon is one of the few states that regulates metal concentrations in fertilizer products.
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