BY STEVE CALANOG AND BIRGITT EVANS • PHOTOGRAPHY AND DESIGN BY JACK PERTSCHUK
Over the years, we have heard many smart, inquisitive gardeners express concern, confusion, and fear about environmental contaminants in their soil. Lead is very often mentioned when talking about gardening. While urban residents may know they should be concerned, rarely do they have much information about the causes of this contamination, how lead poisoning occurs, or what can be done to make their yards and gardens safe.
Lead is one of the most pervasive environmental contaminants in soils that affects human health. In the United States, lead levels in urban residential neighborhoods are, more often than not, elevated and exceeding local, state, and federal safe levels. This is true today despite decades-old laws and programs for removing lead from a wide variety of products. Still lead persists in our environment, and people, especially children, continue to suffer the toxic effects of chronic exposure. The problem will remain for many years into the future, but there are steps you can take now to protect yourself and your family, garden safely, and make your soil healthy again.
In Bay Area yards and gardens, it is typical to find total lead levels in the 300 ppm (parts per million) to 600 ppm range (see “Safe Levels in Soil” on page 58). Homes that are located closer to freeways and other historically high-traffic areas may have levels even higher. Should you be concerned? If you have small children around your home, then yes, you should be concerned. The risk is present and real. Can you make your yard safer and protect children from exposure? Absolutely, yes!
Here’s some practical advice, side by side with in-depth information to help you understand what we’re talking about.
Gardening Safely in Your Soil
Your soil probably contains lead in excess of at least one governmental “safe” level. Since it is not feasible to learn on a yard-by-yard basis how much of that lead is bioavailable (absorbable by the body into the bloodstream and tissues) and could, therefore, pose a health risk, it’s a good idea to simply minimize the risk.
The solution we hear most often is, “I’m going to replace my soil.” This poses a few problems. Consider that it takes 1,000 years to build a single inch of the topsoil and that any soil you bring in might also be contaminated by car exhaust and paint from earlier decades. Furthermore, it is expensive to remove soil, and we only have so many landfills to take it to.
Fortunately, there are better solutions. According to USDA Research Agronomist Rufus L. Chaney, soils containing 500–1,000 ppm of lead can be remediated for safely growing most foods (with the exception of root vegetables). An expert in heavy metal uptake and tolerance by plants, Dr. Chaney has found lead in the fine root hairs of plants and in the skin of root crops, but no evidence of lead uptake in the fruits of plants.
Here are some recommendations for minimizing the risk of lead contamination while still enjoying your garden:
- Get your soil tested. The Alameda County Lead Poisoning Prevention Program’s website and has links to both a directory of certified assessors as well a list of accredited labs, including costs and other information: achh.org/leadpoisoning/testing.htm
- Make/add compost to your soil. This will help raise your soil pH (ideally to pH 6.5–7.5), making it less acidic and minimizing the solubility of lead. In addition, organic matter will bind with the lead in your soil, making less of it available to you or your plants.
- Remediate your soil with a high phosphate fertilizer such as fish bones or bone meal on an annual basis. Lead readily binds with phosphorus to create pyromorphite, a compound that is not readily bioavailable.
- Add a layer of mulch to your soil. Mulch will have the same effects as compost, plus it will provide a physical barrier between bioavailable lead and your plants, shoes, and small hands.
- Plant a groundcover to provide a barrier between you and the soil.
- Grow ornamentals or crops offering edible fruit and/or seeds, since these parts will not accumulate significant concentrations of lead.
- Always wash anything from your garden before you eat it. Greens may not contain lead in their tissues, but lead can splash up onto them when you water your garden. Always peel root crops. Consider filling one raised garden box with clean topsoil to host your roots crops.
- Practice basic hygiene. This includes frequent washing of hands, tools, toys, and other items that come in contact with dirt. Vacuum regularly and take your gardening shoes off outside.
- Repair/replace any peeling paint frequently and do so pursuant to local regulations. Remove any scrap metal that might contain lead—such as old lead pipes, roof flashing, and batteries—and safely dispose of them.
Knowledge is power. With an understanding of lead contamination of soils and how lead gets into our systems, it is possible to make better decisions about the risks and rewards of gardening and to minimize the health risks of lead.•
Human Health Risks and Exposure Pathways
Lead poisoning can cause a variety of effects including neurological and developmental impairments and damage. We become exposed primarily through ingestion, inhalation, and by contact with the skin. In utero cases have been documented as well. The degree to which lead can cause harm depends on how much lead (i.e., the dose), frequency/duration of exposure, and other environmental factors.
Small children are particularly vulnerable to lead poisoning for several reasons: they are smaller in proportion to the exposure than adults would be, they put their hands in their mouths a lot, and their bodies are craving and consuming the building block nutrients for physical development. Children who lack good nutrition, particularly those who are calcium deficient, are at even greater risk as their bodies take up lead almost as a surrogate for the absent calcium. While adult lead poisoning does occur, it is less common. But it should be a great concern to pregnant mothers, who may pass on trace amounts of consumed lead to the fetus.
Roughly half of the lead compounds we find in our yards and gardens occur naturally, existing in combinations such as lead sulfates, lead sulfides, lead carbonates, and lead phosphates. Characterized in chemical terms as “inorganic,” [the majority of] these present little, if any, risk to human health, since they are not generally bioavailable (able to enter the tissues of plants or animals). An example is galena, a pervasive lead-sulfide compound that is prized by gem collectors and spiritual healers. [For an example of one inorganic lead compound that is dangerous, please continue reading below on lead oxide.]
The greatest risk to human health comes from man-made methyl- and ethyl-lead compounds—now mostly banned—that once served a variety of purposes, such as soldering food cans, flashing roofs, preventing engine knock, and making household paint more durable. Various lead compounds were used in pesticides. The list goes on and on. These materials may be toxic outright or they may become toxic as they degrade.
Like all metals, lead rusts when exposed to weather. More specifically, it oxidizes. Oxygen degrades the bonds of the lead compound, recreating [inorganic] lead-oxygen compounds. Known as lead oxides, these degradation products are known to be toxic to humans. Lead paint is a good example. Any home that was built before the 1970s probably contains lead paint. As the paint degrades, chips, and peels, it falls to the ground and continues to degrade in the soil, adding to the load of lead from other sources.
Another very important characteristic of lead is that lead-compounds are not water-soluble, and so they will not migrate by irrigation. They are, however, soluble in acidic conditions and can be taken up by plant roots when soil pH is low.
People sometimes envision visible, soft, gray metal sitting on and mixed into their garden soils and ask, “Why can’t we just pick it out?” In fact, lead particles of most concern range in size from 3 to 250 microns—the average width of a single strand of human hair is 100 microns—so “picking it out” is not really a viable option.
Safe Levels in Soil
Until recently, testing only revealed the total lead value, which includes all types of lead and offers only an implied health risk value. Lead testing available to the public gives a total lead analysis and usually tests topsoil, which presents the highest danger to children. Testing of bioavailability can give a closer approximation of the actual health-risk value. Of the two currently utilized methods, animal testing is rare due to cost and concerns over animal welfare. Better is an in-vitro analysis that aims to replicate the human stomach. Testing in different parts of the country reveals that high total levels of lead don’t always correlate with blood lead poisoning; this is because some types of lead compounds are more bioavailable than others.
So what are safe levels of lead? Regulatory standards at the federal, state, and local levels vary, but are established based on total lead levels for residences. The U.S. Environmental Protection Agency currently has an action level for lead levels at over 400 ppm. The California Department of Health has established level of concern for any residential site that has lead levels in excess of 80 ppm. The challenge with these total lead thresholds is that it is an implied risk which is further compounded by the fact that the “native” levels of lead present in Bay Area soils 100 years ago may have been about 75 ppm. Nonetheless, it is important to know these thresholds and what they mean.