Lead in Urban Gardens
Why is Lead a Problem for Urban Farming?
Lead contamination of soil can occur from air pollution, water pollution and prior use of land.1 Soil in urban and suburban areas have higher lead concentrations than rural areas due to their proximity to industry and automobile exhaust fumes laden with lead particles during the 60+ cars operated with leaded gasoline. Distributed by wind and rain, these lead particles have settled onto open fields, parks, playgrounds, school yards, homes and gardens where it all lies today. With urban agriculture growing in popularity, lead in soil presents a new exposure problem.
The biggest risk with urban gardening is the potential for inhalation and ingestion of lead particles encountered in the process of gardening. Wearing masks and gloves when working in gardens contaminated with lead and apply water to keep down dust are helpful precautions. Thorough washing of hands and the harvested produce will also limit lead intake.
Is Food Grown in Urban Gardens Safe?
The safety of produce grown in urban gardens depends on a variety of factors which include (1) the site’s environmental history, (2) the level of contamination, (3) past remediation efforts and (4) the types of food being produced. Since these factors vary significantly in different locals, there are no blanket safety standards for urban gardens.
Why is the History of Site Important?
Site history is important because many abandoned lots converted into communal gardens were once used as industrial sites where lead and other heavy metals are often found. In some situations the site may have experienced remediation efforts. Although this is a good thing, remediation methods can damage the soil with detrimental effects on its ability to grow fruits and vegetables.
Are Different Plants Safer than Others?
The main concern with foods grown in urban gardens is leaded dust landing on the produce. This can be easily removed by thoroughly washing fruits and vegetables before they are eaten. There are, however, plants which take up lead from the soil they are grown in. For those that do, they tend to have the highest 2
Site history is important because many abandoned lots converted into communal gardens were once used as industrial sites where lead and other heavy metals are often found. In some situations the site may have experienced remediation efforts. Although this is a good thing, remediation methods can damage the soil with detrimental effects on its ability to grow fruits and vegetables.
Are Different Plants Safer than Others?
The main concern with foods grown in urban gardens is leaded dust landing on the produce. This can be easily removed by thoroughly washing fruits and vegetables before they are eaten. There are, however, plants which take up lead from the soil they are grown in. For those that do, they tend to have the highest 2
Although there are over 500 plant species capable of accumulating various heavy metals from the soil, few take up lead.3 And studies show lead does not readily accumulate in the fruiting parts of vegetable and fruit crops (e.g., corn, beans, squash, tomatoes, strawberries, apples). The higher concentrations are more likely to be found in leafy vegetables (e.g., lettuce) and on the surface of root crops (e.g., carrots).4
Generally, it has been considered safe to use garden produce grown in soils with lead levels less than 300 ppm.5 However, the risk of lead poisoning through the food chain increases as lead levels in the soil rise above this concentration. Even at soil levels above 300 ppm, the greatest risk from lead contaminated soil comes from wind blown dust deposits on the plants rather than the uptake of lead by the plant.
What are the Available Solutions?
Several options are accessible to urban gardeners:
Raised Beds
Produce can be grown with safe, imported soil placed into containers or raised beds constructed above the contaminated soil. When using this method it is best to lay a geotextile fabric barrier between the contaminated soil and the raised bed to prevent migration of lead into the clean soil and to keep roots in the raised bed from penetrating into the leaded soil below.
Composting
Adding organic matter into the soil can significantly reduce lead availability to the plants by binding with lead particles to create a secondary compound that is more difficult for plants to absorb. Additives that keep the soil’s pH level maintained above 6.5 will aid the effectiveness of this process.6
Good sources of organic matter include composted leaves, neutral (non-acid) peat, and well-rotted manure. However, organic material obtained along highways or city streets should be avoided as they are likely to contain lead and therefore increase the pollution problem at your location
Bioremediation
This involves a family of methods that use organisms to remove or neutralize pollutants in contaminated sites. One method is phytoremediation which uses plants to absorb the containments and then the harvested plants are collected and disposed of in a safe manner off site. There are also Microbial and Fungal remediation methods which rely on microbes and fungi to degrade the contaminant into a less toxic form.
Rooftop Gardening
A study done in New York shows airborne heavy metal particulates are ten times less at six stories above ground level.7 Thus, rooftop gardens provide a safer haven for urban garden projects.
Hydroponics
This is an emerging technology that avoids soil by growing produce in liquids (usually water) with nutrients of soil-based mediums added into the liquid.
Extraction
This is an expensive method wherein the leaded soil is removed, transported to a proper disposal facility and replaced with new, clean soil.
Soil Washing
This is another expensive soil-removing method which uses liquids (usually water, sometimes combined with chemical additives) and a mechanical process to scrub soil to remove its hazardous contaminants. The process concentrates the contaminants into a smaller volume which can be disposed safely.
How is Soil Tested for Lead?
Lead particulates do not just disappear or become harmless over time, so if you live in urban, suburban or rural areas close to industry or busy highways, it is a good idea to test your soil for lead. For a cost of $25 – $50, there are accredited laboratories that will test samples sent to them. 10
To take a sample, use a clean trowel or large spoon for scooping about half a cup of soil from the top inch of the bare soil you want to test. Try not to scoop up plant leaves, roots, or other large pieces of debris. If there are paint chips in the soil, it is OK to include them in the sample. Place the soil into a clean, plastic sandwich bag. Seal the bag and label it with the label stating where the sample was taken (i.e., Sample # 2 – under children’s swing set). Wash your hands and the spoon with soap and water after each soil sample you take. Lab results usually take only 24 – 48 hours.10
If you require assistance interpreting your results you can contact your state environmental agency, local health agency or USDA Cooperative extension offices for advice.
If soil tests reveal lead levels greater than 300 parts per million, children and pregnant women should avoid contact with this soil.11
References:
Urban Agriculture and Soil Contamination: An Introduction to Urban Gardening, Turner A. Houlihan, University of Louisville, August 2012. Link: http://louisville.edu/cepm/publications/practice-guides-1/PG25%20-%20Urban%20Agriculture%20-%20Soil%20Contamination.pdf/at_download/file
Brownfields and Urban Agriculture: Interim Guidelines for Safe Gardening Practices EPA. Chicago, IL: Region 5 Superfund Division, U.S. Environmental Protection Agency (Summer 2011) Available: http://www.epa.gov/swerosps/bf/urbanag/pdf/bf_urban_ag.pdf
Phytoremediation: Cleaning the Soil with Flowers? Shannon, Trueman, About Education, accessed September, 2014
Lead in the Home Garden and Urban Soil Environment, Carl J. Rosen (Extension Research Soil Scientist), Department of Soil, Water and Climate, University of Minnesota
Concerns About Lead in Urban Gardens, Vanessa Ventola, Growing Culture, accessed September, 2014
Lead in the Home Garden and Urban Soil Environment, Carl J. Rosen, (Extension Research Soil Scientist, Department of Soil, Water and Climate) Regents of the University of Minnesota, 2010
Concerns about Lead in Urban Agriculture, Vanessa Ventola, A Growing Culture, 2012.
Urban Gardens: lead exposure, recontamination mechanisms, and implications for remediation design, HF Clark, DM Hausladen, DJ Brabander, Environmental Research, vol. 107, Issue 3, July 2008, Pages 312-319.
Chemical in Hoses Leach into Water, Jeff Gearhart, Ecology Center, May 3, 2012
Testing Your Home for Lead in Paint and Soil, California Department of Public Health, accessed September, 2014
Soil Lead: Testing, Interpretation, & Recommendations, University of Massachusetts, Department of Plant and Soil Science Fact Sheet Soil Testing, accessed September, 2014
