Articles

Urban Gardening

Why is Lead a Problem for Urban Gardeners?
Lead contaminated 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 the lead-laden exhaust fumes from 60+ years of automobiles burning leaded gasoline. Distributed by wind and rain, these lead particles have settled onto open fields, parks, playgrounds, schoolyards, residential lawns and gardens where it all lies today. With urban agriculture gaining in popularity, lead contaminated soil presents a new exposure problem.

The biggest risk for urban gardeners is the potential for inhalation and ingestion of lead particles encountered during the process of gardening. Wearing masks and gloves when working in gardens contaminated with lead and applying 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 grown. Since these factors vary significantly in different locations, there are no blanket safety standards for urban gardens.

Why is the Garden Site History Important?
Abandoned lots in residential areas can also pose an exposure problem. Demolished dwellings are typically older homes which commonly contain lead paint. Although the structures have been removed, the demolition process often leaves behind lead paint chips in the soil. Garden lots near heavy traffic roadways may also have high concentrations of lead from car exhausts during the many years American automobiles burned leaded gasoline.

Are Some 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 lead concentration in their roots.2

Although there are over 500 plant species capable of accumulating heavy metals from the soil, few take up lead.3 For plant that do, studies show lead does not readily accumulate in the fruiting parts of vegetable or in fruit crops such as corn, beans, squash, tomatoes, strawberries, apples. Higher concentrations are more likely to be found in leafy vegetables such as lettuce and on the surface of root crops such as carrots.4

Generally guidelines suggest it is 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 from car exhausts and will 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 particles 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 Can Clean Soil be Maintained?
Regardless of the abatement method used or if clean soil is imported into the garden, it is important to note the remediated area may still be surrounded by properties containing lead contaminated soil. Wind and rainwater run-off can migrate their dirt and dust onto neighboring properties including those that have experienced remediation efforts.

Studies show that after just 3-4 years remediated soil will be re-contaminated with lead dust blown in from nearby properties.8 In light of this, many urban gardeners cover their beds with mulch or geotextile covers that can be easily removed and replaced as needed. Covering the soil surrounding the garden with grass, mulch or stones will help reduce fugitive dust blowing onto the cultivated area.

Hoses used to water urban gardens can also add lead to remediated soil.9 Hose manufacturers use lead to preserve their durability hosing material which otherwise tends to deteriorate under extended exposure to the sun. Testing shows considerable lead levels will leach from these types of hoses. To avoid this, gardeners can purchase lead-free hoses which are often white with a blue stripe. These products are commonly sold by marine and RV retailers.

Household water pipes can be another source of lead. Those that contain lead will experience some it leaching into the water which is then disbursed onto the soil when watering gardens. To avoid this source of lead, some gardeners use rain water collected in specially designed barrels. These easy to use products are sold by many home and garden retailers.

How is Soil Tested for Lead?
Lead particles do not 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 about $25-$50, there are accredited laboratories that test soil samples sent to them.10

To take a soil sample, use a clean trowel or large spoon for scooping about half a cup of soil from the top inch of the bare soil needing testing. Try not to scoop up plant leaves, roots, or other large pieces of debris. If there are paint chips in the soil it is okay to include them in the sample. Place the soil into a clean, plastic sandwich bag. Seal the bag with a 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 taking each soil sample. Lab results usually take only 24 – 48 hours.10

If you need 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 ppm, children and pregnant women should avoid contact with this soil.11

References
  1. 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
  2. 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
  3. Phytoremediation: Cleaning the Soil with Flowers? Shannon, Trueman, About Education, accessed September, 2014
  4. 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
  5. Concerns About Lead in Urban Gardens, Vanessa Ventola, Growing Culture, accessed September, 2014
  6. 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
  7. Concerns about Lead in Urban Agriculture, Vanessa Ventola, A Growing Culture, 2012.
  8. 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.
  9. Chemical in Hoses Leach into Water, Jeff Gearhart, Ecology Center, May 3, 2012
  10. Testing Your Home for Lead in Paint and Soil, California Department of Public Health, accessed September, 2014
  11. Soil Lead: Testing, Interpretation, & Recommendations, University of Massachusetts, Department of Plant and Soil Science Fact Sheet Soil Testing, accessed September, 2014