22.3: Testing for Soil Contaminants

When evaluating a plot of land for its suitability for urban farming or gardening, the first step is to research its history. Try talking to the property owner, and use the internet, public library, city hall or tax assessor’s office to seek records that would reveal past uses. Useful records include old aerial photos, maps, permits and tax records. Also, visit the site to see whether potential sources of contamination are nearby, such as old houses with peeling paint or a highway. Both cases could mean a high level of lead in the soil. Generally, a site that has a long history as a green space or residential property will have fewer problems than one with a commercial or industrial past (Table 22.1).

After you learn what you can about the property’s history, consult with your state environmental agency, local health department or local Cooperative Extension office to determine the kinds of tests you should perform to accurately assess the condition of the soil. Also, while there are interim guidelines published by the U.S. Environmental Protection Agency (EPA, Brownfields and Urban Agriculture: Interim Guidelines for Safe Gardening Practices, 2011), there are no established federal rules for what soil contaminant levels are considered safe for urban agriculture, so you should work with these qualified professionals to interpret the results of tests and make a plan to recondition the soil. At a minimum, the EPA recommends that urban soils be tested for pH, percent organic matter, nutrients, micronutrients and metals, including lead. Soil testing is described in detail in Chapter 21.

When testing for potential contaminants, you may need to collect samples separately for each contaminant you want to test for, and your sampling procedure for each may vary. For example, you may collect samples at different depths depending on the suspected contaminant (a heavy metal near the surface versus a solvent that may have leached into the soil), or the intended use of different sections of the property (a play area versus a growing area). In addition, contaminants may have been buried in the past.

The distribution of contaminants can be unpredictable, so testing in many locations in the plot may be required. Sections of a property that have obvious signs of potential problems may require separate testing procedures. These can include areas next to old buildings with peeling paint (a higher risk of lead), patches of bare ground where vegetation would otherwise be expected (a sign of compaction or concentrations of toxic compounds), or near stormwater drainage features (which could be bringing petroleum-based compounds, pesticides or other chemicals onto the property from the surrounding neighborhood). Note that the presence of lead in the soil rarely causes physical damage to plants. On the other hand, other metals, such as copper, zinc and nickel can be phytotoxic at high concentrations.

A thorough site assessment of a property should also take into account other conditions that could affect its viability for urban farming or gardening, such as slope and drainage patterns, the presence of aboveground and belowground utility lines, or existing unwanted structures, including possibly the buried foundations of previous buildings. (In the United States, visit www.call811.com, or call 811 to get information on buried utility lines before starting any digging project.)

Seek Out Additional Resources

Due to the potential risk to human health of farming on contaminated soils, it is advised to work with environmental consultants and local Extension specialists with expertise in urban soils when assessing whether to use a site. Depending on the severity of its problems, it can be expensive to assess and clean up a site. The EPA Brownfields Program (www.epa.gov/brownfields) offers grants to state, local and tribal governments, as well as to nonprofits, for these purposes, and might be an option when one or more urban farms seek to partner with a local municipality to clean multiple sites at once. The USDA’s Urban Agriculture Toolkit provides information on how to start an urban farming operation and identifies technical and financial resources that might be available to help with each step.

Further reading on the risks and recommended approaches to site assessment, soil testing and soil management is readily available through state Extension offices and federal agencies, such as:

• Brownfields and Urban Agriculture: Interim Guidelines for Safe Gardening Practices (EPA)
• Evaluation of Urban Soils: Suitability for Green Infrastructure or Urban Agriculture (EPA)
• Gardening on Brownfields series (www.gardeningonbrownfields.org, Kansas State University)
• Gardening on Lead Contaminated Soils (Kansas State University)
• Soils in Urban Agriculture: Testing, Remediation and Best Management Practices … (University of California)
• Minimizing Risks of Soil Contaminants in Urban Gardens (North Carolina State University)

Urban Agriculture and Soil Contamination: An Introduction to Urban Gardening (University of Louisville)