Mitigation vs. Remediation
This factsheet outlines both mitigation and remediation strategies for dealing with contaminated soils. Mitigation (coping) strategies involve reducing human exposure to and/or plant availability of contaminants even though they remain in the soil, such as by safe gardening practices and the use of raised beds. Remediation strategies involve removing contaminants, either through biological treatments that break down contaminants, or through physical treatments that remove contaminants that cannot be broken down by biological practices.
Tips for Dealing with Heavy Metal Contaminants
Once urban farmers have determined their soil safety, and decided to pursue production, there are simple precautions you can take to minimize your exposure to contaminants and toxicity to plants.
- Plant crops away from building foundations, painted structures and heavily traveled roads;
- Remove obvious contaminants such as scrap metal and construction materials;
- Use mulch and cover crops to minimize exposure to contaminated dust, and to maintain high levels of organic matter;
- Because concentrations of heavy metals are highest in roots and leaves, avoid planting and eating leafy or root vegetable crops in soils with heavy metals above typical;
- Lime, compost, or amend soil to keep pH close to neutral, or even slightly alkaline, and ensure adequate draining to reduce the mobility and availability of lead and heavy metals;
- Do not use plants grown in contaminated soil for compost;
- Work in the garden only when soil is moist or damp;
- Wear gloves, long sleeves and pants while gardening to prevent skin exposure;
- Wash hands after gardening;
- Wash all vegetables thoroughly; and
- Remove gardening shoes and garments before entering the home, and wash gardening clothes separately from other clothing.
Mitigation and Remediation Strategies
There are physical, biological, and non-remediation strategies available to urban farmers dealing with contaminated soils. When deciding how to deal with soil contamination, farmers should consider cost, effectiveness, and time frame to determine the strategy that best suits their needs.
“Urban Agriculture and Soil Contamination: A Guide to Urban Gardening” by Allison Houlihan Turner from the University of Louisville provides a helpful overview of different coping and remediation strategies and is available for download from http://louisville.edu/cepm/publications, under Practice Guides.
Non-Remediation (Mitigation) Strategies
The most popular non-remediation strategies for urban farmers dealing with soil contamination are raised bed and container gardening (see Factsheets #11 and #12, Raised Beds and Container Gardening).
One study of gardens in Roxbury and Dorchester, MA, demonstrated the effectiveness of raised beds in reducing exposure to contaminated soil, and found that raised beds require regular maintenance in order to achieve exposure reduction. Recommended maintenance includes removing the top 3-5 cm of soil and replacing it with compost every year (Clark, Hausladen and Brabender 2008).
Soil excavation (removing contaminated soil) is another non-remediation strategy, but is not recommended. The high cost of removing contaminated soil and purchasing clean soil and amendments, often make excavation prohibitively expensive. Additionally, farmers are faced with the added problem of disposing of contaminated soil.
Strategies for physical remediation include excavation, soil washing, and soil vapor extraction. These are often both effective and timely, but are high-cost and unfeasible for most urban farmers. Additionally, disposal of contaminated soil can be difficult and expensive.
Strategies for biological remediation include microbial remediation, phytoremediation, fungal remediation, and compost remediation.
These strategies are often inexpensive, but require an extended time frame. Biological remediation strategies are still being developed by the Environmental Protection Agency and private companies, and their effectiveness still being researched.
Youarethecity, a research practice in New York City, has developed a Field Lab at La Finca del Sur in the Bronx, where team members are planting, monitoring, and harvesting several plant varieties known to remove toxins from soil. This project is intended to make phytoremediation research accessible and pertinent to the public, and to urban gardeners in particular. For more information, including postings of phytoremediation workshops at La Finca del Sur, visit the Field Lab blog at http://newyork.thecityatlas.org/category/atlas-lab/brownfield-remediation/ or http://www.youarethecity.com/.
Youarethecity published “Brownfields to Greenfields: A Field Guide to Phytoremediation,” which is distributed at related workshops. The guide is available for free download at youarethecity.com/pdf/fieldguideyouarethecity.pdf.
For More Information
The Cornell Waste Management Institute, Resource Centres on Urban Agriculture (RUAF), and University of Louisville each provide information on best practices and crops for safe gardening in contaminated soils (see Factsheet #6, Soil Contamination for website links).
The Environmental Protection Agency also has a website on Urban Agriculture & Improving Local, Sustainable Food Systems, which features “Brownfields and Urban Agriculture: Interim Guidelines and Safe Gardening Practices” and downloads and information from the two-part webinar series, “Brownfields and Urban Agriculture Reuse.”
Both are freely available at http://www.epa.gov/brownfields/urbanag/, along with several other resources about brownfield clean up. “Evaluation of Urban Soils: Suitability for Green Infrastructure or Urban Agriculture” (EPA publication number 905R1103, 2011), includes remediation and coping strategies, available for free download at http://water.epa.gov/infrastructure/greeninfrastructure/upload/Evaluation-of-Urban-Soils.pdf.
There are also several organizations that host workshops about soil contamination and managing contaminated soils, such as Cornell Cooperative Extension (http://www.cce.cornell.edu/) and the Northeast Organic Farming Association of New York (http://www.nofany.org/). See their event listings for workshops and details.
Clark, Heather F., Debra M. Hausladen, and Daniel J. Brabender. “Urban Gardens: Lead Exposure, Recontamination Techniques, and Implications for Remediation Design,” published in Environmental Research, Volume 107, Issue 3, 2008.