by Christopher Kelly-Bisson

In every city in Canada there are spaces that have become abandoned and unused. These spaces are shadows of bygone industrial history, where warehouses, factories, and stockyards once stood. Today, such places are known by the rather grey and yellow grasses, mullein, and golden rod that poke up through hard-packed dirt or gravel. These brownfield sites are the forgotten ecosystems of our cities, slowly on the mend from the legacy of industrial exploitation that colonized this country. But where rich condo developers fail to see potential to turn a profit, communities are continuously stepping in to occupy these spaces, and use them for projects such as community gardens.

Abandoned industrial areas however contain significant hazards. Perhaps the greatest of all is soil contamination. For the establishment of community gardens this can be a significant barrier. One that is worth employing whatever limited resources are available to properly develop a plan to mitigate.
This article will cover how to conduct soil testing to determine the safety of soils for the establishment of new urban community gardens. First, detailed steps will be provided for collecting a soil sample in accordance to the Government of Ontario’s standards. Second, a description will be given on how to analyze your soil test results. Finally, this article will cover what strategies can be taken to deal with various levels of contamination.
Conducting a Soil Sample
Officially, the Ontario Department for Municipal Affairs and Housing requires that any testing be carried out by a “Qualified Person”. If you require an official stamp of approval for brownfield revitalization for what you are planning to do, make sure to approach Ontario’s MAH office for further advice. Some community projects are more spontaneous and organic however, and deserve the safety of sufficient testing to rule out significant hazards to growing food on the site. This article covers what communities and individuals can do, but does not replace what the authorities require for certified planning and development.
Equipment

• hand trowel or soil core sampler
• metal pale
• glass jar with lid

Make sure to thoroughly wash all tools of any dirt from other sites before collecting your soil sample. Also make sure not to use any detergents to wash your tools, since they can contaminate the sample.

Method of Soil Collection
The Ontario Ministry of Food, Agriculture and Rural Affairs (OMAFRA) provides farmers with details on how to test the soil in their fields. They suggest that one soil sample be taken for every 20,000 tonnes of soil to accurately represent 25 acres of land. Most communities will find themselves working with less than a quarter acre, so one sample is more than enough.
In order to collect a sample that evenly represents the site you need to collect one trowel-load, or soil core from 20 holes, 6 inches deep. The holes should follow a zig-zag pattern across the site. Soil collected from each hole should then be thoroughly mixed into a metal pale. One sample must consist of two cups, and should be sealed in a glass jar with no header space between the soil and the lid. Samples must then be taken immediately to the lab for testing.
OMAFRA keeps a list of accredited labs that are qualified to conduct soil tests. In Ottawa, I have been using Exova Accutest to test my soil samples. At the lab they can even provide you with glass jars for your sample. For more information about where you can find an accredited lab in your area, consult the OMAFRA website.

Reading Your Soil Analysis
Soil sample analyses are typically ready within one week after submission. The results are then returned digitally as a PDF file by e-mail. The analysis document will only contain numbers without interpretation. The lab can provide some basic interpretations for agriculture properties if requested, such as what crops are best for your soil. They can also provide interpretation for contaminants if documents are provided specifying maximum thresholds for certain minerals and chemicals.
Interpreting results oneself is fairly easy if one know what documents to look for. For my projects I consulted the Ontario Ministry of the Environment’s 2011 index of contaminant thresholds listed as part of the Environmental Protection Act. Contamination is apparent in a soil sample if the quantities of chemicals or minerals revealed in the sample exceeds the levels specified in the Environmental Protection Act index.

Mitigation of Contaminated Soil
A contaminated soil sample requires careful consideration on how to use the site. The safest and surest response would be to contact the proper authorities if you detect that your soil is contaminated. There are some indications however that simple mitigation strategies can be used safely to permit food production on a contaminated site. A document by Toronto Public Health (TPH) outlines a method of assessing what strategy to use.
First, one needs to determine whether the level of concern. TPH suggests that hazardous soil contamination is detectable by scent, historical land use, and tangible evidence (piles of garbage and debris).
If there is no history of site use that could lead to contamination, no noticeable odour, and no visual indication of contaminants then the site is considered low risk. The only considerations for a low risk site is to wash hands after contacting soil, and to wash any vegetables grown.
If there are any indications that contamination could have occurred, then it is suggested that a soil test be taken. Intermediate risk mitigation then involves introducing clean soil and compost from a clean site, adding organic matter to raise soil pH, and adding a deep layer of mulch to prevent exposure of soil.
If contamination is clearly evident, and soil samples confirm this, a more aggressive mitigation strategy is needed. Raised-bed gardens should be built on top of a layer of geotexile topped by a few inches of clean soil, compost, or sand. All exposed soils should be deeply mulched. The only plants that can safely grow in contaminated soils for food production are nut and fruit trees.
In one of my projects we experimented with building a 4-5 ft. tall hugelkultur bed on top of a site contaminated with heavy oil products (diesel and black top leached from a parking lot). The hope was to grow a pollinator garden and inoculate logs with mushrooms in subsequent years after its establishment. The goal was to see if mycchoculture and plant diversity would aid in breaking-down the hydrocarbons found in the soil. This experiment was terminated however because Carleton University has decided to evict our garden from the site in order to build a new student residence.

Sources:

Ministry of the Environment. 2011. Soil, Ground Water and Sediment Standards for Use Under Part XV.1 of the Environmental Protection Act. PIBS#7382e01.
Reid K. 2006. Soil Sampling and Analysis For Managing Crop Nutrients. OMAFRA. AGDEX 533, Order No. 06-031.
Toronto Public Health. 2011. Soil Assessment Guide for New City Allotments and Community Gardens – Summary.