Soils are often compacted, nutrient deficient, and can sometimes contain heavy metals and other contaminants as a result of historical industrial activity, past and present land use and nearness to pollution sources (i.e. a major road, a coal plant). We can be exposed to contaminants through soils and these can have health effects over the long term.

What is a soil contaminant and where does it come from?

A soil contaminant is an element (like lead) or a chemical (like diesel oil, also known as ‘organics’) present in the soil at a level that poses health risks to soil, plant, animal or human health. Contaminants can end up in your soil or on your plants through the air (dust, exhaust), water (rain, groundwater), and direct deposition (from on site or nearby polluters).

**Soil contaminants include: Heavy metals (Inorganic chemicals)

Heavy metals are those elements which are toxic to humans at certain concentrations, including arsenic, copper, lead, mercury, nickel, chromium, cadmium, zinc and aluminum. Some of them, like zinc and copper, are necessary or beneficial to living organisms in small concentrations but are toxic above a certain concentration; others, like lead, cadmium and mercury, serve no known biological function and are always toxic. Heavy metals are naturally occurring, found in rocks, soil systems and bedrock, and in some places a certain metal may be naturally present in higher concentrations. The majority of heavy metal contamination arises from human activity– metal mining and smelting, agrochemical fertilizers and pesticides, sewage sludge, oil and gas operations and fossil fuel burning, improper waste disposal, and fill used in residential development. Unlike organic contaminants, heavy metals cannot be broken down. (Well, except through nuclear fission!) As such, they can continue to build up in soils. But their characteristics may change so that they can be more or less easily taken up by plants or animals. Many of the practices that gardeners, especially organic gardeners, already use in their gardens- such as mulching, feeding the soil with organic matter and compost- can limit the potential for soil contaminants to be taken up in food crops.

Organic Chemicals Chemical/organic contaminants are carbon-based, meaning they are derived or manufactured from something that was once alive, for example, oil and gasoline, which is the remains of plant and animal matter that was compressed for millions of years, then pumped out of the earth and processed. Polyaromatic Hydrocarbons (PAH’s) and other petroleum hydrocarbon contaminants, solvents like trichloroethylene (TCE), dioxins and Polychlorinated Biphenyls (PCB’s) are a few organic contaminants of note. They can be present in soils, particularly on or near historical or present-day auto and machine repair shops, old or leaky oil tanks, busy roads or highways, landfills and dumps, beneath electrical stations and wires, and places where there were building fires or demolished buildings. If you’re concerned you have this type of contaminant because of historical or present land use, or nearness to a busy road or auto-shop, you can get a soil test done for Total Petroleum Hydrocarbons (TPH’s), and /or Polycyclic Aromatic Hydrocarbons (PAH’s) and either avoid growing in areas of contamination, grow in raised beds, or take steps to remediate the soil (see “Earth Repair, by Leila Darwish”).

Should I be concerned about soil contamination?

It is likely that the soil in any urban setting will contain some level of heavy metal and organic contaminants, and long-term exposure can have health risks, particularly for children. Lead and other heavy metals in soil can enter the human body through inhalation or ingestion of dust and soil (on vegetables) and, to a lesser extent, through the consumption of produce grown in contaminated soil, as some plants uptake certain heavy metals. For this reason, it is a great idea to learn more about your soil through the following steps:

• Establish the level of concern. Assess your site, and learn more about the site history. If your site was an infill area, an orchard, a landfill, a commercial property, or near a railway, it can likely be considered “medium concern.” If it was or is nearby a gas station, dry cleaner, printing or autobody shop, rail line, industrial site, demolished buildings or renovation of old homes, or had garbage dumped or burnt, it may be considered “higher concern.” Read more about how to establish levels of concern in the City of Toronto’s “Guide for Soil Testing in Urban Gardens,” available online (see “Resources”).

• Test the soil. For information on how to sample and test the soil for contamination, see activities below.

• Take action to reduce risks.

Questions

Activities