U of T Study looks at hazards of gardening close to traffic in Toronto

Wednesday, April 11, 2012 7:45:00 PM


Clare Wiseman
(Photo:Rita Leistner)


A University of Toronto study led by Professor Clare Wiseman of the Centre for Environment is looking at the hazards of gardening in close proximity to traffic in Toronto.  Since 2010, common garden plants such as oregano and eggplant were cultivated at four locations: two locations on the U of T's downtown St. George campus (on Galbraith Building's rooftop and at the corner of St. George St. and Hoskin Ave.) and two in the west end of Toronto near the Gardiner Expressway.  Dr. Wiseman will give a presentation on this research at the Centre for Environment's Research Day on Wed April 18.  Her research was also featured in the Spring 2012 issue of U of T magazine.

The study seeks to strengthen our current knowledge of the fate of traffic-related trace metal emissions and their uptake by plants.  The goal is to elucidate how soil trace metal behavior and bioaccessibility is influenced by stabilization processes, as determined by primary physico-chemical soil parameters and changing field conditions over time. The ultimate aim is to assess the hazard potential of gardening in close proximity to traffic and provide a better knowledge basis for more informed decision-making in the establishment of community gardens in Toronto.

Preliminary results from the first phase of this study using the application of a microwave-assisted acid digestion procedure and ICP-MS for both plant tissue and soil samples suggest several important trends. Firstly, the location of gardens matters. Plants grown on soils close to higher volumes of traffic generally contain higher levels of trace metals. Oregano grown close to the Gardiner Expressway, for instance, had significantly higher concentrations of lead compared to a residential location with a low volume of traffic (an average of 4.67 mg/kg d.w. vs. 0.84 mg/kg d.w.). Secondly, metal uptake by plants is strongly species dependent. Eggplant grown at the roadside St. George campus location had higher levels of toxic cadmium on average (0.22 mg/kg d.w.) compared to oregano grown in the same bed (0.13 mg/kg d.w.). Thirdly, trace metal concentrations in the rhizosphere of cultivated eggplants were significantly higher compared to the bulk soil. This was particularly apparent with cadmium, which reflects the high environmental mobility of this metal. While cadmium concentrations were decreasing in the bulk soil over the 2010 growing season, they were simultaneously increasing in the soil in close contact with eggplant roots. At harvest, mean cadmium concentrations in the eggplant rhizosphere were 0.30 mg/kg. This was twice the amount measured for this metal in bulk soil, which indicates the importance of the plant rhizosphere and related exchanges between plant roots, soils and microorganisms in influencing the trace metal soil chemistry.

The results of this study strongly suggest that soil quality guidelines based on bulk soil concentrations are inadequate for they do not account for the rich, dynamic processes occurring at the soil-plant root interface which may ultimately influence the fate and uptake of trace metals. Overall, the study results thus far indicate that the extent of contamination in existing Toronto soils is comparatively low for many toxic metals. This holds true for even roadside soils, in close proximity to moderate levels of traffic, which had demonstrated lower levels of most trace metals such as cadmium compared to an organic triple-mix soil bought from a local garden supplier. The next phase of this research will continue to explore current trends, as well as examine the role of mineralogical controls in governing the fate of metals from soil to plants.

For more information, please contact Dr. Wiseman at clare.wiseman@utoronto.ca.