An assessment of trace metals in the soil, vegetation and atmospheric deposition of urban areas in Vancouver. Oka, Gladys Azaria
Urbanization of recent decades has motivated the expansion of urban agriculture as a means to address growing concerns of food security, climate change mitigation and community building, especially in areas facing socio-economic challenges. Community gardens are often relegated to brownfield sites which may have contained some degree of soil contamination prior to remediation. The intrinsic placement of gardens in areas of high industrial exposure poses a concern for atmospheric deposition as another source of contaminants. Metals are of particular interest because they have large anthropogenic contributions and persist in soils for very long periods of time. This study investigates metal concentrations in the native soil and atmospheric deposition of three sites, which represent a range in crop production, site history and industrial exposure. Metal accumulation in the rhizosphere soil, root and shoot of Kentucky bluegrass was assessed. Study sites include the UBC Farm, the 16 Oaks community garden and a brownfield in the Strathcona neighbourhood. Field sampling of topsoil and vegetation took place in the fall. In addition, wet and dry deposition were collected over a period of five months. HCl and aqua regia extraction were performed to determine the labile and total fractions of metals in the soil, vegetation and deposition. During this time Zn, Pb, Ni, Mn and Cu were found at detectable concentrations at all sites. Total metal concentrations were highest at 16 Oaks and lowest at the Farm. Dry deposition was the main mechanism for atmospheric metal contributions and was largest at the Brownfield and lowest at the Farm. Ni and Mn seem to largely originate from parent material while Zn, Pb and Cu may be considerably influenced by atmospheric deposition. High mobility into root and shoot were observed for all metals with large variability at 16 Oaks and the Brownfield. This may be attributed to site heterogeneity, lack of plant preference for accumulation into vegetative parts and large variability in foliar uptake. Future siting of community gardens needs to address the potential additive effects of native soil contamination and atmospheric deposition, as parent material, site history and current deposition trends seem to be complementary to overall soil and vegetative health.
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