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(P902a) Soil-air exchange of organochlorine pesticides in the Fraser Valley, British Columbia.
Bidleman, Terry*,1, Leone, Andi1, Van Vliet, Laurens2, Szeto, Sunny3, Ripley, Brian4, Harner, Tom1, Shoeib, Mahiba1, 1 Meteorological Service of Canada, Downsview, ON, Canada2 Agriculture and Agri-Food Canada, Agassiz, BC, Canada3 Consultant, Vancouver, BC, Canada4 University of Guelph, Guelph, ON, Canada
ABSTRACT- The Fraser Valley is one of the main vegetable-growing regions of Canada, and several organochlorine pesticides were applied during the 1960s and 70s. Residues persist in the soils today, especially in the organic-rich muck soils. This study was carried out to determine the potential for residues in the soil to volatilize into the atmosphere. Air samples were collected in August 2000 and 2001 at 40 cm height above six fields representing three soil types with varying levels of organic matter (muck, silt loam and sand loam). Ambient air samples were also collected within the region but not directly over agricultural fields. The air samples and soils were analysed for DDT compounds, chlordanes, dieldrin and lindane. Concentrations of pesticides in the air directly above the fields were positively correlated with soil residues. Soil/(soil + air) fugacity fractions (calculated from pesticide concentrations in soil and air, the fraction of organic matter in the soil and the octanol-air partition coefficient, Koa) were >0.5 in many cases, indicating the potential for net volatilization. Strong log-log correlations were found between the Koa (positive) or liquid-phase vapour pressure (negative) of the pesticide and the soil/air concentration (Cs/Ca) at the two muck farms, showing the influence of physicochemical properties on volatilization. Enantiomer fractions, EF = (+)/[(+) + (-)], of trans- and cis-chlordanes in air-above-soil samples matched those in muck soils where net volatilization was predicted from fugacity ratios. Differences between soil and air EFs were observed above silt loams with lower chlordane residues and less potential for volatilization.
Key words: pesticides, soils, atmospheric transport, chiral compounds
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