Measurement of DDT Fluxes from a Historically Treated Agricultural Soil in Canada

Organochlorine pesticide (OCP) residues in agricultural soils are of concern due to the uptake of these compounds by crops, accumulation in the foodchain, and reemission from soils to the atmosphere. Although it has been about three decades since DDT was banned for agricultural uses in Canada, residues persist in soils of some agricultural areas. Emission of DDT compounds to the atmosphere from a historically treated field in southern Ontario was determined in fall 2004 and spring 2005. The ΣDDTs concentration in the high organic matter (71%) soil was 19 ± 4 μg g^-1 dry weight. Concentration gradients in the air were measured at 5, 20, 72, and 200 cm above soil using glass fiber filter−polyurethane foam cartridges. Air concentrations of ΣDDTs averaged 5.7 ± 5.1 ng m^-3 at 5 cm and decreased to 1.3 ± 0.8 ng m^-3 at 200 cm and were 60−300 times higher than levels measured at a background site 30 km away. Soil−air fugacity fractions, f_s/(f_s + f_a), of p,p‘-DDE, p,p‘-DDD, and p,p‘-DDT ranged from 0.42 to 0.91 using air concentrations measured above the soil and ≥0.99 using background air concentrations, indicating that the soil was a net source to the background air. Fractionation of DDT compounds during volatilization was predicted using either liquid-phase vapor pressures (P_L) or octanol−air partition coefficients (K_OA). Relative emissions of p,p‘-DDE and p,p‘-DDT were better described by P_L than K_OA, whereas either P_L or K_OA successfully accounted for the fractionation of p,p‘-DDT and o,p‘-DDT. Soil-to-air fluxes were calculated from air concentration gradients and turbulent exchange coefficients determined from micrometeorological measurements. Average fluxes of ΣDDTs were 90 ± 24 ng m^-2 h^-1 in fall and 660 ± 370 ng m^-2 h^-1 in spring. Higher soil temperatures in spring accounted for the higher fluxes. A volatilization half-life of ∼200 y was estimated for ΣDDT in the upper 5 cm of the soil column, assuming the average flux rate for 12 h d^-1 over 8 months of the year. Thus, in the absence of other dissipation processes, the soil will continue to be a source of atmospheric contamination for a very long time.