Air−Soil Exchange of Organochlorine Pesticides in Agricultural Soils. 1. Field Measurements Using a Novel in Situ Sampling Device

Abstract

Initial results are presented for in situ measurements of soil−air partitioning for a range of organochlorine (OC) pesticides in two contaminated agricultural soils. A soil survey was conducted and used to identify high levels of several OC pesticides in two regions of southern Ontario that are known for their intensive agriculture, the Tobacco Belt and the Holland Marsh. Experiments were conducted at one field in each region by sampling air very close to the soil surface using a disc-shaped sampler. The equilibrium status of the sampled air was tested by comparing the chiral signature of the soil with the signature in air sampled by the device and ambient air. Although results showed that 104% of trans-chlordane (TC) and 96% of cis-chlordane (CC) in the air under the sampler originated from the soil, the propagated errors in these results (34% SD for TC and 26% SD for CC) are too large to provide conclusive evidence for equilibrium. Therefore, a soil−air quotient (QSA) is reported here instead of the soil−air partition coefficient (KSA). This value is an approximation of the “true” KSA. Results show a linear relationship between log QSA and log KOA and fit in with the relationship KSA = 0.411ρφOCKOA where ρ is the soil density (kg L-1). Using this relationship, fugacities were calculated in air and soil. Results of this calculation identify a strong disparity that favors soil-to-air transfer. This gradient is confirmed by measurements at different heights over one of the fields. Soil−air exchange is a key process in the overall fate of OC pesticides. The results from this study will improve our ability to model this process and account for differences between soils.