Air-soil exchange of organochlorine pesticides in agricultural soils. 2. Laboratory measurements of the soil/air partition coefficient (Ksa).

This is the second of two papers that investigate soil−air exchange of organochlorine (OC) pesticides. In the present paper, a fugacity meter was used to measure soil−air partition coefficients (KSA) under controlled laboratory conditions for OC pesticides in three different soils as a function of temperature and soil organic carbon content. A strong temperature dependence of KSA was observed for all three soils. A linear relationship between KSA and the octanol−air partition coefficient (KOA) was observed for all soils. The enthalpy of soil−air exchange (ΔHSA) was calculated and shown to be slightly greater than the enthalpy of octanol−air exchange or the enthalpy of vaporization, possibly because of interactions between the pesticides and the organic matter in the soil. ΔHSA values were very similar for all soils, irrespective of organic matter content/type. An expression for predicting KSA values on the basis of KOA, fraction of organic carbon, and soil density (Karickhoff relationship) was tested against the chamber measurements and found to produce consistent values of KSA. A small offset was observed between field-derived values of QSA (soil−air quotient, an approximation of KSA) and chamber-derived values of KSA. This was attributed to several factors including possible incomplete equilibrium during the field measurements and fluctuation of meteorological parameters that govern soil−air exchange. Ultimately, the findings of this work and the previous field study of soil−air exchange indicate that the Karickhoff relationship is useful and valid from a modeling standpoint where it is necessary to compromise accuracy in exchange for simple relationships that utilize readily available input data