The influence of aged and unaged adsorption and desorption constants on the assessment of xenobiotic leaching potential

Mathematical models are being increasingly used to determine the behaviour of xenobiotics in soil, and to conduct risk assessments. Estimation of leac hing potential using models has traditionally been conducted with two input values only, the soil adsorption constant K-oc and the first order kinetic half-life value. It is increasingly apparent that this is an over simplifi cation of the processes involved. When determining potential movement to de eper soil layers both desorption characteristics and hysteresis effects, to gether with their dependence with time have an important role. Both adsorption and desorption phenomena can vary according to the chemical properties of the xenobiotic, with desorption characteristics probably hav ing greater importance. The effect of increasing sorption with time, which can commence immediately the chemical comes into contact with the soil, has been shown to be significant. Also the influence of 1/n values in the Freu ndlich equation can significantly influence the modelling results. It is therefore evident that in order to improve the predictions of xenobio tic movement in soil these factors should be included in the models.