MECHANISMS AND VELOCITIES OF ANTHROPOGENIC PB MIGRATION IN MEDITERRANEAN SOILS

The isotopic composition of Pb measured in soil samples was used to de termine rates and mechanisms of anthropogenic Pb migration in the soil . Petrol-Pb found in soluble halogenated aerosols migrates into the so il and is retained in the soil by the stationary soil particles. Lead infiltration velocity is approximately 5 x 10(-1) cm/year, and its ret ardation factor is estimated to be on the order of 1 x 10(3). The infi ltration of Pb into the soil is best described by the advection-disper sion equation under the assumption that the time scale of the longitud inal dispersion is much longer than the time scale of advection. There fore, the contribution of dispersion to the solution of the advection- dispersion equation is negligible. As a result, the soil profile of pe trol-Pb resembles the time-dependent input function of petrol-Pb. The estimated petrol-Pb penetration velocity and the isotopic composition profile of Pb in off-road soil are used for the computation of the fra ction of anthropogenic Pb in this soil. It is calculated that the frac tion of anthropogenic Pb in the acid-leached soil samples and in the s oil residue of this soil profile drops from 60 and 22% near the surfac e to 6 and 0% at a depth of 33 cm, respectively. The downward migratio n velocity of Pb in soils of the studied area, which are typically 50 to 100 cm deep, implies a residence time of Ph in the soil of 100 to 2 00 years, (C) 1998 Academic Press.