Sequential Extraction Analysis of Lead in Michigan Roadside Soils: Mobilization in the Vadose Zone by Deicing Salts?

Sequential extraction analysis was used to characterize the geochemical partitioning of Pb in roadside soils of the Detroit, MI area. Four soil profiles (10, 30, 60, and 100 m from road) were sampled at 15-cm intervals to a depth of 75 cm at each of five locations along an urban-rural transect. The observed concentrations increase with increasing traffic volume and proximity to the highway, indicating that vehicle emissions are the principal source of Pb. Concentrations in roadside soils of urban sites (>100,000 vehicles per 24 h) are 10 to 15 times greater than the background level of 12 to 22 mg/kg found at rural sites. Sequential extraction data show that the Pb is mainly in carbonate and oxide forms in the most heavily contaminated soils; organically bound forms tend to predominate elsewhere. Both surface and subsurface horizons of soils are polluted at urban sites where translocation in the vadose zone is suggested by elevated levels of Pb to depths of 75 cm. Mobilization is attributed primarily to partial dissolution of organic matter by excess NaCl derived from highway deicing salts and complexation of Pb with organic chelates. Mobilization appears to be occurring in soils up to 60 m from the highway at urban sites.