Deposition and migration of atmospheric Pb in soils from a forested silicate catchment today and in the past (Strengbach case): Evidence from 210Pb activities and Pb isotope ratios

210Pb activities, Pb isotope ratios and elemental abundances of leachates from soils were determined to identify the origin of atmospheric Pb and characterize its migration in mineralogically and chemically different soil systems of a forested silicate catchment. Leaching experiments were performed to separate the leachable, mobile reservoir of the soil system – mainly containing atmosphere derived Pb – from that of the residual bedrock reservoir. Pb isotope data allow recognition of an evolution in the soil leachates with increasing depth pointing to at least two major Pb sources. Surface samples collected in 2001 show a Pb isotopic signature which is comparable with that of particulate matter (PM) collected in 1995 and 2005/2006 in an urban environment close to the catchment and hence corresponds to a very recent isotopic signature of the atmosphere. The more radiogenic Pb isotopic signature in soil leachates from 30 to 40 cm depth corresponds to older airborne Pb probably derived from historical mining and smelting close to the catchment (< 5 km). 210Pb inventories confirm trace element data and indicate that the soils are important sinks of atmospheric Pb in the uppermost part (40 cm) of the soil profile. The migration speeds derived from these inventories for the different depths (0.5 cm y− 1, 0.2 cm y− 1, 1.6 cm y− 1) suggest that the atmospheric Pb needs less than 150 years to reach the depth of 40 cm. Thus, the Pb enrichments above this depth might be related to a period of local mining activities which ceased in the mid-20th century. The soil solutions are not in isotopic equilibrium with the corresponding soil leachates. Soil solutions from 5 to 60 cm depth have Pb isotopic compositions similar to those of stream and spring waters and soil leachates from 10 to 20 cm depth. They carry a Pb with current atmospheric Pb isotopic compositions. The presence of anthropogenic Pb in spring waters might suggest that some of it migrated rather quickly through the soils.

► Pb is strongly enriched in the uppermost 40cm of the soil profiles.
► Leaching experiments indicate that this Pb is controlled by organic matter, phosphates and Fe oxides.
► Pb isotope ratios point to a complex pollution history of the soils which carry Pb with at least three different origins.
► The migration speeds of Pb derived from 210Pb inventories for the soil profiles range between 0.5 cmy-1 and 1.6 cmy-1.