ICP-MS-based tracing of metal sources and mobility in a soil depth profile via the isotopic variation of Sr and Pb

The isotopic compositions of lead and strontium and the total concentration of 11 elements (Rb, Sr, Ca, Ba, Cu, Zn, Mn, Fe, Pb, Cr, Cd) in NH4NO3 extracts (reflecting the mobile, bioavailable fraction) and total digests of soil fractions along a soil depth profile were analyzed using sectorfield ICP-MS. This information allows distinguishing between recent external (atmospheric deposition) and primary geogenic sources. Both isotopic systems indicate a clear difference between the surface composition and the composition of lower geogenic levels within the investigated soil profile. An increase of the Sr-87/Sr-86 isotope ratio can be observed in the total digest down to a depth of about 120 cm for Sr, whereas Pb shows a stable isotopic composition starting from a depth of about 60 cm. We found increased isotopic ratios of 17 SO 86 Sr in the mobile fraction of the topsoil compared to the subsoil and to the total digest in the topsoil. This observation and the increased Sr levels in the mobile fraction of the topsoil are an indicator for recent deposition of Sr along with the dust derived from the paving of the forest roads in the study area. The significant change of the Sr-87/Sr-86 ratios in the total digest at 120 cm depth can be related to a variation in the lithology of this layer, which was confined by a concomitant change of the conventionally applied Ti/Zr ratio. This lithogenic inhomogeneity was not apparent from the fairly homogenous distribution of total Sr in the soil profile. Similarly, we were able to distinguish the accumulation of anthroponic heavy metal sources and their recent mobilization within the topsoil front long-term leaching of geogenic sources in deeper soil layers. (c) 2004 Elsevier B.V. All rights reserved.