Lithologically inherited variation in Pb isotope ratios in sedimentary soils in The Netherlands

Abstract

Knowledge on the lithologically inherited variation in present day Pb isotope ratios in soils is remarkably limited. Such information is essential to determine the anthropogenic Pb fraction and anthropogenic Pb sources in Pb-polluted soils. This study presents results of a survey of subsoil samples from approximately 350 rural locations covering the entire Netherlands, for which the bulk geochemical and Pb isotope composition was determined. The sample density was approximately 1 site per 70 km2. The aim was to establish a geochemical reference for the lithologically inherited variation in Pb isotope ratios in Dutch soils based on the subsoil samples, with which to compare the topsoils (companion paper in this journal issue).

The lithologically inherited variation in Pb isotope ratios of the subsoils in The Netherlands is established at 1.175–1.221, 2.441–2.494 and 0.478–0.492 for 206Pb/207Pb, 207Pb/208Pb and 206Pb/208Pb respectively. The four main lithologies distinguished, sand, clay, peat and loess, have distinct Pb isotope signatures. No significant difference in isotope signature was found between marine and fluviatile clays. Multiple regression analysis established that the observed variation can be primarily explained by the textural and mineralogical variation within Dutch subsoils, with Al and Zr content representing useful predictors for the observed Pb isotope variability. Clay soils are characterised by a radiogenic Pb isotope signature that is notably low in 207Pb. Soils with a high Zr content are especially high in 206Pb. Although the vast majority (∼90%) of the Pb isotope variation in the subsoils appears to be controlled by lithological inheritance, some subsoils (mainly peats) are suspected of containing a component of non-lithologically derived Pb.