Revisiting estimates of diffuse contamination of agricultural soils by Pb.

Knowledge of overall anthropogenic impacts on soil contamination by lead (Pb) remains insufficient despite decades of research. Estimates of diffuse Pb contamination in European soils vary by an order of magnitude. Previous research has been limited by the lack of a mathematical definition of scattered and diffuse contamination, a limited understanding of the altitudinal control of Pb emission loads, an unknown spatial extent of point contamination sources, and neglect of the natural variability of soil-forming bedrock. To address these challenges, the dataset of topsoil Pb from the Czech national monitoring and from the own soil profile sampling were processed. The soil monitoring dataset was analysed using probability density functions and functional data analysis. Altitudinal control of topsoil Pb was examined using geographically weighted and quantile regressions. Soil depth profiles from selected areas of the Czech Republic were analysed for total Pb; the stratigraphy of selected profiles was checked by gamma spectrometry. The measurable extent of point sources of Pb emission is up to ca. 30 km. Scattered contamination from more remote sources produced an altitudinally controlled increase in topsoil Pb concentrations in topographic barriers. True diffuse contamination increases with altitude and contributes between approximately 3 mg kg-1 at about 500 m a.s.l. and 16 mg kg-1 at about 800 m a.s.l. to Pb in the top 30 cm of soil, which is lower than most previous estimates. This contribution is smaller than the background Pb concentration range of 16 to 34 mg kg-1 and is of little toxicological concern.