Two and three-dimensional quantification of lead contamination in alluvial soils of a historic mining area using field portable X-ray fluorescence (FPXRF) analysis

Two- and three-dimensional element distributions in soils and sediments may be important for the identification of geomorphic processes and the reconstruction of landscape evolution. Field portable X-ray fluorescence (FPXRF) analysers provide new possibilities in the detection and quantification of these element distributions. Results from a research project studying the heavy metal contamination of alluvial soils in a historic mining area at the Vils River near Freihung (Eastern Bavaria, Germany) exemplify how FPXRF can be applied to quantify two- and three dimensional element distributions in a floodplain environment on different scales. The results show that two-dimensional lead distribution of topsoil bulk samples (0 to 20 cm depth) clearly depends on the geomorphology of the study area. Concentrations of more than 1000 mg kg^{- 1} lead are restricted to the river floodplain. On a fluvial terrace lead concentrations of the topsoils are consistently far below 500 mg kg^{- 1}. Within the floodplain environment the lead contamination is not homogeneously distributed but forms areas of higher and lower lead concentrations. Micro fluvial structures (channels, ridges) are polluted on different levels, depending on the composition of the sediments or the water contents. This demonstrates that accurate assessments of floodplain metal contamination can only be made if small scale spatial variability has been considered. Analysis of the three-dimensional lead distribution shows that concentrations of the in situ measurements range from 500-6000 mg kg^{- 1} subject to depth, geomorphology and distance from the Vils River. Although the absolute contents are higher, three-dimensional lead distribution derived from laboratory measurements is identical with the plotted results of the in situ investigations. By calculating a correction factor for further investigations, water content as one of the main causes for differing results between field and laboratory analyses can be diminished. In situ FPXRF analysis as a robust technique could be used to investigate metal contamination on floodplain soils at a range of spatial scales. It allows detailed sampling, which can yield important information on the geomorphological controls on metal contamination in such environments.