Robust assessment of moderate heavy metal contamination levels in floodplain sediments: A case study on the Jizera River, Czech Republic

• Assessment of background values and heavy metal enrichment in floodplain soil
• Methods to handle facies bias of enrichment factors in floodplain fines
• An approach that integrates geography, geophysics and geochemistry
• Natural enrichment by heavy metals must be considered when enrichment factors are ~ 1.5.

Enrichment factors for Cr, Cu, Ni, Pb and Zn in floodplain fines from the middle and the lower courses of the Jizera River (a tributary of the Elbe River in the Czech Republic) were evaluated to compare the original contamination profiles with post-depositional and pedogenic changes. Background concentrations of heavy metals were assessed from uncontaminated sediments (soils) in the study area that belong to the same sedimentary facies and were not affected by reductimorphic processes. Facies assignment is accessible by geophysical imaging combined with core analysis. Sediments from point bars and channel banks in direct contact with riverine water are more heavily polluted than overbank fines from the distal floodplain. The point pollution source, a century-old battery and car production facility in the city of Mladá Boleslav, has certainly been responsible for Ni and Cr pollution, contributed substantially to Cu and Pb pollution, and had a less significant effect on the Zn enrichment factor. Although the use of soil enrichment factors has been criticized, these factors help to manage hydraulic sorting and recognition of post-depositional migration in soil profiles of floodplain sediments. When moderate pollution (enrichment factor about 1.5 for Cu, Pb and Zn) is found, background concentrations must be carefully evaluated and natural enrichment must be taken into account. Studies of such small enrichment factors contribute to the understanding of the dispersal and fates of pollutants in floodplains.