Granulometric distribution of metals in road-deposited sediments by using different sieving methods

Abstract

Road-deposited sediments (RDS) contain metals that can pollute receiving waters during precipitation events. The concentration of particle-bound metals tends to increase with decreasing particle size. In terms of metal load, the highest loads were often located in the medium range of particle sizes. Many studies have used varying or undocumented sieving techniques without showing the quality of the separation process. This is important as different particle size classes are documented with different pollution contents and loads. This knowledge builds the foundation for the selection of best management practices to improve receiving water quality. Based on the knowledge that the separation of particle sizes by the commonly applied dry sieving can be methodologically biased, this study compares particle size classes obtained by dry and wet sieving methods. The aim of the study was to show the methodological influence of the sieving process on the particle-size distribution (PSD) with focus on the metal distribution within RDS. Therefore, concentrations of Cu, Zn and Pb were determined and loads were calculated per sieve fraction after wet and dry sieving. Wet sieving causes a shift of the PSD to finer particles and both metal concentrations and loads are significantly higher especially in the smallest sieve fractions. Fine, highly polluted particles remained in coarser sieve fractions after dry sieving. This leads to diverging metal contents and especially loads in the fractions > 250 µm and thus lead to varying conclusions from the results. The results of the comparison of sieving methods however confirmed the relevance of the fine particle fractions in terms of best management practices. The fraction < 40 µm showed highest concentration and load in RDS after wet sieving. The results also showed the considerable influence of the sieving method used and the need to document laboratory methods as part of good laboratory practice.