Standardized particle size for monitoring, inventory, and assessment of metals and other trace elements in sediments: <20 microm or <2 microm?

Despite the manifold factors that determine the composition of a river sediment (e.g., geology, soil type, rural or highly industrialized and densely populated, as well as forested areas, concentration of carbonates and organic matter), the linear regressions between the 9 metals determined in the fractions <2 microm and <20 microm have high coefficients (R2 after Pearson) for Cr and Cu (both 0.94), followed by Pb (0.90), Cd (0.82), Zn (0.81), Ni (0.76), and Mn (0.72). Low and very low coefficients are found for Hg (0.51) and Fe (0.22). In addition, the histograms of the metal ratios <2 microm:<20 microm (i.e. the slope of a linear regression) show that--with the exception of Cd (1.47)--all median ratios of the other metals fall within a narrow range (1.24-1.35). Both "fine" (<2 microm and <20 microm) fractions are able to fulfil the requirements for monitoring, inventory, and assessment of metals in sediments. Preference should be given to the rapid, simple, and economic <20 microm separation by sieving; this fraction corresponds fairly closely to the former suspended load of a riverine transport.