Particle Size and Geochemistry of Suspended Material and Its Influence on Nephelometric Turbidity

ABSTRACT In this study, suspended sediment from seven alpine rivers was collected for analysis of their textural and geochemical properties to determine what effect differences in geochemistry, particle shape, and organic matter have on suspended sediment turbidity ratings. Small variations in the suspended sediment shape were shown to have a greater effect on turbidity rating than differences in sediment mineralogy. The geochemistry of SPM was consistently elevated in Al 2 O 3 , CaO, K 2 O, FeO, MnO, and TiO 2 relative to riverine bed sand, because higher density minerals are likely to be preferentially transported as silt fragments, rather than as bedload. Although sourced from lithologically similar terranes (schist and semi‐schist) samples with higher weight percentage concentration of mineral oxides (Al 2 O 3 , K 2 O, FeO, MnO) yield lower specific turbidity, which may cause higher NTU‐SPM rating slopes (beta), but this effect was generally weaker than other optical properties of suspended sediment. These data show that nephelometric ratings may not be transferable between catchments of similar lithological terranes (e.g., regionalization approaches) if there are differences in suspended sediment geochemistry.