Measuring spatial and temporal variation of the chemical composition of suspended sediment of the river Rhine using a handheld XRF

Abstract

The chemical variation of suspended sediment over space and time in rivers has rarely been studied. Moreover, measuring the chemical composition of suspended sediment in rivers usually requires the collection of large volumes of river water to obtain sufficient suspended sediment for analysis. In this thesis, the spatial and temporal chemical variation of suspended sediment in the Rhine River was studied using a handheld X-Ray Fluorescence scanner (XRF), since this device can measure element concentrations of suspended sediment extracted by filtering water samples. Because this approach is not widely used, the measured concentrations were verified with data obtained from the Dutch Ministry of Infrastructure and Water Management. The temporal and spatial variability in the Dutch part of the Rhine (Lobith – Oudendijk) was analysed by collecting synoptic and time-integrated suspended sediment samples. Moreover, fieldwork was carried out in the German Rhine (Basel – Rheinberg), to analyse the spatial variation of the chemical composition of suspended sediment and bed sediment. The concentrations measured from the water samples needed to be corrected, otherwise the concentrations between the differently obtained samples could not be compared. The temporal variability of the measured element concentrations provided a subdivision into three categories: (1) concentrations of Ca, Mn, and Zn declined with increasing water level and showed a negative relation with changing water height. The predominant process that explained this trend was dilution of point-sources from which these elements originated. (2) The concentration of Ti, Fe and Rb increased with increasing water height and showed a positive relation with changing water height. Remobilization of bed sediment into the stream due to erosion and reduced organic matter concentrations in the water were the main processes causing increased trace metal concentrations during high water events. (3) The concentrations of Cr, Cu, Sr and Pb in the suspended sediment showed no distinct reaction with respect to the increasing water level. The chemical composition throughout the Dutch Rhine showed little spatial variation. The concentrations measured in the suspended sediment in the German Rhine obtained by suspended sediment samples showed no major fluctuations and the concentration stayed relatively constant, whereas the bed sediment showed increasing concentrations in downstream direction. The measured concentrations in the bed sediment were substantially higher than the concentrations found in the suspended sediment at the same location. The main processes that caused the three major reactions in the chemical composition of suspended sediment in the Dutch part were dilution, remobilization and reduction of organic matter content.