Trace metal distribution in the bed, bank and suspended sediment of the Ravensbourne River and its implication for sediment monitoring in an urban river

Abstract

Purpose: This study aims to identify a suitable sediment compartment for sediment quality monitoring by: (a) studying the concentration of trace metals (Cd, Cu, Ni, Pb and Zn) in the bed, bank and suspended sediment compartments of the Ravensbourne River to establish any differences in trace metal concentrations with compartment; (b) determining the influence of sediment particle size fractions (< 63 μm and 63 μm–2 mm), organic matter and mineralogy on any differences; and (c) examining if metal concentration in each sediment compartment complies with the draft UK sediment quality guidelines. Materials and methods: Here, we make a comparison of metal concentrations in the bed, bank and suspended sediment compartments of the Ravensbourne River collected using different sampling techniques. We distinguished between two particle size fractions—the < 63 μm fraction (suspended, bed and bank sediment) and the 63 μm–2 mm fractions of bed and bank material with the aim of comparing concentrations between the two fractions. Particle size analysis, metal speciation, organic matter content by loss on ignition and mineralogy using X-ray diffraction were also carried out on each sediment compartment. Results and discussion: The results showed variations in trace metal concentrations with sediment compartment and with particle size. The mineralogical characteristics were comparable for all sediment compartments, and the relationships between organic matter content and metal concentrations were significant in the majority of the bank sediment samples. There were no significant differences (p > 0.05) in the concentrations of metals between the suspended sediment and the < 63 μm bed sediment fraction, but there was a significant difference (p < 0.05) between the suspended sediment and the < 63 μm bank sediment fraction. There were also significant differences between the concentrations of metals in the < 63 μm and the 63 μm–2 mm fractions. Generally, the Ravensbourne River did not comply with the draft UK sediment quality guidelines for the metals analysed. Conclusions: This study shows the importance of identifying a suitable sediment compartment to sample for compliance with sediment quality standards. The bed and suspended sediments are the most widely used sediment compartments for sediment monitoring, but collecting sufficient mass of the < 63 μm sediment fraction for monitoring presents a challenge for urban gravel bed rivers like the Ravensbourne River. It seems appropriate to establish individual monitoring regimes for different rivers.