The use of sediment colour measured by diffuse reflectance spectrometry to determine sediment sources: Application to the Attert River catchment (Luxembourg)

Abstract

Summary Colour coefficients measured by diffuse reflectance spectrometry were used to quantify suspended sediment sources using the fingerprinting approach. The investigations were undertaken in the Attert River catchment (NW Luxembourg), where time-integrated suspended sediment samples and samples of potential sediment sources (topsoil and channel bank samples) were collected, in order to test the ability of colour to provide a fingerprint property for sediment source tracing. Sediment colour coefficients were computed from diffuse reflectance spectrometry measurements (ASD FieldSpec-II spectrometer, 0.4–2.5 μm) taken over the visible wavelength range. The linearly additive behaviour of these colour coefficients, which is an important requirement for its use in the mixing models, was tested in the laboratory by means of artificial mixtures. Model prediction uncertainty associated with the spatial variability of source tracer properties was assessed using an inclusive approach to mixing models based on Bayesian error estimation and employing Monte-Carlo simulation. The results provided by the colour measurements were compared with those obtained using the classical fingerprinting approach in the same catchments (i.e. using geochemical, radionuclide and organic tracers). Even though neither of the two approaches is definitive, and the results involve considerable uncertainty, the consistency between the approaches encouraged the extension of the approach to include measurements on the particles retained on glass fibre filters. This permitted use of small samples collected by an automatic sampler to investigate source variability during a storm runoff event. Preliminary results obtained using this approach demonstrated that colour provides a useful property for use in sediment source fingerprinting investigations, which is both fast and easy to measure. It also provides a rapid and cheap means of investigating hillslope-to-channel coupling and the transfer of eroded material during storm runoff events, even at low concentrations. High spatial and temporal variability of the characteristics of suspended particles was observed. The use of sediment colour measured by diffuse reflectance spectrometry is seen to represent a useful development, in terms of reducing analytical costs associated with composite fingerprinting procedures.