Radiometric sand–mud characterisation in the Rhine–Meuse estuary: Part A. Fingerprinting

The composition of the underwater sediment bed is often defined in terms of the mud (<63 μm) or sand (>63 and <2000 μm) content. Mud and sand differ considerably in their content of (natural) radionuclides. Radionuclide concentrations are highest in the clay minerals that form a significant part of the mud fraction. A study was initiated in the Hollandsch Diep and Haringvliet, two semi-stagnant fresh water basins in the southwestern part of Rhine–Meuse Estuary (The Netherlands), to investigate whether radionuclides can be utilised to determine the sand and mud content of aquatic sediments. The radiometric fingerprint was assessed based on sediment samples with different composition, provenance and age. Differences in provenance (Rhine versus Meuse) could be discriminated by the 40K content of the sediment. Differences in age or composition, in terms of mineralogy, organic matter or carbonate content, only slightly affected the radiometric characteristics based on 232Th and 238U. On the other hand, no reliable correlation between 40K and the mud content was obtained; therefore, 40K is excluded from the fingerprint. The fingerprints based on the 232Th and 238U contents characterise mud (<63 μm) and sand (>63 μm) as: 46.2±1.9 Bq/kg 238U and 9.3±0.9 Bq/kg 238U and 45.6±1.9 Bq/kg 232Th and 9.7±0.9 Bq/kg 232Th, respectively. The assessed radiometric fingerprint allows a quantitative interpretation of the mud and sand content through the total 238U+232Th activity. This interpretation is confirmed by the high correlation (R2=0.96) for the mud percentages obtained from radiometric analysis and those by laser diffraction in the laboratory. It can be concluded that radiometric sedimentology provides a tool for reliable sediment characterisation. New perspectives are opened when radiometric fingerprints are combined with in situ radiometric characterisation of sediments, which will be presented in part B of this paper (this issue).