Development of a sediment tagging method

Development of a sediment tagging method: Important for the success of the flume experiments was a method for tracking sediment resuspension, transport and mixing and for monitoring, contemporaneously, the partitioning of exchangeable metals. On the basis of previous work at Plymouth involving platinum group elements, rhodium (Rh) and platinum (Pt) were chosen as tracers because they have a strong affinity to bind to estuarine sediments and their environmental concentrations are too low to cause baseline effects. Nickel (a metal on the EU List of Priority Contaminants) and Zn were chosen as exchangeable metals because of our previous work on their partitioning behaviour. The first phase of the project was developing and testing of a method for tagging estuarine sediments with tracers and exchangeable metals.

The flume experiments required a relatively small plug of natural sediments tagged with the tracers, either side of which was “background” sediment of fine commercial sand (median diameter d50= 130 µm) or natural sieved estuarine sediment. The plug consisted of estuarine sediments from either the Plym or the Mersey that had been wet sieved to obtain particles with a similar texture to the fine commercial sand. In practice, the isolation of about 10 kg of natural sediment of appropriate texture required the wet sieving of about 120 kg of sediment over several days. Although the textures of the two sediment types were similar, the natural sediments contained particulate organic carbon (~0.8%), an important constituent for labelling the particles with Rh and Pt. The tracers were adsorbed onto about 10 kg of sieved natural sediment, with a steady state being achieved after 9 days. A similar approach was used to tag the sediments with the exchangeable metals, Ni and Zn. Prior to a flume run, several samples of the doped plug sediment were taken to establish the initial concentrations of the tracers. The sediment and SPM samples were analysed for Rh, Pt, Ni and Zn by inductively coupled plasma-mass spectrometry (ICP-MS) following extraction with concentrated nitric acid. Dissolved Ni and Zn were also determined by ICP-MS. Laboratory analyses showed that the fine commercial sand had a low specific surface area and no detectable particulate organic carbon and trace metals.

Therefore, when mobilised and mixed with the plug sediment during a flume experiment, it did not compete for the uptake of Ni and Zn from the dissolved phase and there was no evidence of re-partitioning of particulate metals between the two sediment types.