Dynamic interactions between cohesive sediment tracers and natural mud

PurposeCohesive sediment tracers have been developed to improve our understanding of fine sediment transport in the aquatic environment. However, there is little understanding of their physical and dynamic characteristics compared to the natural sediments they are intended to mimic. This work focuses on a labelled clay mineral tracer examining its dynamic characteristics and determining whether it flocculates and interacts with natural estuarine mud.Materials and methodsGross floc characteristics (size and settling velocity) were measured using video image analysis. Floc density, porosity and mass settling flux were then calculated. Fine-scale floc internal structure and composition were observed using transmission electron microscopy (TEM). The interaction of the tracer and natural mud was examined by observing tracer and natural mud mixtures.Results and discussionThe tracer formed macroflocs (>160 μm) that could not be distinguished statistically from natural mud, whilst tracer microflocs (<160 μm) were smaller and settled more slowly. Gross settling characteristics and TEM analysis indicate that the tracer and natural mud do not interact on a primary particle-to-particle basis, although microflocs of pure tracer and mud do interact. The physical and dynamic floc properties of tracer and natural mud mixtures were different from both pure tracer and pure natural mud due to the irregular packing of differently shaped natural mud and tracer flocs.ConclusionsThe cohesive tracer flocculates and has similar physical and dynamic properties to natural mud; however, when the tracer interacts with natural mud, it forms flocs with significantly different characteristics. These mixed flocs exhibit different transport characteristics (e.g. settling velocity) to natural muddy material. Therefore, cohesive sediment tracers may not accurately predict cohesive sediment transport pathways, and this has implications for the use of cohesive tracers to understand natural mud transport and to develop sediment transport models.