3-dimensional floc structure and dynamics

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Suspended particulate matter (SPM) plays a fundamental role in the impact, and eventual fate of sediment, pollutants, pathogens, nutrients and manufactured nanomaterials in aquatic environments. Suspended particulate matter is important to the aquatic ecosystem, transferring material from the catchment to coast, contributing significantly to the biogeochemical cycling of nutrients, and is the main vehicle for downward carbon flux in the ocean. Too much suspended sediment in the water column can be considered a pollutant; reducing water quality by increasing nutrient loads and reducing dissolved oxygen concentrations with impacts for ecological status and the UK's compliance with the EU Water Framework Directive. Once settled, fine sediment can reduce aquatic biodiversity, for example by smothering organisms and spawning gravels. Sediment can also block river channels and ports with impacts for navigation. For example, in the UK we dredge c. 40M tonnes of sediment every year costing millions £GBs. Consequently, in order to make evidence-based sediment management decisions, there is an urgent need to understand and make accurate predictions of SPM fate and transport in all aquatic environments.Suspended particulate matter exists in aquatic systems as flocs. A floc is a very fragile, loosely bound aggregate of fine sediment particles, bacteria, organic matter and fluid-filled pore space. Flocs have very complex shapes and structures, and very low density. As a floc settles out of suspension it continually changes shape, size and structure as it breaks apart and reforms many times in the turbulent water. The delicate nature of these flocs and their size (a few microns to a few millimetres) means that they are very difficult to sample and analyse. Current analytical techniques e.g. optical or electron microscopy can either look at whole flocs, but with limited detail, or sub-micron sections of a floc, and they can only look at a 2-dimensional cross section. Therefore, many numerical models that predict fine sediment transport rely upon 2-dimensional simplifications of complex 3-dimensional shapes and structures, or have to use mathematical approaches to describe floc structure e.g. they assume flocs are fractal and that their structures are self-similar irrespective of scale. Therefore, our understanding of floc structure and behaviour is very limited and unless we can sample and analyse flocs at a range of spatial scales we can neither support nor challenge these mathematical assumptions.We will address this research gap and sample, observe and quantify sediment floc micro-structure for the first time in 3-dimensions and at multiple, correlated spatial scales from c. 10 nanometres to millimetres. This will address the fundamental question of how 3-dimensional floc micro-structure influences suspended particulate matter transport. This project integrates sampling techniques from the biomedical sciences that are more commonly used to examine fragile biological cell tissues and analysis used in materials science and the earth sciences to analyse the microstructure of concrete, alloys and rocks (focussed ion beam nanotomography FIB-nt/SEM and X-ray microtomography). We will then apply this novel approach to the study of delicate, flocculated sediment in the aquatic environment.