In situ characterisation of complex suspended particulates surrounding an active submarine tailings placement site in a Norwegian fjord

Suspended particle measurements over flood and ebb transects of a Norwegian fjord (Frænfjorden) with an active submarine mine tailings placement are presented. Measurements focused on characterisation of spatial variability in particle and floc size, and determination of suspended mine tailing concentrations. Such information is crucial for understanding transport and flocculation dynamics, which in turn can contribute to improvements of numerical models, and thus to more accurate predictions of settling fluxes and transport processes. Multiple instruments were used to ensure that all relevant particle sizes could be observed. These included a LISST-100x, a LISST-Holo and a bespoke particle imaging system, which were deployed together on a profiling frame at multiple stations transecting the fjord. Measurements were obtained close to the high-concentration plume from the tailings discharge and extended several kilometres horizontally. We observed a combination of few-micron fines, densely packaged flocs and large string-like flocs, several mm in length; a range which could not be captured by a single instrument. Such a range of floc sizes and complexities has significant implications for the transport and settling of the material suspended within the fjord.