Surface sediments of Richards Bay Harbour, South Africa – potential pollutants (heavy metals, persistent organic pollutants, microplastics) and grainsize distribution

Richards Bay harbour, on the Indian Ocean coast of South Africa, is one of the largest coal export facilities in the world. Since its founding in 1976, the bay and its associated estuary (the Mhlatuze) have undergone fundamental changes linked to the development and expansion of the port. Today, the industrial impact in Richards Bay is centred on coal export, aluminium smelters and fertilizer plantations and its associated ship operations add to a heavily impacted environmental system, especially in the managed harbour basin.               Based on surface sediment samples, this study analyses the harbour-system as final sink for pollutants. Sedimentological aspects are combined with environmental influence indicators such as potential pollutants (e.g., heavy metals, persistent organic pollutants, microplastics), eutrophication indicators (total organic carbon, biogenic silica) and erosion indicators (e.g., grain size).      The harbour-interior is periodically dredged to maintain a constant water depth for the ocean going vessels, which is reflected in its bathymetry and grainsize distribution patterns. Multiple parameters infer a hydrodynamically controlled environment, due to their correlation with the harbours bathymetry, such as grainsize or elemental distribution. Short sediment cores from the entire harbour interior are of very young age and therefore indicate very high sedimentation rates.     Analyses reveal a link between the distribution of Polyethylene terephthalate (PET) measured in bulk sediment samples and remains of microplastics. Additionally the distribution of microplastics shows strong similarities to the hydrodynamic regime in the harbour system as seen in bathymetry and grainsize distributions. In contrast, the distribution of certain environmental pollutants, e.g., cadmium and chromium, appear to be influenced by point sources, such as the main bulk port or the small craft harbour. Results of elemental concentrations complement previous studies, but reveal increased maximum concentrations values (e.g., max. Cu concentrations in Wepener & Vermeulen (2005): 53.5 mg*kg-1; current study: 353 mg*kg-1). Additionally, the measured maximum concentration values exceed findings of other comparable studies on South African ports (e.g., max. Cu concentration in Fatoki & Mathabatha (2001) for Port Elizabeth: 68.5 mg*kg-1 and East London: 106 mg*kg-1). The otherwise even distribution pattern of Organochlorine pesticides (OCPs) indicates a sink for pesticide pollution within the harbour centre. OCP values suddenly decrease towards the harbour mouth and imply their discharge by (tidal) currents towards the Indian Ocean. Wepener, V., & Vermeulen, L. A. (2005). A note on the concentrations and bioavailability of selected metals in sediments of Richards Bay Harbour, South Africa. Water SA, 31(4), 589-596.Fatoki, O. S., & Mathabatha, S. (2001). An assessment of heavy metal pollution in the East London and Port Elizabeth harbours. Water SA, 27(2), 233-240.