The impact of urban pollution on the metal contamination of the Zandvlei estuary, Cape Town, South Africa

Metal contamination of aquatic systems is a global concern, negatively impacting ecosystems and human health. However, limited information is available on metal concentration levels in South African estuaries, including the Zandvlei estuary in Cape Town, a highly urbanised system with an extensive stormwater drainage network. The objective of this study was to determine spatial and temporal variations of metal concentrations (aluminium, zinc, lead, and copper) in water, sediment, and macroalgae (i.e. Enteromorpha spp.) in the Zandvlei estuary. Samples were collected from five sites over four seasons in 2017 and analysed using Inductively Coupled Plasma Mass Spectrometry (ICP-MS), with metal levels were compared to water and sediment quality guidelines. Results revealed significant spatial and temporal variations, influenced by the proximity to pollution sources, physicochemical parameters, river inputs, sediment characteristics, weather conditions, and anthropogenic activities (i.e. industrial activities and urban development). In water samples, metals were most concentrated in the order Al > Zn > Cu > Pb; in sediment and macroalgae, the order was Al > Zn > Pb > Cu. Notably, Enteromorpha spp. accumulated higher metal concentrations than water and sediments, and some metal levels exceeded guideline thresholds, indicating contamination risks. These findings highlight the importance of Enteromorpha spp. as a biomonitoring tool for metal pollution in urban estuarine environments. This study addresses significant knowledge gaps in South African estuaries, supporting enhanced environmental monitoring and conservation strategies.