Spatial and temporal distribution of metals in the Salt River catchment, Cape Town

The Liesbeek and Black Rivers are two major urban tributaries in the Salt River catchment. The Salt River enters the ocean at Paarden Island, Table Bay, Cape Town. A multitude of pollution sources are found along the banks of these rivers, including industries, household waste, sewage treatment plants, golf course and informal settlements, which continue to threaten the water and sediment quality of these rivers. The catchment has been severely modified and canalized, increasing flooding events and sediment loads downstream. Despite the multitude of pollution sources surrounding the heavily urbanised Salt River catchment and the risk pollutants such as metals pose to aquatic ecosystems, no comprehensive study on metal pollution in this catchment has been published to date. This study investigated the spatial and temporal distribution of metals (Al, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn) in water and sediments the Liesbeek, Black, and Salt Rivers by comparing new data (2022) with unpublished 2006 data, as well as by comparing wet and dry seasons and selected sampling stations. The objectives of the study were to assess contamination levels, identify the potential sources of pollution, and evaluate the spatial and temporal trends in metal concentrations over a sixteen years period. Two sampling stations were selected in each of the Liesbeek and Black Rivers, and one in the Salt River (due to most parts of the latter being canalised). Physicochemical parameters were measured at each sampling station by means of a handheld water multimeter. Sediment grain size analysis was also done for each sampling station, using sieves, a sieve shaker and the Wentworth scale (1922). Five replicates of water and sediment samples were collected at each of the sampling stations for metal analysis. Samples and blanks were digested in a heat block, using nitric acid and then filtered through 6 mm and 0.45µm filter paper. An Inductively Coupled Mass Spectrometer (ICP-MS) was used to determine metal concentrations at Stellenbosch University Central Analytical Facility. Results showed that physicochemical parameters increased downstream, with higher values observed in the lower Black River and Salt River, compared to the Liesbeek River. This trend is likely due to varying seasonal water levels, urban runoff, pollutants and canalisation. The study highlighted metals of particular concern in the Liesbeek, Black, and Salt Rivers that exceeded the recommended quality guidelines. According to the new risk-based freshwater quality guidelines (Odume et al. 2024), Al, Cu and Fe in this study were classified under the critical impact category and Mn and Cd under the moderate impact category. This indicated that the water quality is unacceptable and could lead to high mortality rates among aquatic organisms. Al, Cu, Fe, Mn and Pb, exceeded permissible levels of the Canadian Interim Sediment Quality Guidelines (CCME, 2002) at most of the lower sampling stations, which may have long term consequences on aquatic species’ and human health. Urbanization, industrial discharges, household waste, sewage, stormwater runoff, and illegal dumping were identified as potential sources of metals. There were no clear temporal trends of increasing or decreasing metal concentrations observed across years (2006 and 2022). Distinct seasonal (wet and dry) patterns were however noted, with higher concentrations of metals such as Al, Fe, and Pb during the wet season of 2022 and a shift towards higher concentrations of metals like Cd, Cr, and Mn during the dry season of 2006. These patterns are likely due to a combination of seasonal environmental conditions (e.g. rainfall, flow speed, water level and evaporation), and increased input from surrounding sources. No clear spatial trends were observed for metals in river water, but some of the sampling stations, especially the Liesbeek River station 1 and areas downstream in the Black and Salt Rivers, showed high metal concentrations associated with household waste, atmospheric deposition, road runoff and industrial activities. The highest sediment metal levels were observed at the Black River station 2, located near industrial areas, roads, and a wastewater treatment plant and at the Liesbeek River stations, which has fewer major metal sources. The latter indicates that domestic pollution, litter and atmospheric deposition are important sources of metal contamination in urban rivers. Even though there were no clear long-term trends observed in this study, the findings highlight the importance of continued monitoring and pollution management strategies to mitigate the risks posed by metal contamination in the Salt River catchment and urban catchments in general. Attention should be given to smaller urban rivers and tributaries, such as the Liesbeek River, as they may be at greater risk than previously assumed.