Contamination sources and concentrations of metal pollutants in the Eerste River, Cape Town, South Africa

Abstract

Rivers provide essential supporting, regulating, provisioning and cultural services to people. However, an 11% decline in South African river condition from 1999–2011 has been reported, largely attributed to increased water pollution. The decline in river condition is of great concern as it significantly undermines the ability of rivers to deliver valuable services to people. Anthropogenic activities generate large quantities of waste, such as heavy metals, which has the potential to cause long term effects on the ecosystem. Therefore, this study focused on the Eerste River and its associated estuary. The research objectives for this study were to determine whether various metals are present at environmentally significant levels along the length of the river in the water and sediment; and to determine whether there is a link between concentrations of various metals present at environmentally significant levels along the length of the river and the sources of contamination. To achieve the objectives, water and sediment samples were collected at different sites along the Eerste River in March 2021 (dry season) and August 2021 (wet season). The samples were prepared using an acid digestion procedure, and the water and sediment samples were analysed using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Five metals are presented in the study, namely aluminium, manganese, iron, zinc and lead. Statistical analyses were conducted using the SigmaPlot 14 software. Statistically significant differences in metal concentrations between sampling sites were evaluated using a Kruskal–Wallis One-Way ANOVA on Ranks and Student Newman Kuels Method for post hoc tests. Statistically significant differences in metal concentrations between sampling seasons were evaluated using the Mann Whitney Rank Sum Test. In terms of the water sample results, the mean concentrations of the metals in the water are ordered from highest to lowest concentration in the dry season as follows: Fe>Al>Zn>Mn>Pb. The mean metal concentrations in the wet season in the water are ordered as follows: Al>Zn>Fe>Mn>Pb. In terms of the sediment sample results, the metal concentrations in the dry and wet season are ordered as follows: Fe>Al>Mn>Zn>Pb. The mean metal concentrations were generally higher in the wet season than in the dry season. The variations in the concentrations can be attributed to non-point source pollution, metal-containing road dust, wildfires and exhaust fumes, stormwater runoff and runoff from farms, and WWTW effluent discharges. Majority of the metals were significantly higher at site 4, which not only receives pollution from upstream sources, but also receives a vast amount of pollution from the Kuils River tributary. When comparing the metal concentrations against national and international water quality and sediment quality guidelines, it was observed that the mean aluminium concentrations in water exceeded the South African Water Quality Guidelines (SA WQG) and the Australia and New Zealand Guidelines for Fresh and Marine Water Quality. The mean manganese and iron concentrations in water did not exceed any guidelines. However, the iron concentration in the Eerste River estuary exceeded the South African Water Quality Guidelines for Coastal Marine Waters (SA WCG – CMW). No sediment quality guidelines have been developed for aluminium, iron, and manganese nationally or internationally. The mean zinc concentrations in water exceeded the SA WQG, the Canadian Water Quality Guidelines for the Protection of Aquatic Life (CWQG) and the SA WQG – CMW. The mean zinc concentrations in sediment did not exceed any guidelines. The mean lead concentrations in water exceeded the SA WQG but the mean lead concentrations in sediment did not exceed any guidelines. It is concluded that the Eerste River is polluted, although there are significant differences in metal concentrations in both water and sediment along the length of the river. It is further concluded that WWTW effluent from the Macassar plant is not a main source of contamination, but it is contributing to some degree, along with several other sources such as stormwater runoff, natural occurrence, and non-point source pollution. It is recommended that further research be undertaken to better distinguish the sources of metal pollution in the Eerste River as well as to determine the impacts of metal contamination on river biodiversity. Further research should also be conducted to generate more data to develop South African sediment quality guidelines. Additional recommendations include continuous monitoring of metals in the river and in WWTW effluent to locate vulnerable areas and apply appropriate remediation/abatement measures.