The impact of urban pollution on the contamination of water, sediment and algae in the zandvlei estuary, cape town, South Africa

Abstract

Estuaries are some of the most productive areas of the coastal environment as they provide essential ecosystem goods and services. However, as a result of rapid urbanisation coupled with population growth, these systems are under increasing pressure. The Zandvlei estuary in False Bay, Cape Town is an example of an estuarine system in the cool-temperate biogeographic region of South Africa that is affected by urban pollution emanating from various land uses (i.e. industrial activities). Metal contamination remains a concern in many parts of the world as it negatively affects aquatic ecosystems due to their persistent and toxic nature which could negatively affect human health. Little information is available on metal concentration levels in South African estuaries. As a result, there is a need for further investigation as metals have been identified as an emerging pollution pressure that requires attention. This study aimed to determine the concentration levels of six metals (i.e. Aluminium (Al), Manganese (Mn), Iron (Fe), Zinc (Zn), Lead (Pb), Copper (Cu)), including their spatial and temporal variations in water, sediment, and algae (i.e. Enteromorpha spp.) within the Zandvlei estuary. The information gathered in this study forms the basis for further metal pollution investigations within the study area. Samples were collected in the main body of the estuary on four separate occasions from five sampling sites. The collected water, sediment, and algae samples were acid digested and the concentrations determined by means of ICP-MS. Statistical analysis was conducted using the SigmaPlot (version 14.0) software by SYSTAT Software Inc. Statistical differences between the different sampling sites and sampling occasions for the subject metals were analysed and evaluated using the Kruskal-Wallis One-Way Analysis of Variance (ANOVA) on Ranks and Student Newman Kuels Method for post hoc tests. In cases where there were missing or unequal datasets, the Dunns Method was utilised for post hoc tests. The condition for the significant differences was set at p<0.05 for all statistical analyses. Spatial variations in the recorded metal concentrations showed significant differences at sites near stormwater drainage outlets as well as the influent rivers and this could be an indication of a potential source of contamination within these areas. Physicochemical parameters such as salinity, pH, and temperature also played an important role. Areas where there were standing waters due to the low gradient of the estuary (which consists of deeper channels as a result of dredging) and due to the closing of the estuary mouth also recorded metal concentrations that were relatively higher in some instances. Temporal variations in the recorded metal concentrations revealed a range of differences that were influenced by rainfall, salinity, and changes in anthropogenic inputs, among other factors. The highest mean concentration in water was recorded for Al (2.0438±2.7055 mg/ℓ). Fe recorded the highest mean concentration in both sediment (6744.90±5130.36 mg/kg) and macroalgae (4764.45±606.10 mg/kg). Metal concentrations were generally low in water samples, however, Al, Fe, Zn, Pb, and Cu exceeded one or more of the following water quality guidelines: South Africa’s Department of Water Affairs and Forestry (1996), Canadian Council of Resource and Environment Ministers (1987), Canadian Council of Ministers of the Environment (2001), and Australian and New Zealand Environment and Conservation Council (2000). Metal concentrations in water samples decreased in the following sequence: Al > Mn > Fe > Zn >Cu > Pb. In sediment, Zn, Pb, and Cu concentrations from one or more of the sampling sites exceeded sediment quality guidelines provided by CCME (1999) and/or ANZECC (2000). Metal concentrations in sediment decreased in the following sequence: Fe > Al > Zn > Pb > Mn > Cu. Concentrations in macroalgae (Enteromorpha spp.) were also relatively high and were between “moderately contaminated” to “uncontaminated” for Cu and Pb. Macroalgae as a biomonitor recorded relatively higher concentrations when compared to the other mediums with higher concentrations recorded between spring (SO4) and summer (SO1) and this could be attributed to the metabolic processes including the presence of sufficient algae biomass. Metal concentrations in Enteromorpha spp. decreased in the following sequence: Fe > Al > Mn > Zn > Pb > Cu. In light of the above, it is necessary that source identification and continuous monitoring of metal concentrations in the Zandvlei estuary be undertaken.