time, but at much lower levels than either the lake inflow or outflow, indicating that the canal is not a major contributor of faecal coliforms.
Figure 3.6: Percentage exceedance of faecal concentrations in the lake inflow, lake outflow and canal (October 2001 to February 2008). Data sourced from Ekurhuleni Metropolitan Municipality.
Organic enrichment, for example through sewage, can have important impacts on a number of other water quality variables including a change in pH, reduction in dissolved oxygen (DO) and an increase in turbidity and suspended solids, temperature and bacterial contamination (Dallas & Day 2004).
A reduction in DO, caused by organic enrichment, can have severe impacts. As Davies
& Day (1998: 190) stated, “The concentration of dissolved oxygen is probably one of the most important abiotic determinants of the survival of most aquatic organisms”.
This is because DO is necessary for aerobic respiration while many toxic elements, such as ammonia, cadmium, cyanide and zinc become increasingly toxic under reduced levels of DO (Dallas & Day 2004). The degree of turbidity determines light penetration, with far reaching effects on the aquatic biota as photosynthesis and vision both depend on light (Dallas & Day 2004). Suspended solids, causing turbidity also smother surfaces, such as habitats and gills, absorb toxins and can change community composition to those species most able to adapt to such conditions (Dallas & Day 2004).
In 2008 there was an extensive fish kill due to the anaerobic conditions resulting from leaking sewage. An informant described the lake (at that time) as a sewage farm, which parallels the data of counts at 1 million per 100ml.
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Facecal colliform (counts/ 100 ml)
% greater than given value
Chemical Oxygen Demand (COD)
Chemical oxygen demand (COD) measures the capacity of water to consume oxygen in chemical oxidation reactions and is therefore a useful surrogate of chemical pollution (Dallas & Day 2004).
Figure 3.7: Chemical oxygen demand at the Boksburg Lake inlet, outlet, and canal feeding the lake (October 2001 – February 2008). The ideal and unacceptable ranges based on the Klipriver catchment management forum (CMF) guidelines are plotted; gaps in the graphs indicate missing values (Gordon 2008).
Water samples were analysed for COD from October 2001 to February 2008, as shown in figure 3.7. This data indicates a degree of temporal variation in COD but also a trend of increasing levels, which signifies a growing chemical pollution problem. There are two periods with particularly high COD levels, namely March 2003 to December 2003 and June 2007 to February 2008. During both these periods levels exceeded 100mg/l on a consistent basis and peaked above 200mg/l.
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Oct-01 Dec-01 Mar-02 Jun-02 Sep-02 Dec-02 Mar-03 Jun-03 Sep-03 Dec-03 Mar-04 Jun-04 Sep-04 Dec-04 Mar-05 Jun-05 Sep-05 Dec-05 Mar-06 Jun-06 Sep-06 Dec-06 Mar-07 Jun-07 Aug-07 Nov-07 Feb-08
Date
COD (mg/l)
Lake inflow Lake outflow
Canal Unacceptable (Klipriver CMF guidelines)
Ideal (Klipriver CMF guideline)
Figure 3.8: Percentage exceedance of chemical oxygen demand in the lake inflow, lake outflow and canal (October 2001 to February 2008). Data sourced from Ekurhuleni Metropolitan Municipality.
Figure 3.8 indicates that both the lake inflow and canal are significant contributors of COD. At both these sampling points levels exceeded the unacceptable Klipriver CMF guidelines 70% of the time. For 10% of the time the levels increase dramatically to exceed the unacceptable guidelines by considerable amounts, especially when one considers that a logarithmic scale has been used.
Copper
Copper, although a micronutrient, is toxic at low doses and has been known to cause brain damage in mammals and to change the species richness and composition of invertebrate communities (DWAF 1996; Dallas & Day 2006). The DWAF guidelines recommend that values of copper levels should be below the chronic effect level if ecosystems are to remain healthy. The toxicity of copper increases with a decrease in water hardness and dissolved oxygen and when combined with other metals, while it decreases with higher alkalinity and chelating agents (DWAF 1996). Applicable anthropogenic sources of copper include mining and industrial activities (DWAF 1996).
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COD (mg/l)
% greater than given value
COD
Lake in6low Lake out6low Canal
Unacceptable (Klipriver CMF guidelines) Acceptable (Klipriver CMF guidelines)
Figure 3.9: Copper concentrations at the Boksburg Lake inlet, outlet, and canal feeding the lake (January 2006 – March 2008). The SA chronic and acute effect values are plotted; gaps in the graphs indicate missing values (Gordon 2008).
Water samples were analysed for copper between January 2006 and March 2008, as shown in figure 3.9. The levels of copper fluctuated dramatically throughout this period, varying from 0 to over 100ug/L, which is almost 100 times the SA acute effect value. This could point to the temporal discharge of copper into the environment from a variety of point sources.
Figure 3.10 indicates that the canal is the largest contributor of copper, with levels exceeding the SA acute effect value over 60% of the time. The lake inflow is also an important contributor, with levels exceeding the SA acute effect value about 40% of the time. What is of note is the extent to which these levels are exceeded, varying between 100 to 200 times the acute effect value. For 26% of the time the lake inflow exceeds the SA acute effect value, meaning that 14% of copper levels are remaining in the lake. Boksburg Lake, therefore acts as a sink for copper, which will be accumulating in the sludge. These values indicate that copper constitutes a noteworthy risk to the Boksburg Lake aquatic ecosystem.
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Jan-06 Feb-06 Apr-06 Jun-06 Jul-06 Sep-06 Oct-06 Dec-06 Feb-07 Mar-07 May-07 Jul-07 Aug-07 Oct-07 Dec-07 Jan-08 Mar-08
Date
Copper (ug/L)
Lake inflow Lake outflow
Canal SA chronic effect value (DWAF 1996)
SA acute effect value (DWAF 1996)
Figure 3.10: Percentage exceedance of copper concentrations in the lake inflow, lake outflow and canal (January 2006 to March 2008). Data sourced from Ekurhuleni Metropolitan Municipality.
Nickel
Nickel is toxic even in small quantities where it damages the lungs of mammals, can lead to mortality of amphibians, fish and insects and has carcinogenic effects (Davies
& Day 1998; Dallas & Day 2004). It is five times as toxic when combined with zinc due to synergistic effects (Dallas & Day 2004). Industrial wastewater can be a source of nickel.
Water samples were analysed for nickel concentrations during 2006 (figure 3.11).
During this year nickel concentrations exceeded the SA acute effect value by 5x or more on a regular basis. Figure 3.12 indicates that the lake inflow is an important source of nickel which exceeds the SA acute effect value 100% of the time. For 22% of the time it exceeds the SA acute effect value 10 times. From this one can infer that the Boksburg Lake catchment as a whole is a significant source of nickel. Nickel from the canal also poses a risk, with levels exceeding the SA acute effect value 55% of the time, leveling off at 100 ug/l, which is five times the SA acute effect value.
Interestingly the lake outflow has high levels of nickel that exceed both the lake inflow and canal, being at 200 mg/l 44% of the time (20% more often than the lake inflow).
When one considers that only a year of data was sampled, this could indicate that the concentrations of nickel in the lake inflow and canal were higher in the previous year.
These values point to nickel constituting a notable risk to the Boksburg Lake aquatic ecosystem. This is especially so when one considers the high levels of zinc in Boksburg Lake, as discussed below, and its synergistic toxic effects with nickel.
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