Contextual orientation
3.2 Boksburg Lake’s urban water catchment
3.2.3 Summary of biophysical characteristics 2001-2008
The condition of water bodies reflects human activities in a catchment (Davies & Day 1998) and as a result can be considerably altered from their base condition. Water quality, water quantity and the geomorphological characteristics can all be impacted.
The particular focus of this section is the quality of Boksburg Lake’s water. Figure 3.3 is a Google Earth image of Boksburg Lake and its catchment. It indicates the mining, commercial and industrial activities that take place close to Boksburg Lake and that the
lake’s two main drainage lines flow past Cason mine dump and Anderbolt Industrial Park.
Figure 3.3: A Google Earth image of Boksburg Lake and its catchment (indicated by the black line), including drainage lines. It shows the Cason mine dump, the East Rand Mall, the central business district, the industrial areas of Anderbolt Park and Boksburg East Industrial Park and Boksburg North and Plantation residential areas, as well as the sampling points used by Ekurhuleni Metropolitan Municipality are indicated in black text for the canal, lake inflow and lake outflow.
Dallas and Day (2004) explained that acceptable water quality is specific to a particular user (such as naturally occurring aquatic species, recreational users, agricultural users, industrial users) and is determined by the physical properties of a sample of water. This includes physical constituents (turbidity, suspended solids and temperature) and chemical constituents (including non-toxic: Total Dissolved Solids, conductivity, pH, nutrients, organic enrichment, dissolved oxygen and salinity; and toxic: trace metals and biocides). These variables may have favourable or detrimental results on aquatic organisms, which can be heightened if variables counteract or reinforce each other’s effects (Dallas & Day 2004). Pollution occurs when physical and/or chemical constituents of the water have been altered to the detriment of aquatic organisms and/
or users (Davies & Day 1998).
Trace metals, including mercury, aluminium, cadmium, lead, nickel, copper, chromium, selenium and zinc (Davies & Day 1998) are classified as toxic because under natural conditions they typically occur in very low concentrations (Dallas & Day 2004); they cannot be broken down and are therefore persistent, becoming more concentrated higher up the food chain (Davies & Day 1998). Any increase thus exposes aquatic organisms to concentrations they are not adapted to and leads to a reduction in species richness and diversity and change in species composition (Dallas
& Day 2004). Contamination of water bodies by trace metals therefore requires proper management and careful monitoring (Dallas & Day 2004). This is important to note, when considering the intensive mining and industrial activity that occurs in Boksburg, which are two common sources of trace metals (Davies & Day 1998; Dallas & Day 2004).
Ekurhuleni Metropolitan Municipality regularly tested the water quality of Boksburg Lake, at the lake’s inlet, outlet and a point in the canal feeding Boksburg Lake, from 2001. See figure 3.2 for the location of these sampling points. The following water quality variables were monitored: pH, chemical oxygen demand (COD), electrical conductivity, faecal coliforms, nitrite nitrogen, phosphates, ammonia, aluminium, copper, iron, nickel, manganese, magnesium and zinc. Gordon (2008) analysed this water quality data to establish trends over time for each water quality variable.
Gordon’s (2008) analysis has been used as an initial summary of the biophysical condition of Boksburg Lake between 2001 and 2008. Gordon (2008) compared concentrations and values at the lake inflow, lake outflow and canal points with the Klipriver catchment management forum (CMF) water quality guidelines (Klipriver Forum 2003) that indicated an ideal and unacceptable range. Guidelines for inland aquatic ecosystems (DWAF 1996) were used in cases where the Klipriver CMF water quality guidelines were not available for a particular water quality variable (figures 3.9, 3.10, 3.11, 3.12, 3.13, 3.14). The DWAF (1996) guidelines for the protection of aquatic ecosystems indicate quantitative chronic and acute effect toxicity values for different variables. A chronic effect value refers to situations when the target water quality range (TWQR) has been exceeded and where non-lethal effects of aquatic organism exposure to lake water are measurable. The TWQR specifies the desired concentration range for a particular constituent (DWAF 1996). An acute effect value indicates that lethal effects in aquatic biota are measurable, ecosystem health is threatened if the situation persists, even for a short period, and urgent management attention is required (DWAF 1996). The identification of the chronic and acute toxicity range for each constituent provide broadly protective ecosystem goals but this does not adequately account for the complexity of ecosystem interactions, where interacting water quality
variables may have additive, synergistic or counteractive effects (DWAF 1996; Dallas
& Day 2004).
Variables that exceeded to varying degrees the Klipriver CMF guidelines and, where applicable DWAF guidelines, included faecal coliform counts, COD, copper, nickel, zinc, aluminium and iron. The analysis of these variables is presented below. pH is also included as its value influences the toxicity of other variables. Data was collected intermittently and gaps in the data set are evident in each graph.
Data from Ekurhuleni Metropolitan Municipality, as made available by Gordon (2008), was re-analysed to determine the percentage time that a given value was exceeded for the different water quality variables (faecal coliform counts, COD, copper, nickel, zinc, aluminium and iron) at the lake inflow, lake outflow and canal.
Results pH
pH is an important water quality variable as it determines the availability and potential toxicity of a variety of substances, particularly many heavy metals. Heavy metals most likely to have toxic effects under acidic conditions include aluminium, copper, manganese, nickel and zinc (Davies & Day 1998; Dallas & Day 2004). Under natural conditions, most freshwaters in South Africa have pH ranges between 6 and 8 (DWAF 1996). Human induced acidification generally results from acid precipitation, industrial effluents and mining activities (DWAF 1996; Dallas & Day 2004).
Figure 3.4: pH measured at the Boksburg Lake inlet, outlet, and canal feeding the lake (October 2001 – February 2008). The ideal range based on the Klipriver catchment management forum (CMF) guidelines is plotted; gaps in the graphs indicate missing values (Gordon 2008).
The pH values at all sampling points from 2001 to 2008 have fallen within the ideal range (Klipriver CMF guidelines), except three times (figure 3.4), which is surprising considering the amount of industrial and mining activity in the catchment. In September 2002 the lake inflow sampling point dropped below 6 and in May 2003 and November 2007 it went above 9. The pH range of Boksburg Lake will therefore buffer the toxicity of aluminium, nickel, zinc, manganese and copper that occurs in the lake.
Faecal coliforms
The most common source of pollution of water bodies is organic enrichment through sewage, measured by the number of faecal coliforms present (DWAF 1996; Dallas &
Day 2004). This pollution changes species composition with an increase in those tolerant to such enrichment and a decrease or elimination of those that are sensitive, as well as an overall reduction in species diversity (Dallas & Day 2004). Humans may also be affected as faecal coliforms indicate the possible presence of pathogens in a water body, which can transmit infectious diseases such as cholera, typhoid fever and dysentery, to name a few (DWAF 1996).
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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
pH
Lake inflow Lake outflow Canal
Ideal range (Klipriver CMF guidelines)
Figure 3.5: Faecal coliform counts measured 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).
It is locally known that Boksburg suffers from sewage leakages and spills, probably attributable to deteriorating sewage infrastructure. This has contributed to the highly variable monthly faecal coliform counts over the eight years sampled at Boksburg Lake that often exceeded the Klipriver CMF guidelines for unacceptable amounts, as figure 3.5 indicates. DWAF’s guidelines for full contact (swimming) and intermediate (boating and fishing) are more stringent, being 100 counts per 100ml and 1000 counts per 100ml respectively. Between late 2002 and early 2006 the coliform counts rarely exceeded the unacceptable range. However, from mid 2006 to 2008 faecal coliform concentrations exceeded the unacceptable level on a regular basis. Seven times levels were close to 100 000 counts per 100ml and nearer 1 million counts per 100ml five times.
Figure 3.6 shows the percentage exceeding given values of coliforms. Data indicates that the lake inflow is the largest source of faecal coliforms and one can infer that faecal contamination comes from sewage leaks occurring in the Boksburg Lake catchment as a whole. At the lake inflow, the unacceptable Klipriver CMF guidelines were exceeded 20% of the time, levels were as high as 100 000 counts per 100ml for 10% of the time and exceeded 1 000 000 counts per 100ml for about 2% of the time.
Data from the canal exceeds the unacceptable limit of 10 000 per 100ml for 10% of the
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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
Faecal coliforms (counts/100 ml)
Lake inflow Lake outflow
Canal Unacceptable (Klipriver CMF guideline)
Ideal (Klipriver CMF guideline)
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