Aluminium

Materials and Methods

3.5 Toxic constituents seldom occur in high concentrations in unimpacted systems (DWAF 1996c). Criteria are given as single numerical values associated with a specific

3.5.1 Aluminium

Aluminium is the most abundant metallic element in the lithosphere, but has little or no known biological function (Gensemer and Playle 1999). Aluminium can exist in a number of forms some of which may be soluble in water and some insoluble in water. It occurs as ionized aluminium (Al³⁺), as ionised aluminium complexes, such as Al^2-(OH)6

and insoluble compounds (Dallas and Day 2004). Aluminium is probably not an essential nutrient in any organism and is potentially one of the more toxic metals as soluble ionised aluminium is toxic to fish and aluminium complexes have been implicated in fish-kills (Dallas and Day 2004). The aluminium minerals, particularly the silicates of aluminium, are widespread. Some important minerals containing aluminium are hydrated aluminium oxide, magnesium aluminium oxide and the various aluminium silicates (DWAF 1996b).

Aluminium occurs in water in two main phases, as suspended aluminium minerals, and

80

as dissolved aluminium (DWAF 1996c). Aluminium is strongly influenced by pH levels and its solubility and toxicity is strongly pH-dependent. At alkaline pH levels it is present as soluble but biologically unavailable hydroxide complexes, while under acid conditions, aluminium occurs as the soluble, available and toxic species (Dallas and Day 2004).

The TWQR, CEV and AEV values of aluminium at different pH levels (DWAF 1996c) are indicated below:

Aluminum concentration (mg l^-1) pH< 6.5 pH > 6.5

TWQR (mg l^-1) 0.005 0.01

Chronic Effect Value (CEV) 0.01 0.02

Acute Effect Value (AEV) 0.1 0.15

Aluminium is known to be toxic to various invertebrates and to plants and can interfere with the calcium ion metabolism (Dallas and Day 2004). Increases in acidity result in changes in the chemical structure of soils with the release of minerals such as aluminium into runoff water, which may enter lakes and fish farms. High concentrations of soluble aluminium may also be found in natural waters affected by acid rain and acid mine drainage (Dallas and Day 2004).

Table 3.21: Seasonal aluminium values (in mg Al l^-1)of the four sampling sites Surveys Site A Site B Site C Site D

Winter 0.001 0.02 0.01 0.02 Spring 0.01 0.01 0.02 0.01 Summer 0.01 0.01 0.01 0.01

Autumn 0.01 0 0 0.01

Mean 0.008 0.01 0.01 0.0125

At sites A and C, the aluminium concentrations recorded during the Winter survey were within the TWQR of 0.01mg l^-1 at pH >6.5 for aquatic ecosystems (DWAF 1996c), while the aluminium concentrations at sites B and D were equal to the CEV of 0.02mg l^-1 (Table 3.21). During the Spring survey, only site C recorded a CEV concentration of 0.02 mg Al l^-1 (Table 3.21 and Figure 3.21). The aluminium concentrations were constant (0.01mg Al l^-1) at the four sampling sites during the Summer and Autumn

81

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02

mg l-1

Site A Site B Site C Site D

Aluminium

W inter Spring Summer Autumn

Figure 3.21: Seasonal aluminium concentrations of the four sampling sites

surveys except for sites B and C (Autumn) where the aluminium concentrations were too low to be determined by the spectrophotometer (Figure 3.21). The highest mean value was 0.0125mg Al l^-1 recorded at site D (Table 3.21).

Dallas and Day (2004) stated that aluminium is pH dependent and at low pH values aluminium is largely in the aqua form which is soluble and very toxic. As the pH increases, hydrolysis of aluminum results in a series of increasing insoluble hydroxyl ions complexes (Al(OH)²⁺ and Al(OH)2+

). Taking this into consideration the aluminium concentrations recorded during this study were not toxic as the pH values ranged between 7.7 and 8.82 (Table 3.3) during this study. However, a CEV concentration of 0.02 mg Al l^-1 (Table 3.21 and Figure 3.21) was recorded at three occasions which might be a matter of concern.

82 3.5.2 Copper

Copper is one of the world's most widely used metals and it occurs naturally in most waters (DWAF 1996c). Copper occurs in four oxidation states, the two most common forms are cuprous copper (I) and cupric copper (II). Cuprous copper is unstable in aerated aqueous solutions and will normally be oxidized to cupric copper (DWAF 1996c). Copper is a common metallic element in the rocks and minerals of the earth's crust, and is commonly found as an impurity in mineral ores and CuFeS2 is the most abundant of the copper minerals (Villarroel 2000). Igneous rocks contain more copper than sedimentary rocks. According to DWAF (1996c) the occurrence of natural sources of copper in the aquatic environment is due to weathering processes or from the dissolution of copper minerals and native copper. Metallic copper is insoluble in water, but many copper salts are highly soluble as cupric or cuprous ions (DWAF 1996c).

Dallas and Day (2004) reported that copper is a micronutrient and an essential part of cytochrome oxidase and various other enzymes involved in redox reactions in cells. It is toxic at low concentrations in water and is known to cause brain damage in mammals (Olaifa et al. 2004). The toxicity of copper decreases when the total water hardness of an aquatic ecosystem increases (DWAF 1996c). Furthermore, the toxicity of copper increases as the pH and dissolved oxygen concentrations decrease (Benedetti et al.

1989; Greenfield 2004). According to DWAF (1996c), copper in aquatic ecosystems is correlated with water hardness as follows:

Water Hardness (mg CaCO₃ l^-1) < 60 60-119 120-180 > 180 TWQR (mg l^-1) Cu mg l^-1 0.0003 0.0008 0.0012 0.0014 Chronic Effect Value (CEV) 0.0005 0.0015 0.0024 0.0028

Acute Effect Value (AEV) 0.0016 0.0046 0.0075 0.012

During this study copper concentrations exceeded the TWQR of 0.0014mgl^-1 at a total water hardness of >180mg CaCO3 l^-1 at all sites, as suggested by DWAF (1996c).

Except for site D, the mean total water hardness was >180mg CaCO3 l^-1 at all sites (Table 3.8). The copper concentrations were high during the Winter and very high during the Autumn surveys at all sites (Table 3.22 and Figure 3.22). The highest

83

Table 3.22: Seasonal copper values (in mg Cu l^-1)of the four sampling sites

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

mg l-1

Site A Site B Site C Site D

Copper

Winter Spring Summer Autumn

Figure 3.23: Seasonal copper concentrations of the four sampling sites

recorded concentration was 0.19mg Cu l^-1 at site C with a mean value of 0.055mg Cu l^-1 (Table 3.22). The mean concentrations exceeded the CEV and AEV (DWAF 1996c) for copper at a water hardness of >180mg CaCO3 l^-1at all sites, but the toxicity of copper is reduced by the high water hardness levels (Table 3.8) as well as the alkaline pH vales recorded at all sites (Table 3.3). The high copper concentrations recorded during this study can mainly be attributed to industrial effluents and mine tailings (Von der Heyden Surveys Site A Site B Site C Site D

Winter 0.02 0.01 0.022 0.021 Spring 0.01 0.02 0.005 0.009 Summer 0.01 0.018 0.004 0.002 Autumn 0.092 0.067 0.19 0.125 Mean 0.033 0.029 0.055 0.039

84

and New 2004) at sites B and C and geological weathering and atmospheric pollution (Moore and Ramamoorthy 1984) at sites A and D.