Materials and Methods
3.2 Non-toxic inorganic constituents are defined as those water parameters which may have a toxic effect on aquatic life at extreme concentrations (DWAF 1996c), but
3.3.2 CATIONS: calcium, magnesium, potassium and sodium .1 Calcium
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tailings contain fluoride which might also be the reason for very high and potentially toxic fluoride levels at site B.
3.3.2 CATIONS: calcium, magnesium, potassium and sodium
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3.11). These higher calcium concentrations at the latter sites are due to effluent from the mine waste water system. The lowest calcium concentration of 13mg l-1 was recorded during Autumn at both sites A and D with the lowest mean at site A (18.5mg Ca l-1).
0 50 100 150 200 250
mg l-1
Site A Site B Site C Site D
Calcium
Winter Spring Summer Autumn
Figure 3.11: Seasonal calcium concentrations of the four sampling sites
Calcium is one of the cations associated with the weathering of rocks (Chapman and Kimstach 1996) and the high calcium concentrations can thus be associated with the volcanic rocks of the Phalaborwa Igneous complex which is underlain by pyroxenite rocks, hosting calcium. Moreover, calcium is present in high concentrations in tailings water (Wikipedia) explaining the higher concentrations recorded at sites B and C.
3.3.2.2 Magnesium
Magnesi um is common in natural waters as Mg2+, and along with calcium, is a main contributor to water hardness (DWAF 1996c). Magnesium arises principally from the
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weathering of rocks containing ferro-magnesium minerals and from some carbonate rocks (Chapman and Kimstach 1996). According to Chapman and Kimstach (1996), it is an alkaline earth metal which reacts with oxygen and water to form magnesium oxide and magnesium hydroxide, respectively. The solubility of magnesium in water is governed by the carbonate/bicarbonate equilibrium and hence, the pH. Common minerals of magnesium are magnesium carbonate and various magnesium silicates (DWAF 1996a). Magnesium hydroxide is relatively soluble at a pH of 7, but gradually becomes less soluble as the pH increases. Magnesi um bicarbonate, chloride, nitrate and sulphate are very soluble in water, whereas magnesium carbonate, silicate and phosphat e are insoluble (DWAF 1996c).
According to Dallas and Day (2004), very little is known about the effects of magnesium on aquatic organisms although magnesi um is also an essential nutritional element since it is found in relatively high concentrations, and as such it is unlikely to act as a limiting nutrient or toxin. According to DWAF (1996a) the concentration of magnesium in freshwater is between 4 - 10mg l-1 with theguidelines for domestic use 30mg l-1and a TWQR between 50 – 70mg l-1 will have no health effects on the consumers.
Table 3.12: Seasonal magnesium values (in mg Mgl-1)of the four sampling sites Surveys Site A Site B Site C Site D
Winter 25 221 173 27
Spring 31 232 183 25
Summer 11 56 142 6
Autumn 18 175 225 6
The magnesium concentrations ranged from 6mg l-1 to 232mg l-1 during the four sampling surveys (Figure 3.12 and Table 3.12). The concentrations were much higher at sites B and C during this study and higher than the TWQR as suggested by DWAF (1996a), but according to Dallas and Day (2004) not toxic. The magnesium levels at site A were always higher than those at site D, except during the Winter survey. During the
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0 50 100 150 200 250
mg l-1
Site A Site B Site C Site D
Magnesium
Winter Spring Summer Autumn
Figure 3.12: Seasonal magnesium concentrations of the four sampling sites
Summer survey the magnesium levels were the lowest at all sampling sites probably due to the dilution effect of the summer rains. The mean magnesium values of sites B and C were higher than that of sites A and D (Table 3.12). These high concentrations can also be associated with weathering of rocks at the PIC where the geological formations of sites B and C contain high levels of magnetite which host magnesium (Otto et al. 2007).
3.3.2.3 Potassium
Potassium usually occurs in natural waters in concentrations one-half to one-tenth than that of sodium (Trombley 2001). Even though potassium is fairly abundant in many of feldspars, and feldspars are among the most common silicate minerals, the potassium released by the weathering of such minerals appears to be rather quickly re- incorporated into clay minerals which largely resist further weathering (Chapman and Kimstach 1996). Potassium always occurs in water in association with anions, usually chloride, but can also occur with sulphate, bicarbonate or nitrate (DWAF 1996a). High concentrations of potassium may occur in runoff from irrigated lands and from fertilizer
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production as well as domestic wastes. Potassium tends to remain scarce in most natural waters (Chapman and Kimstach 1996). There is no TWQR for potassium available for aquatic ecosystems. The TWQR for domestic use is 0–50mg K+ l-1 (DWAF 1996a).
Table 3.13: Seasonal potassium values (in mg K l-1)of the four sampling sites
Surveys Site A Site B Site C Site D
Winter 0.1 155 158 0.1
Spring 10 182 101 10
Summer 22 106 128 17
Autumn 0.1 165 137 0.1
Mean 8.05 152 131 6.8
0 20 40 60 80 100 120 140 160 180 200
mg l-1
Site A Site B Site C Site D
Potassium
Winter Spring Summer Autumn
Figure 3.13: Seasonal potassium concentrations of the four sampling sites
Potassium concentrations were generally low (i.e. less than 1mg K l-1) at sites A and D during the Winter and Autumn surveys (Table 3.13 and Figure 3.13). Very high concentrations of potassium were recorded at sites B and C throughout the four surveys. The highest value of potassium was recorded at site B (182mg K l-1) during
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Spring with a mean of 152mg K l-1 at this site (Table 3.13). The high potassium concentrations recorded at sites B and C can be attributed to mining activities (tailings), weathering of rocks (geological) and high concentrations of potassium may also occur in runoff from irrigated lands and from domestic wastes. Potassium always occurs in water in association with anions, usually chloride, but can also occur with sulphate, bicarbonate or nitrate (Dallas and Day 2004). During the present study the mean potassium concentrations were much lower than that of sodium which is in accordance with their natural occurrence in freshwater.
3.3.2.4 Sodium
Sodium is an alkali metal which reacts with water to form highly soluble, positively- charged sodium ions. It is found in the ionic form (Na+) in plant and animal matter (DWAF 1996a). It is an essential dietary element important for the electrolyte balance and the maintenance of many essential physiological functions (DWAF 1996a). Sodium is ubiquitous in the environment and usually occurs as sodium chloride, but sometimes as sodium sulphate, bicarbonate or even nitrate (DWAF 1996c). Sodium is found as solid sodium chloride (rock salt) in areas where geological deposits occur (Otto et al.
2007). The levels of sodium in surface waters are generally low in areas of high rainfall but high in arid areas with low mean annual precipitation (Trombley 2001). Sodium is highly soluble in water and does not precipitate when water evaporates, unless saturation occurs. Hence, water in arid areas often contains elevated concentrations of sodium. Industrial wastes, especially processes that give rise to brines, contain elevated concentrations of sodium (DWAF 1996a). There is no TWQR for sodium available for aquatic ecosystems. The TWQR for domestic use of 100–200mg Na l-1 (DWAF 1996a) was used to discuss the results obtained during this study.
Table 3.14: Seasonal sodium values (in mg Na l-1)of the four sampling sites Surveys Site A Site B Site C Site D
Winter 39 213 158 47
Spring 59 275 221 30
Summer 56 216 344 24
Autumn 21 195 208 12
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0 50 100 150 200 250 300 350
mg l-1
Site A Site B Site C Site D
Sodium
W inter Spring Summer Autumn
Figure 3.14: Seasonal sodium concentrations of the four sampling sites
Higher sodium concentrations were recorded at sites B and C throughout the sampling period compared to the other two sites (Table 3.14 and Figure 3.14). The highest concentration of sodium was 344mg Na l-1 recorded at site C in Summer which is much higher than the TWQR for domestic use of 100–200mg Na l-1 as suggested by DWAF (1996a). Very low sodium concentrations were recorded at sites A and D with the lowest concentration (12mg Na l-1) recorded at site D during Autumn (Table 3.14). The sodium levels were always higher at site A than site D except during the Winter survey. Sodium levels recorded from the latter two sites are related to the geology and agricultural runoff. The high concentrations of sodium at sites B and C can be related to mining activities at these sites.