4.2 Physical parameters and concentrations of PAHs in the Msunduzi
4.2.1 Conductivity, pH and temperature of the Msunduzi River
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CHRY 26.78 228 226; 228
*NA-d8: naphthalene-d8, NA: naphthalene, ACY: acenaphthylene, FLUO: fluorene, PHEN: phenanthrene, ANTH:
anthracene, PYR: pyrene and CHRY: chrysene.
As shown in Table 4.2, all of the PAHs were detected at m/z values corresponding to their molecular ions, with few fragmentation peaks being observed. This is consistent with the chemical stability of the compounds owing to their aromaticity and delocalisation of electrons within their entire conjugated rings. Thus, the PAH molecules survive bombardment by the 70 eV electron beam directed in their paths.
A single electron is removed from the highest molecular orbital of each PAH molecule to form a stable and an odd-electron radical molecular ion. The m/z values of each PAH were used to identify (as complementary data to the retention times from the TIC) the 7 PAHs, which were selected for quantitative analysis (see also the figures presented in Appendix 3 for some confirmation ions of PAHs).
To confirm the identity (and complement the retention data from the TIC) of the 7 PAHs and to subsequently quantify them, GC-MS in SIM mode was used to identify and quantify the selected PAHs. In this mode, the percentage recovery of each PAH was calculated by subtracting the amount measured from the unspiked duplicates measured at set molecular ion peaks (Table 4.2).
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Table 4.3 Physical parameters of the Msunduzi River water.
HD (Henley Dam), CD (Camps Drift), DuTV (Du Toit), DWWTP Inlet (Darvill Wastewater Treatment Plant Inlet), DWWTP Outlet (Darvill Wastewater Treatment Plant Outlet), AA (Agriculture Area), MT (Msunduzi Town), JUM (Joining Point of Msunduzi and Umgeni Rivers, NDA (Nagle Dam)
Seasons
Parameters
Sampling sites
HD CD DuTV DWWTP
Inlet
DWWTP Outlet
AA MT JUM NDA
Autumn of 2014
pH of water 7.00 6.65 6.31 5.67 6.70 5.86 8.10 5.72 7.71
Conductivity (µS/cm) at 25 ºC 103.8 187.9 144.1 655.0 698.0 279.0 348.0 30.0 91.8
Water T (ºC) 16.8 16.6 17.8 23.0 24.0 18.3 18.7 18.3 20.1
Ambient T (ºC) 28.2 25.3 25.5 29.3 29.2 29.2 21.6 18.3 24.1
Winter of 2014
pH of water 7.99 7.85 7.79 7.34 7.39 8.69 9.02 8.84 7.80
Conductivity (µS/cm) at 25 ºC 120.4 225.0 169.6 946.0 703.0 387.0 435.0 318.0 90.3
Water T (ºC) 12.9 16.3 19.9 22.2 16.7 17.8 19.3 19.5 20.3
Ambient T (ºC) 26.6 31.0 32.5 21.1 21.4 33.8 34.7 33.3 30.4
Summer of 2014
pH of water 7.60 7.42 7.39 7.39 7.77 7.68 7.88 8.10 8.33
Conductivity (µS/cm) at 25 ºC 83.9 132.9 125.2 865.5 543.0 300.3 329.0 279.5 105.6
Water T (ºC) 25.0 27.1 23.1 28.7 27.7 27.9 27.8 27.2 28.7
Ambient T (ºC) 27.5 29.0 31.8 31.4 31.4 32.8 37.7 33.0 37.0
Spring of 2015
pH of water 6.78 7.79 7.67 7.21 7.37 8.64 8.32 6.74 8.54
Conductivity (µS/cm) at 25 ºC 103.7 384.2 267.2 1165.0 928.2 253.5 231.7 355.7 119.0
Water T (ºC) 15.3 21.5 16.2 19.7 16.0 25.0 17.9 20.0 26.2
Ambient T (ºC) 16.3 17.1 18.4 16.1 20.6 25.2 19.5 26.8 28.5
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The pH of surface water indicates the acidity and alkalinity of the natural water (Gupta & Sharma, 1996). In cases where there is a minimum external injection of acidic or basic wastes, the pH of the natural water depends strongly on the equilibria involving absorption and the evolution of CO2 as well the HCO3-/CO32- buffer system (Hutchinson, 1975). The latter comes from the dissolution of limestone and shale (Ca/MgCO3), which make up the bedrock of most river basins. However, the overall pH of the water is also affected by input loads of organic contaminants (humic and fulvic acids) from surface runoff (Sahu et al., 1998).
During the autumn season, the pH of the river water and wastewater presented the results of normal water as shown in Table 4.3. Normally, a decrease of pH in river water may be due to the dissolution of minerals into the river water while its increase may be due to the depletion of inorganic compounds in water.
During the winter season, the pH of water varied between 7.32 and 9.02. The river water presented a pH within the range of a weak base. A slight decrease was found from the mouth of the river to the DWWTP Outlet sampling site. The water in the lower reaches of the river, such as the JUM and the NDA sampling sites, had higher pH values, which may be attributed to the biochemical processes that occurs in these areas during this season.
The electrical conductivity of water is primarily ascribed to the dissolved ions derived from the putrefied plant material (Singh et al., 2013) and an input of inorganic as well as organic waste (Wright & Hamilton, 1982).
The electrical conductivity (EC) generally increased from the source of the river up to the DWWTP Outlet sampling site and decreased gradually up to the mouth of the river. This can be explained by the variation of dissolved ions and pollutants in the water as the river flows and its presence is also due to the variation in ambient temperature as EC is temperature dependent.
The EC values varied between 90.2 and 946 µS/cm. The higher range of EC observed in winter can be related to the higher than normal ambient temperature values recorded during this season. Research has been reported that the temperature of water
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can affect its conductivity by increasing ionic mobility as well as the solubility of many salts and minerals, especially during the day. For example, an increase of 1 ºC can cause an increase of the electrical conductivity of water from 2 to 4% (Whipple, 2002).
Indeed, the electrical conductivity in the water is normally affected by the compartment of the ionisable organics on which different organic pollutants such as PAHs can adsorb and get stabilized in water (SFM, 2015).
During the spring season, the pH and the EC varied slightly in the surface water from the source to the mouth of the river. The only exception in EC was observed in the wastewater, where the DWWTP both Inlet and Outlet presented high values of EC around 1000 µS/cm. This may be due to the higher concentrations of different types of dissolved ions and pollutants released into the wastewater from the city of Pietermaritzburg during this season. The presence of organic pollutants such as PAHs in the aquatic system depends on physical parameters of water, such as dissolved solids as well as EC and the suspended particulate matter on which they can adsorb.
Winter and spring seasons presented higher averages of the EC, which could be a result of the breakup of industrial wastes in the water. This is because in these two seasons there is generally little rain to wash away these wastes, so they spent a long time in the river water thus increasing its electrical conductivity. Normally, the electrical conductivity increases with an increase in water temperature. The warmer temperatures tend to increase solubility of the ions and solids, resulting in higher dissolved solids which in turn contribute to increased conductivity. Therefore the warmer the water, the higher the electrical conductivity. Conductivity is therefore reported as conductivity at 25 ºC (Wetzel, 2001; SFM, 2015).
During the summer season, the pH varied gradually as the river flowed. The pH ranged from 7.39-8.33 while the electrical conductivity of water was very low in the middle reaches as shown in Table 4.3. The pH in the autumn, spring and summer seasons was between 6 and 8, which meant that the Msunduzi River water was in the normal range of stream water (falling in the range of drinking water from 6 to 9).
During the winter season it was in the range of weak base or seawater pH. The average electrical conductivity in this study was 281.98±0.07 μS/cm for the autumn,
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376.89±0.06 μS/cm for the winter, 422.77±0.05 μS/cm for the spring and 305.75±0.05 μS/cm for the summer seasons.
The average temperature of water was 18.78±0.02 ºC for the autumn, 17.87±0.03 ºC for the winter, 19.93±0.01 ºC for the spring and 27.01±0.05 ºC for the summer season, whilst the ambient temperature was 25.87±0.02 ºC for the autumn, 28.96±0.05 ºC for the winter, 21.02±0.03 ºC for the spring and 31.86±0.05 ºC for the summer seasons. The abnormal average of ambient temperature observed during the winter season resulted in the fact that samples were taken during a sunny (warm) day.
The average pH obtained in this work was 6.63 for autumn, 8.07 for winter, 7.67 for spring and 7.72 for summer. The winter season appeared to have a higher average pH, which might be related to the lack of rainfall (normal rainfall has a pH of 5.60, slightly acidic due to CO2 from the atmosphere) during this season.