Mapping the dispersion of inorganic contaminants in surface water in the vicinity of Potchefstroom
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South Africa is a country with fewer water resources, as large parts of the country are semi-arid, and the scarcity of water is exacerbated by the pollution of surface water, largely by anthropogenic sources, especially mining activities. In the North West Province, the effluent of acid mine drainage from mining activities is regarded as the major source of pollution in the aquatic environment, as it contains metals such as iron and sulphate and has a low pH (< 2.5). Various mines, especially the gold mines, are situated near the Wonderfonteinspruit (WFS) catchment, which is a tributary to the Mooi River upstream of Potchefstroom. During heavy rainfalls the Mooi River receives large volumes of wastewater from the WFS, which reduces the quality of the surface water. The Mooi River in turn contaminates the Vaal River downstream of Potchefstroom. However, these rivers also receive water pollution from other sources such as the municipal sewage works, industries and agricultural land. The main objective of this study was to determine the main sources of pollutants and the extent of contribution to the contamination of the surface water in the vicinity of Potchefstroom. This objective was achieved by systematic sampling along the Mooi and Vaal Rivers. The measured physico-chemical parameters were compared with the SANS guideline for drinking and irrigation water, in order to assess the suitability of the surface water for human consumption and agricultural uses. A statistical analysis was also done in order to obtain a good knowledge of the relationship of the water quality indicators. About 44 water samples were collected seasonally (wet and dry) to be analysed for physico–chemical parameters. In addition, 9 sediment samples were selected in the dry season and assessed using a four-stage sequential extraction method. The reason for collecting the water samples seasonally was to assess the variation in chemistry between the two seasons. A pH combined electrode with an integrated temperature probe was used in situ to measure parameters such as temperature (˚C), pH, electrical conductivity (EC), dissolved oxygen (DO) and oxidation-reduction potential (ORP). At the laboratory, a COD and Multiparameter Bench Photometer HI 83099 were used to analyse the water for sulphate, nitrate and cyanide. The total content of metals was analysed using inductively coupled plasma spectroscopy (ICP-OES). Parameters such as chloride and the total alkalinity were measured through the titration method. The sediment samples were characterised using the X-ray diffraction (XRD), X-ray fluorescence (XRF) and Fourier transformed infrared (FTIR) spectroscopy to determine the mineralogical composition, major components, and functional groups, respectively. The total organic carbon was also determined in the sediments by using the Walkey Balk titration method. Results obtained from the water analyses showed high concentrations of trace metals (Ca, Mg, As, Cd, Fe, Pb, U) and major anions (SO42-, CN-, NO3- and Cl-) mostly in sampling points situated near the mining activities. A decreasing trend of concentrations was observed when moving downstream of the study area. The significant decrease observed in Potchefstroom can be explained by two factors. Firstly, it can be due to the dilution effects by the Mooi River and secondly, it could be because mining activities are much less evident in this area. However, the concentration increases again when moving downstream of Potchefstroom. The elevated concentrations recorded at these points can be attributed to the mining activities at Orkney and its vicinity or wastewater from municipal and domestic sewage. A significant decrease of pollution was observed in the dry season, suggesting that the level of pollution was mainly influenced by large effluents caused by heavy rainfall. The PHREEQC data results revealed that high percentages of Ca and Mg were present as free hydrated species, whereas the Fe, Pb, As and U concentrations were mostly present as carbonate or hydroxide species in both the wet and dry seasons, respectively. However, some percentages of Fe were also present as free hydrated species and thus likely to cause toxicity in the surface water. The sequential extraction results revealed that metals such as Ca and Mn were mostly associated with the exchangeable fraction while iron and manganese are dominantly found in the oxide fraction. Mg, Fe and Cr were mostly bound to the residual fraction. Metals associated with the exchangeable fraction show higher bioavailability and these metals are likely to increase the level of toxicity in the water, while metals associated with the residual fraction are strongly bound to the sediments and less susceptible to mobilization. The XRF and XRD results showed that the sediment samples consisted predominantly of SiO2, CaO, Fe2O3, MgO, MnO, Cr2O3 oxides and quartz minerals, respectively. FTIR results revealed that the sediments mostly contained inorganic materials such as clay and quartz, and this was substantiated by the bands at 788.40 cm-1, 690.00 cm-1, 688.33 cm-1, 629.56 cm-1 and 629.25 cm-1, which represented AA, BB, CC and DD, respectively in all the sediment samples. In conclusion, it was observed that the water quality in the studied areas was poor, rendering it unfit for drinking and irrigation purposes according to the SANS/WHO guidelines. Elevated pollution was observed in the sampling areas situated near the mining area. However, the runoffs from agricultural land and municipal sewages have an impact, especially during the wet season, as some elevated concentrations were observed in the sampling points situated near these areas.
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