Water quality assessment of the Koekemoerspruit : bintegrating water physico-chemistry and phytoplankton assemblages

Abstract

Midvaal Water Company, situated on the banks of the Vaal River in the North West Province, supplies potable water to the Greater Municipality of Matlosana, as well as mining and industrial undertakings in the area. Water is abstracted downstream from the confluence of the Koekemoerspruit (KMS) and Vaal River. The KMS represents an affected (miningand urbanisation associated pollution) water resource and the middle-Vaal River system acts as the receiving water body. This emphasised the need to assess the water quality of the KMS and its influence on the Vaal River. The main aim of this study was to integrate the use of phytoplankton assemblages and water physico-chemistry, hypothesising that it would provide a more accurate and comprehensive means to determine and assess water quality. The descriptive statistics revealed that nutrient enrichment and salinity, as the result of urbanisation and gold mining, contributed most to the deterioration of water quality in the KMS. Nutrient enrichment at Site 1, reflecting water quality impacts from the informal settlement of Khuma, was indicated by the high mean values for TOC (9.82 mg/ℓ), Faecal coliforms (3444.46 cfu/100mℓ), NH4 (22.80 mg/ℓ) and PO4 (3.19 mg/ℓ). Salinisation at Site 3 reflected mining impacts and was indicated by high mean values for turbidity (40.54 NTU), EC (238 mS/m), Na (258.05 mg/ℓ), Cl (182.76 mg/ℓ) and SO4 (932.95 mg/ℓ). In the Vaal River Site 6 was chosen to reflect water quality downstream of the confluence with the KMS and to show any impact on the Vaal River. Descriptive statistics revealed that trace metals contributed most to deteriorating water quality at this site. The impact of trace metals at Site 6 was indicated by the high mean values for Fe (3.39 mg/ℓ), Mn (7.29 mg/ℓ) and As (2.94 mg/ℓ) and was the result of heavy rain experienced during March 2014 that caused a sudden influx of polluted runoff from nearby tailings dumps. The phytoplankton data confirmed that Cyanophyceae dominated in the KMS, except at Site 3 (canal) that was dominated by Chlorophyceae. This confirmed that Chlorophyceae is more tolerant to saline conditions and that Cyanophyceae is influenced more by the availability of nutrients. In addition, the application of the Shannon-Wiener Diversity Index, Pielou’s Species Evenness Index, Margalef’s Species Richness Index, and Palmer’s Algal Genus Pollution Index to the phytoplankton data, concluded that the KMS is more organically polluted than the Vaal River. Chlorophyceae dominated in the Vaal River, and Cryptophyceae and Dinophyceae were only present in the Vaal River. Amongst other techniques, the data were subjected to multivariate statistical analysis. The principal component analysis (PCA) did not succeed in reducing the amount of variables, but could be used to explain variability within the data more effectively. Projections of the variables on a component-plane (that plotted the variables captured by the 1st and 3rd principal components), as well as the PCA ordination, were however successful in separating the KMS from the Vaal River, combining both phytoplankton and physicochemical data. It was also possible to assign individual sites to specific sets of variables that correlated with the descriptive statistics, clearly illustrating that the KMS was mostly impacted by nutrient enrichment (Site 1) and salinity (Site 3), and that Site 6 (after KMS) in the Vaal River was separated from the other sites by grouping it along with the trace metals. Most important, the fact that a clear distinction can be made between the KMS and Vaal River concluded that the KMS does not have a significant impact on the water quality of the Vaal River.