Abstract
The main purpose of this study was to determine a simple inorganic chemistry index that can be used for all surface waters in South Africa, in order to characterise the inorganic chemistry of surface waters. Water quality data collected up until 1999 from all sample monitoring stations (2 068 monitoring stations, 364 659 samples) in South Africa were transformed into an Excel dataset and subsequently quality screened using the stoichiometric charge balance, after which 196 570 (41%) of the water analyses could be used. Using an inorganic chemistry index (ICI) a more detailed analysis can be performed. The ICI shows that the surface water chemistry in South Africa is dominated by chemical weathering, chloride salinisation and sulphate contamination. Based on the importance of these factors, primary catchment areas in South Africa can be subdivided into 3 groups, each with their own characteristic water chemistry. The results of this study allowed the development of a ternary diagram in which the 3 factors are represented. This diagram can be used to investigate the variation of the inorganic water chemistry of South African surface waters in time and space.
Highlights
Rivers provide most of the water to South African users (e.g. Day, 2010)
Maximum and minimum inorganic chemistry index (ICI) values, to distinguish weathering-dominated from non-weathering dominated rivers, were established using the dataset from Gaillardet et al (1999). This dataset comprises chemical data of 62 world rivers of which 53 are weathering-dominated, having a median ICI of 30%, and 9 are non-weathering dominated with a median ICI value of 70%. These ICI values are subsequently used as a maximum (≤30%) and minimum (≥70%) value to characterise South African surface waters to be weathering- or non-weathering dominated, respectively (Fig. 2)
Elevated chloride concentrations are primarily caused by ling factors determining South African surface water chemsaline soil and groundwater, as a result of natural and istry, a ternary diagram can be developed as a visual aid
Summary
Rivers provide most of the water to South African users (e.g. Day, 2010). Due to climate conditions water is limited and, in addition, pollution caused by urbanisation, mining, and agricultural activities makes freshwater even scarcer (e.g. Walmsley et al, 1999). After removal of the blanks and incomplete analyses, 364 358 chemical analyses remained (Table 1) The quality of this dataset was subsequently screened by calculating the stoichiometric charge balance (SCB), according to the equation (e.g., Appelo and Postma, 2005): SCB (%) = 100 × (∑ [cations] − ∑ [anions]) / (∑ [cations] + ∑ [anions]), where the concentrations (absolute values) of the cations and anions are expressed in the equivalent scale. Chemical analyses of which the stoichiometric charge balance was < −5% or > +5% (e.g., Zhu and Andersen, 2002) were removed from the dataset, after which 196 570 (41%) useful chemical analyses remained (Fig. 2, Table 1) These data were used in this study. The inorganic chemistry of South African rivers is, to a large extent, controlled by chemical weathering of rocks
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