Natural and anthropogenic influences on the water quality of the Orange River, South Africa

The natural condition of the Orange River should be evaluated in order to develop baseline information so that it can be used for comparison, monitoring and informed decision-making. This will also allow for further research to take place. Although there is a lot of data available on the Orange River, little work has been done on the evaluation of the natural conditions that influence the inorganic water chemistry Inorganic data, from 1986 to 2006, obtained from the Department of Water Affairs and Forestry (DWAF) was evaluated for six sample stations along the Orange River (D1H009Q01, D3H008Q01, D3H012Q01, D3H013Q01, D7H008Q01 and D7H005Q01). Climate data (1986-2006) was obtained from the South African Weather Service. The following water quality data was used in the evaluation: pH and the concentrations of major elements (all in mg/L). The major elements include sodium (Na ), potassium (K ), calcium (Ca ), magnesium (Mg ), silica (Si ), fluoride (F ), orthophosphate (PO ), chloride (Cl ), total alkalinity (TAL) assumed to be bicarbonate (HCO ), sulphate (SO ), nitrate (NO ) (assuming that NO >>> NO ) and the total dissolved solids (TDS). Various geochemical techniques were used to analyse the data. The results of this study show that the water chemistry of the Orange River is controlled by: 1. Chemical weathering of siliceous sediment, intrusive igneous rocks and metamorphic rocks (Na , K , Mg , Ca , HCO , F and Si ). 2. Input from agricultural and urban activities affecting, in particular, the concentrations of PO , NO , SO and Cl . There is an increase in cation and anion concentrations from 1986-2006. The concentration of cations and anions increases downstream from D1H009Q01 to D7H005Q01 i.e. from a colder wetter climate to a drier hotter climate. Based on the chemical characteristics, two groups were identified. The stations in each group include: Group 1: D1H009Q01, D3H013Q01, D3H012Q01 and D3H008Q01 and Group 2: D7H008Q01 and D7H005Q01. For group 1 the degree of pollution is generally consistent (between 10 and 30 percent). The element concentration was plotted against the total annual runoff and the visual trend shows a decrease in Na , K , Mg , Ca , HCO and F as the annual runoff increases. This is because the dilution effect is stronger than the release of cations and anions due to chemical weathering. This decrease is typical for weathering of rock types such as granites, shale and metamorphic rocks. Si shows an increase in concentration as total annual runoff increases. This indicates that feldspar is the dominant mineral that is being weathered. Chemical weathering of feldspars (specifically Na- and K-feldspars) releases more Si compared to other species. PO and SO show no visual trend and Cl and NO show a possible increase in concentration with an increase in total annual runoff. This is most likely due to greater input of these species from the surrounding agricultural and urban areas when the runoff increases. For group 2 the degree of pollution is generally higher and shows a greater fluctuation compared to group 1. The visual trend for the concentration of Na , K , Mg , Ca , Cl , SO , HCO and F shows a sharp decline at low runoff, dilution is the dominant process. The concentration of these then increases as chemical weathering becomes more dominant, the effect of chemical weathering becomes greater than the effect of dilution. After this the effect of dilution becomes greater and the species concentrations show a steady, slight decrease, similar to group 1. Si shows a sharp increase as total annual runoff increases to 5000 million m3, thereafter it decreases slightly. The increase of the Si concentration is due to chemical weathering which is stronger than the dilution effect. The increase in NO and PO is most likely due to agricultural activities and urbanisation in the immediate area. As surface runoff increases it increases transport of these chemical species into the river. Both the agricultural/urban input and the release of cations and anions by chemical weathering are severely influenced by the stream runoff. Any evaluation of the inorganic chemistry from the Orange River should include runoff. The variation of the annual runoff affects the percentage of pollution, especially for the lower Orange River.Prof. J.M. Huizeng