Coverage of data on TDS is extensive. Figure 3.17 indicates that major problem areas, where TDS exceeds 450 mgR^-1, are the Western Cape Rivers, the Eastern Cape rivers, northern KwaZulu/Natal and the Vaal River. High TDS levels in the Cape are probably naturally occurring and no cause for alarm.

Coverage for nitrate is also extensive (1995 and 1996 figures). Figure 3.18 indicates that nitrogen pollution is currently at too low concentrations to be a problem in South African surface waters.

Figure 3.19 shows total phosphorus (TP) for 1985 and 1986, as well as 1995 and 1996. The greater sampling frequency in 1985/86 may be due to the introduction at that time of the 1 mg l^-1 phosphorus standard. The number of stations sampled for (TP) has dropped dramatically in the 1990s. Nonetheless, the areas that are sampled seem to have similar characteristics as in 1985/86.

Because of the size of the Orange River catchment, conditions along this river may be used as an indicator of water quality for about 50 % of the area of South Africa. A longitudinal section of the river from the headwaters of the Vaal to the Orange River mouth shows the spatial variability of TDS, total phosphorus and nitrate Figure 3.20a-c.

Little information is available on the extent of groundwater pollution in South Africa, particularly with regard to microbiological quality. Salinisation is recognised as a threat (Figure 3.22), particularly in the eastern and southern Cape and the dry, western section of the Northern Cape, and the Northern Province. Nitrate risk areas (nitrate concentration >10mgl^-1 as Nin >20% of samples) include the Northern Cape, and parts of the Northern Province, while fluoride risk areas (fluoride concentration >1.5mgl^-1 as F in >20% of samples) are the western section of the Northern Cape, the Eastern Cape and Mpumalanga.

Salinisation

Salinity refers to the total dissolved inorganic compounds in the water and is measured by total dissolved solids (TDS). Contributions to salinity due to human intervention includes: discharge of municipal and industrial effluent; irrigation return water; urban storm-water runoff; surface mobilisation of pollutants from mining and industrial operations and seepage from waste disposal sites, mining and industrial operations. Effects of increased salinity include salinisation of irrigation soils; reduction in crop yields; increased scale formation and corrosion in domestic and industrial water conveyance systems, increased requirement for pre-treatment of selected industrial water uses, and changes in biota.

Enrichment by plant nutrients

Enrichment by plant nutrients

The most essential nutrients required by plants are nitrogen and phosphorus in various forms (NO₂, NO₃, NH₄, PO₄, orthophosphates). The accumulation of nutrient compounds in excess of natural requirements results in nutrient enrichment eutrophication and may impact on the composition and functioning of the natural aquatic biota. The direct impact is the excessive algal and macrophyte growth leading to impacts on the attractiveness for recreation and sporting activities; the presence of toxic metabolites; the presence of taste- and odour-causing compounds, and difficulty in treating the water for potable and industrial use.

Microbiological quality

Microbiological quality is the presence of disease causing micro-organisms and parasites. Recreational and potable users of water rely on access to safe water. Theirs is a major concern in the national drive to provide reliable and safe sources of water to all. Human settlements and overloaded sewage systems are the major source of deteriorating microbiological water quality. Micro-organisms and parasites may enter the water system in partially-treated sewage effluents, seepage and run-off from inadequate sanitation and waste disposal.

Sedimentation and silt migration

The settlement and silt load carried by streams and rivers typically reflects the natural geophysical and hydrological characteristics of the upstream catchment. Many South African rivers carry a naturally high suspended-solids load. This has been further increased through construction activities; poor agriculture and silviculture practices; over-grazing; destruction of the riparian vegetation, and the physical disturbance of land by industry and urban development (see Terrestrial Ecosystems section). High suspended-solids loads decrease light penetration of water; change natural productivity; affect the natural balance of predators and prey in biotic communities; smother habitats and organisms and change the viability of riverine vegetation. Additionally, high sediment loads are captured in impoundments, reducing their storage capacity.

Acidification

Industrial effluents

The pH of natural waters is determined largely by geological and atmospheric influences. Freshwater resources in South Africa are relatively-well buffered. However, human-induced acidification, from industrial effluents, mine drainage and acid precipitation (see Climate and Atmospheric Change section), can cause a lowering of the pH, leading to mobilisation of elements such as iron, aluminium, cadmium, cobalt, copper, mercury, manganese, nickel, lead and zinc. This may impact the biota, as well as mining, domestic, industrial and agricultural users.

Other water quality components that may become problems in the future, or are localised in extent are: acidity, trace elements, oxygen-consuming compounds and thermal pollution.