What circumstances can affect the pH levels and, in turn, the water quality in South Africa?
Pictured: acid mine water from a coal mine in Mpumalanga.
4 Key Factors that affect pH in Water:
1. The concentration of carbon dioxide in the water
Carbon dioxide (CO2) enters a water body from a variety of sources, including the atmosphere, runoff from land, release from bacteria in the water and respiration by aquatic organisms. This dissolved CO2 forms a weak acid. Natural, unpolluted rainwater can be as acidic as pH 5.6, because it absorbs CO2 as it falls through the air. Because plants take in CO2 during the day and release it during the night, pH levels in water can change from daytime to night.
2. Geology and soils of the watershed
Acidic and alkaline compounds can be released into water from different types of rock and soil. When calcite (CaCO3) is present, carbonates can be released, increasing the alkalinity of the water, which raises the pH. When sulfide minerals, such as pyrite are present, water and oxygen interact with the minerals to form sulfuric acid. This can significantly drop the pH of water. Drainage from forests and marshes is often slightly acidic, due to the presence of organic acids produced by decaying vegetation.
Map of the soil pH of South African soils
3. Drainage from mine sites
Mining for gold, silver, and other metals often involves the removal of sulfide minerals buried in the ground. When water flows over or through sulfidic waste rock or tailings exposed at a mine site, this water can become acidic from the formation of sulfuric acid. In the absence of buffering material, such as calcareous rocks, streams that receive drainage from mine sites have low pH levels – acid mine water.
4. Air Pollution
Air pollution from car exhaust and power plant emissions increases the concentrations of nitrogen oxides and sulfur dioxide in the air. These pollutants can travel far from their place of origin, and react in the atmosphere to form nitric acid and sulfuric acid. These acids can affect the pH of streams by combining with moisture in the air and falling to earth as acid rain or snow.
pH levels in water catchment areas around South Africa
According to the National Water Resource Quality Status Report – First Edition June 2002, pH values vary across South Africa.
- pH was low in the Klip Spruit (of the Olifants WMA) and would likely result in irritation of the mucous membranes of water users in this area. It is likely that the source of the low pH is the acid mine drainage from the coal mines and mine dumps in the area. A notable effect of the low pH would be “burning eyes” with the use of the water for recreational purposes.
- The Thukela WMA had high pH values, with the Upper and Middle Vaal WMAs having high EC values. The South Western Cape (Breede and Berg WMAs) had low pH values evident in some cases and elevated SAR, EC and Cl concentrations, again limiting the potential for growing salt sensitive crops.
- From an irrigated agriculture use perspective, there were high pH levels in the Luvuvhu and Letaba, Crocodile West and Marico, Olifants, Usuntu to Mhlatuze, Mzimvubu to Keiskamma, Upper Orange and lower Orange WMAs.
- The Fish to Tsitsikamma, Gouritz and South Western Cape (Breede and Berg) WMAs had low pH values, making irrigated agriculture in these areas more challenging and limiting crop selection to more salt tolerant crops.
Updated August 2019.