GEOGRAPHICAL PATTERNS, AND THEIR ORIGINS, IN THE DOMINANCE OF MAJOR IONS IN SOUTH-AFRICAN RIVERS

The proportions of ions in rivers of South Africa and Lesotho were inv estigated using Maucha diagrams, which allow a visual assessment of th e ionic proportions of any water sample. Four broad categories of ioni c proportions show clear geographical distribution patterns linked to the geological and climatological character of the country. Category 1 (dominant ions Ca2+, Mg2+ and HCO3-; Na+ < 25% of cations) is restric ted to the regions of the high-altitude basalt cap of Lesotho/KwaZulu- Natal and the Dolomite and Pretoria Series of the Northern Transvaal. Category 2 (dominant ions Ca2+ Mg2+ and HCO3-; Na+ > 25% of cations) m ostly encircles category 1 at lower altitudes; it occurs on Karoo and Waterberg sedimentary rocks and igneous rocks of the Basement Complex and the Bushveld Igneous Complex. Category 3 (Na+, Ca2+, Mg2+, NCO3-, Cl-more or less co-dominant) is widespread and not apparently associat ed with any particular geological formations. Category 4 (Na+ and Cl- dominant) occurs ill the southwestern Cape on Table Mountain Sandstone s, in the western arid regions on Karoo sediments, and in coastal Kwa- Zulu-Natal on a variety of substrata. Waters in categories 1 and 2 are 'rock dominated'; dilute waters in category 4 are 'precipitation (rai nfall) dominated' and concentrated waters in category 4 are 'evaporati on-precipitation (crystallization) dominated'. It is shown that dilute rainfall-dominated waters can become directly evaporation-dominated i n more arid areas.