Mineralisation Problems in the PWV Complex – The Identification of Viable Solutions by Means of a Suite of Mathematical Models

Abstract

The Pretoria-Witwatersrand-Vereeniging (PWV) complex, the most economically active region in southern Africa, depends for its water supply upon the Vaal river, which serves also as the sink for much of the pollution emanating from the complex. Over the past 45 years, the annual increase in water demand by the urban consumers has exceeded 7%, and the current rate of growth shows no sign of diminishing. The region is not richly endowed with waer resources and already considerable quantities of water have to be transferred into the Vaal basin from the adjacent Tugela basin. Mineral pollutant loads, both from point and from diffuse sources, have risen to intolerable levels. With the onset of the rainy season, there is a pronounced “first flush” phenomenon, whereby salts accumulated in the catchment are washed off, causing unacceptably high TDS peaks in the primary water supply source. The problem is compounded by the feed-back effect whereby effluents are recycled into supply with consequent build- up of salinity. Without some form of numerical model it would not have been reasonably possible to plan measures to improve the position. A suite of numerical models having meteorological data as basic input has been developed. The models simulate at key points in the system, for any demand horizon, monthly or daily salt concentrations associated with a wide variety of alternative management strategies. The models have been successfully calibrated and verified. Incorporated in the models are the economic parameters relating salinity in the water supply to the costs incurred by consumers in having to use water of poor quality. A method has been evolved whereby the costs and benefits to consumers resulting from various water resources and pollution control planning and management options can be objectively compared. Both present-day and probable levels of future pollution and of demand are taken into account in the evaluation of options.