Modelling the long-term effect of irrigation with gypsiferous water on soil and water resources

The use of gypsiferous mine water for irrigation of agricultural crops is a promising technology that could solve problems related to both shortage of irrigation water and disposal of effluent mine drainage. The long-term effect of irrigation with lime-treated acid mine drainage on soil properties and catchment salt load was investigated. The soil water-salt balance-crop growth model (SWB) and the CLIMGEN weather data generator were used to simulate 30 years of irrigation with gypsiferous mine water for different irrigation management scenarios, followed by 20 years of dry land summer cropping, to determine if the problem of salt disposal was merely being postponed. Generated weather input data were for Bethal (South Africa), soil input data for a Plinthic Ferralsol (FAO-UNESCO) and crop input data for a rotation of pearl millet (Pennisetum glaucum cv. SA Standard) and oats (Avena sativa L. cv. Overberg). The soil appeared to act as an effective salt sink, with large quantities of calcium sulphate (340-404 Mg ha(-1)) being precipitated in 30 years, and with negligible amounts of remobilization thereafter. Dissolved salts in the soil solution increased during winter when rainfall is negligible. The highest concentrations occurred deeper in the profile, which is desirable because rooting is less dense there and plant growth is thus less affected. Due to large amounts of gypsum precipitation near the surface, the ions in solution were mainly Mg(2+) and SO4(2-). Simulated root weighed, saturation electrical conductivities indicated that summer cropping should present no problems, but for certain winter crops a leaching fraction should be applied to obtain maximum yields. Drainage water quality was variable, depending heavily on rainfall. Peak salt levels were around 9.6 g L(-1). Depending on irrigation strategy, between 418 and 636 Mg ha(-1) of salts was leached over 30 years. Once irrigation ceased, very little leaching occurred. Annual irrigation depended on rainfall, but averaged at about 1100 mm per annum. About 350 mm of percolation occurred, resulting in net utilization of around 750 mm annually. About 40% of the salts added through irrigation could be immobilized in the soil profile. It is concluded that year round, high frequency irrigation, with a leaching fraction in winter should be an effective and economical means of utilizing large quantities of gypsiferous water without causing irreparable damage to soil resources.