Modeling Irrigation with Gypsiferous Mine Water: A Case Study in Botswana

Abstract.

The possible use of mine effluent for irrigation of agricultural crops was investigated for BCL Ltd. nickel mine (Selebi Phikwe, Botswana). The aims were to estimate the long-term water and salt balance under irrigation with gypsiferous mine water, as well as to make recommendations on cropping and management options. A simple but mechanistic edynamic soil water-salt balance generic crop growth model (SWB, Soil Water Balance) and a weather data generator (CLIMGEN) were used for long-term predictions. Eleven years of irrigation with BCL effluent were simulated for different cropping systems and irrigation management practices, followed by 39 years of irrigation with freshwater or dry land cropping. The long-term simulations indicated that between 661 and 2017 mm of mine effluent could be used annually through irrigation, depending on crop choice and irrigation management. Between 18% and 32% ot the total amount of salt added through irrigation was predicted to leach in 11 years, the remainder being precipitated in the 1.1 m deep soil profile in the form of gypsum. The rate of gypsum dissolution and leaching after 11 years of irrigation with mine water depended on management. Simulated root density-weighted electrical conductivities at saturation indicated favorable conditions for growth of summer crops, but a leaching fraction is recommended in winter. A lucerne perennial pasture was the most suitable cropping system from the point of view of mine water utilization and salt precipitation in the soil profile. Deficit irrigation (room for rain) followed by dry land cropping was the most suitable management strategy in order to limit salt leaching and possible ground water contamination. The acceptability of the proposed strategies will depend on Botswana's standards for environmental protection, as well as social and economic issues.