Crop response to irrigation with acidic and neutralized, saline mine water

Abstract

The cessation of mining activities and concurrent abandonment of mines in many South African mining areas has left the state with the responsibility of caring for and rehabilitating ownerless mines, which are prone to flooding and discharge acid mine drainage (AMD) to surrounding environments. In the Witwatersrand Goldfields, AMD arising from abandoned mines is currently being treated by High-Density Sludge (HDS) neutralization and released into surrounding natural water bodies as part of a short-term intervention. Long term management of the water would require the construction of more HDS plants and an additional desalination step, which is estimated to cost the state several billions of Rands in capital costs and R1 billion in annual running costs. Alternative cost-effective AMD management strategies are being considered and irrigation is one of the strategies being proposed. One of the options being suggested is to irrigate with neutralized AMD, which would eliminate the need for desalination. Another option is to irrigate with untreated AMD, which would eliminate the need to construct treatment plants altogether. There is, however, a concern that the mine waters do not comply with the 1996 South African Water Quality Guidelines for Irrigation, which are generic and outdated. Another concern is that continuous irrigation with the water may have detrimental long-term environmental effects. The South African Water Quality Guidelines for Irrigation have, however, recently been updated to a software-based, risk-based, generic and site-specific decision support system (DSS) that allows users to run long term irrigation simulations. The aim of this study was to ascertain if acidic and neutralized, saline Goldfield mine water can be used successfully for crop irrigation, using the waters from the Witwatersrand Goldfields, as water quality references. The study also aimed to investigate the role played by crop tolerance and soil type in mitigating the effects of the mine waters on selected crops. An additional aim for this study was to assess the fitness-for-use of mine water for irrigation under site-specific conditions and to investigate the potential use of the South African Water Quality Guidelines for Irrigation decision support system (SAWQI-DSS) in predicting long term effects of irrigating with acidic and saline mine water. A greenhouse pot trial was undertaken in 2015, in which several crops, selected for their tolerance/sensitivity to acidity and salinity, were planted and irrigated with municipal water as well as with synthetic acidic and neutralized saline mine waters. A red sandy soil loam with theoretically low acid neutralizing capacity, and a vertic soil with a theoretically higher acid neutralizing capacity were used as growth media. Water loss was supplemented to each pot’s individual field capacity so that each crop’s unique water requirements were met. Crops were initially watered on a weekly basis and then twice a week as crop water requirement increased. Each treatment had three replicates, set up in a completely randomized design on a rotating table. Site-specific fitness-for-use assessments of acidic and neutralized saline mine water for irrigation were performed using the SAWQI-DSS. Long term simulations (45 years) were conducted for irrigation of a salt-sensitive summer crop and a salt tolerant winter crop grown on a sandy loam or clay soil, with the application of different leaching fractions. The field chosen for the simulations received some rainfall. Most crops irrigated with the mine waters did not show signs of foliar injury. However, crops irrigated with acidic mine water did show symptoms of nutrient imbalances, phytotoxicity and drought stress. Crops grown on the red sandy loam soil, typically showed symptoms of drought and manganese toxicity, whereas those grown on the vertic soil typically showed signs of nutrient imbalances when irrigated with municipal water. Most crops irrigated with acidic mine waters accumulated phytotoxic levels of Al and Mn, and excessive levels of Fe. Accumulation of Al, Fe and Mn was lower in crops irrigated with neutralized mine water, however, crops grown on red sandy loam soil typically accumulated phytotoxic levels of these metals when irrigated with neutralized mine water. Irrigation with mine waters had no significant effects on the growth and grain yield of most crops grown on the vertic soil. On the other hand, crops grown on the red sandy loam soil had less growth and grain yield. The grain of crops grown on the red sandy loam soil and the vertic soil presented potential health risks associated with Al and Mn toxicity when irrigated with mine waters. However, the Al, Fe and Mn content of this grain did not exceed the tolerance thresholds for livestock, indicating that the grain would be suitable for use as livestock fodder. Fodder safety evaluations indicated that crops grown on the red sandy loam soil, when irrigated with mine waters, are more likely to present potential health risks associated with Al, Fe and Mn zootoxicity in livestock than those grown on the vertic soil, particularly if their foliage is consumed. Health risk assessments indicated that the grain of these crops would not be safe for consumption by individuals who are vulnerable to Al, Fe and Mn toxicity, particularly the grain of crops that are consumed in large amounts. Grain Al and Mn content of most crops was, however, below zootoxicity thresholds for livestock. The differences in crop response to irrigation with mine water, particularly the difference between crops grown on the red sandy loam soil and those grown on the vertic soil, can be attributed to the higher CEC and organic matter content of the vertic soil than in the sandy loam soil, which can buffer against the effects of acidity, salinity and render potentially toxic elements less mobile, and therefore, less available for crop uptake. The evaluations indicated that the exceedance of food safety thresholds for humans were more a result of high consumption of the crops and less a reflection of high levels of potentially toxic metals in the crops. SAWQI-DSS fitness-for use assessments predicted that long term irrigation with acidic saline mine water would have detrimental effects on root zone salinity and crop yield. Root zone salinity was predicted to be higher on the clay soil than on the sandy loam soil. The model used in the SAWQI-DSS accounts for differences in water and solute movements between soil types, as influenced by soil texture. Coarse-textured soils are typically easier to leach than fine-textured soils, hence, the model predicted that sandy loam soils would be less saline. The assessments indicated that increasing leaching fraction would have an influence on rootzone salinity, however, clay soils required a higher leaching fraction to reduce salinity than sandy loam soil. Al, Mn and Fe were predicted to reach the accumulation threshold in less than 100 years, with accumulation being more rapid in sandy loam soil. The assessments also predicted that the salt-tolerant crop would perform better than the salt-sensitive crop. SAWQI-DSS fitness-for use assessments predicted that long term irrigation with neutralized saline mine water would not have detrimental effects on root zone salinity and crop yield. Irrigation with acidic saline mine water has major cost saving implications, as it means that there would be no need to build mine water treatment plants. Irrigation with neutralized mine water is also a promising alternative for mine water management, as it would eliminate the need to construct desalinization facilities and the running costs associated with the process. This study has demonstrated that crops can be produced successfully through irrigation with mine waters, especially neutralized mine water, which showed positive results regardless of the soil type used. Further studies are, however, required to determine the effects of irrigation with such waters in field conditions for different soil types. In the case of acidic saline mine water, the influence of lime application, irrigation management (i.e. type of irrigation system, irrigation timing, etc.) and leaching on soil and crop responses are required. In the case of the neutralized mine water, the effects of leaching and irrigation management on soil and crops. Further studies to establish relevant health risk assessments with regard to the trace element content of foods, in the South African context, are also required. The SAWQI-DSS has displayed great potential for use in evaluating long-term effects of irrigating with water that would typically be considered undesirable. The option to introduce site specificity gives users the flexibility to assess the effect that alternative options, for managing the water they have available, will have when used for irrigation. The SAWQI-DSS does, however, have limitations in assessing the fitness-for-use of acidic mine waters as it does not give a comprehensive account of the effects of irrigation water acidity on soil quality, and crop yield and quality. For instance, the fitness-for-use assessments do not indicate the degree of soil acidification or foliar injury associated with the direct effects of irrigation water acidity. Furthermore, the assessments do not indicate how relative yield would be affected by trace element toxicity as it does for Na, Cl, and B. Further studies are therefore required to account for foliar injury resulting from irrigation water acidity, similar to what has been done for foliar injury resulting from irrigation water salinity. There is also a need to incorporate the effects of trace element toxicity on relative yield, similar to what has been done for Na, Cl and B. In cases whereby toxicity thresholds and yield reductions associated with trace element toxicity have not been established, experimental trials should be conducted to fill those knowledge gaps. In addition, the incorporation of health risk assessments associated with trace element accumulation in grain is required.