The impact of irrigation with coal mine water on groundwater resources

English: Huge volumes of mine water, impacted on by acid mine drainage, are presently produced as a result of mining activities. These waters are mostly neutralised either as a result of seepage through neutralising geological strata, or artificially by the addition of lime. Mine drainage is, therefore, often saturated with gypsum. Gypsiferous mine water can thus be regarded either as one of the greatest problems associated with mining, or as a potential asset. Large amounts of waste water could be made available to the farming community, and utilised for the irrigation of highly productive soils in the coalfields. Concentrating the gypsiferous soil solution through evapotranspiration, thereby precipitating gypsum in the soil profile, will reduce environmental pollution, as these salts are removed from the water system. However, the regulatory authorities have raised some concerns regarding the possible impact of largerscale irrigation on water resources. This research attempts to provide meaningful answers. Five different irrigation sites were selected for research. Monitoring indicates that very little salinity applied on surface during irrigation reaches the water table. Soil analysis indicates that most of the salts are captured in the upper one or two meters of the soil profile. The data indicate that the clay layers, which playa major role in the vertical flux of the water, also have an influence on the salt distribution through the soil profiles. The porous cups data suggest that the majority of the salts is currently contained within the uppermost portions of the soil profile, showing a steady decrease in sulphate concentrations with depth. Geochemical modelling indicates that soil water in the uppermost meter is saturated with gypsum, and precipitation occurs. Virgin solts: The chloride values at all the different irrigation sites showed a decrease value with depth, indicating bimodal flow. The saline water moving through the soil profile contributes approximately 15% or less to the mixing process of the water. Lateral flow in the highly conductive F zones on the contact of the weathered zone with the solid rock, results in salinity dilution. Twenty-five percent of the total irrigation water volume reaches the water level. There is a direct correlation between the clay content of the soil and the diffusive component of the bimodal flow. Due to the moist conditions at irrigation sites, swelling of the clay results in the very low vertical conductivity value of 10-4 mid. The average estimated moisture content with depth is available on the full text document. The salt balance revealed that 80% of the sulphate applied during irrigation is retained in the soil profile, most of which occurs in the upper two meters. 80% of this retained sulphate occurs in the soil water. After irrigation ceases, it can take more than 100 years to leach the sulphate from the soil profile, due to bimodal flow and low recharge. Conclusion: In the short to medium term, irrigation is feasible on virgin soils, as most of the salinity is captured, and little is released in the groundwater. Spoils: The tests and models indicate the great importance of the reactive nature of the spoils on which irrigation will occur. Where mine water of relatively consistent quality is used, the changes in longer-term quality will be far less dramatic. Based on the slow salinity build-up at the different sites, it is recommended that irrigation be alternated (i.e. between two pivots over time) if site criteria selection has been adhered to, and that mine-water irrigation should not be done for periods exceeding 10 years in any particular area. A non-negotiable prerequisite is that appropriate monitoring must be in place.