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Store-and-release (SAR) covers are often installed over sulphidic mine waste to limit the generation and transport of acidic and metalliferous drainage by reducing water percolation to these wastes. SAR covers act to reduce both the net volume and the frequency of percolation events by buffering the underlying waste from smaller rainfall events that would have otherwise penetrated to the waste and by enhancing evapotranspiration of stored pore-water. While water content has been shown to significantly impact the rate of sulphide oxidation in numerous studies, there is currently no established relationship between percolation volume and acidity load. This project investigates whether reducing incident water percolation with a SAR cover simply results in more-concentrated deep drainage containing a comparable acidity load, with no net benefit from SAR installation.
The relationship between percolation volume and acidity load was tested by leaching three columns of a composite marcasite:quartz material (1:300) weekly with deionised water to achieve regular drainage via vacuum extraction equivalent to pore volumes of 0.3, 0.5 and 0.7. A fourth column was run at the mid-range drainage rate but leached fortnightly to investigate the impact of varying percolation event frequency. Vacuum extraction of pore water enabled precise regulation of column water content to ensure comparable oxidation conditions between the four columns in the inter-leach period. Normal operation of columns ceased after 11 weeks. The twelfth and final week consisted of a large flush event for all four columns, where the equivalent of 1.05 pore volumes of drainage was extracted.
Drainage after the first percolation event stabilised around pH 2.5 for all columns. Analysis of drainage quality found that a 60% reduction in drainage volume over the 11 weeks of normal operation resulted in a 13% decrease in cumulative acidity load and a 30% reduction in cumulative sulphate (SO4) load. The primary mechanism for this reduction in solute load with reduced percolation volume was the capacity of discrete infiltration events to flush oxidation products and expose fresh sulphide surfaces to reaction. Altering the percolation event frequency from weekly to fortnightly resulted in 47% and 41% reductions in realised acidity and SO4 loads, respectively, indicating that changes to the frequency of percolation events had a stronger impact on acidity and SO4 load than the net volume of discrete percolation events. Acidity loads in the final high volume flush event were 1.1–2.8 times greater than the previous week, illustrating that significant generated acidity was not mobilised at previous lower percolation volumes.
Results from this study illustrate that solely decreasing the volume or frequency of discrete percolation events may reduce the net acidity load experienced in deep drainage. SAR covers act to enhance both of these factors whilst also reducing material water content, with these three factors all acting in concert capable of significantly lowering the acidity load reporting to waste pile deep drainage.
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