Waste Rock Characterisation versus the Actual Seepage Water Quality

Proper characterisation is needed to predict the long term behaviour of mine waste material and to successfully close a waste facility. The objective of this study was to compare the performance of commonly used prediction methods with the actual seepage water quality from closed or active mine sites. This was carried out by characterising waste rock material and seepage waters from seven mine sites. The prediction methods included static tests, NAG leachate, shake flask test and hot Aqua Regia. Differences were observed between the methods predicting acid production potential (APP) and the actual acidity of the seepage waters. The study indicated that the laboratory tests were often too pessimistic in comparison with the pH values measured from the seepages. Therefore, the use of several different methods, as well as mineralogical data, is recommended for the APP prediction. According to the results of the different examined leachability tests, the hot Aqua Regia extraction had the best correspondence with the actual seepage water quality in predicting which elements will be present in the effluents. In general, performance of the NAG test leachate was reasonable, but it underestimated the metal concentrations when NAG leachate pH was higher than around 4. The shake flask test was observed to be the most unsuitable for the effluent quality prediction. The results obtained from the Aqua Regia extraction and NAG leachate can be used to predict the elements that will appear as elevated concentrations in the effluents, considering that the concentrations are only approximate, not exact.