Determining the weathering characteristics of a waste dump with field tests

Prediction of the relative rates of acid generation and neutralization is required to determine if and when acidic drainage will occur for mining waste. Results of laboratory kinetic tests are routinely used to predict the long-term weathering rates of a waste dump. More specifically, an estimate of the ‘time to acidity’ (lag time) is needed to predict if exposed waste rock will become acidic before underwater disposal. A meaningful prediction requires consideration of the differences between the conditions in the laboratory and an actual waste dump. Field tests, which are less commonly conducted, provide site-specific weathering conditions but return results that are difficult to interpret. The results from field tests were compared to those obtained from laboratory kinetic tests. The field rates of weathering were determined to be an order of magnitude greater than the adjusted laboratory results, which were corrected for both surface area and temperature. The difference between the field and laboratory weathering rates is believed to be caused by inadequate hydrogeological assumptions and deficiencies in the experimental protocols. Insights were gained into building and operating kinetic field tests, interpreting results and understanding the limitations and advantages of field test data for prediction purposes. Field test data were found to be very useful for predicting depletion rates of neutralization potential and for validating sulphide oxidation rates obtained from laboratory tests.