An evaluation of methods for acid rock drainage prediction

Acid rock drainage (ARD) represents a significant environmental concern arising from the excavation of sulfide-bearing rocks, which, upon exposure to atmospheric conditions, catalyzes sulfide oxidation, leading to the release of sulfuric acid and heavy metals. The reactions known to cause low pH, coupled with the leaching of specifically elevated aluminum concentrations in the Agder region, have been observed to significantly impact the water quality and contribute to substantial losses in species diversity. Preventing such ARD events necessitates reliable prediction methods to ensure proper waste management. However, the currently applied guidelines in the Agder region in Norway exhibit inconsistencies and conflicting classifications among the parameters utilized to characterize the samples. A robust test must be developed to assess ARD characteristics accurately. Consequently, a comprehensive evaluation encompassing the limitations, reliability, and applicability of diverse methods and guidelines for predicting ARD characteristics and the long-term leaching behavior of the samples was conducted. Subsequently, the most reliable method was identified based on these assessments and corresponding test outcomes. The long-term leaching behavior of the rock samples was investigated through column experiments and water chemistry analyses to assess the potential for acid generation, buffering capacities, and ion leaching. The column tests revealed acid-generating characteristics in three columns, whereas two columns exhibited non-acid forming potential. Mineralogical analyses of thin sections were conducted to predict ARD characteristics through the textural parameters suggested by the ARD Index (ARDI). Concurrently, the applicability of these parameters was also assessed for the characterization of the acid potential. The parameters yielded accurate characterizations when compared to the leaching experiment results from current and prior research as references for field weathering behavior. Five rock samples were classified as acid-generating, while three were categorized as non-acid forming. Furthermore, the mineralogy was also evaluated through XRD. However, the results were associated with uncertainties due to the high detection limit and the low concentrations of both acid and neutralizing minerals within the samples. In particular, the XRD overestimated the sulfide content, especially in samples determined as non-acid forming. Simultaneously, the carbonate neutralization potential could not be determined using lithogeochemistry due to calcium also being bound within host rock minerals. Nevertheless, whole rock analysis can provide insights into elemental concentrations that may be of environmental concern during an ARD event. Furthermore, three guidelines, NS-EN 15875:2011, the Agder guidelines, and the Single Addition NAG test, were compared against the mineralogical approach utilizing ARDI parameters and the results from the column leaching experiments. The NS-EN 15875:2011 identified all samples containing sulfur as acid-generating, primarily due to the low neutralization potential from carbonates. Additionally, the application of the Agder guidelines resulted in several conflicting classifications among replicates, with the method being highly dependent on the prediction of accurate sulfur content due to the structure of the guidelines. Finally, the interpretations and results of the Single Addition NAG test provided the most reproducible, reliable, and accurate classification of the rock samples while simultaneously revealing conflicting classifications compared to acid and base calculations. Uncertain classified samples could be further characterized by thin-section analysis. Consequently, the Single Addition NAG test was determined to most accurately predict the ARD potential, combined with textural ARDI parameters to confirm the predicted ARD characteristics.