Prediction of Source Term Leachate Quality from Waste Rock Dumps: A Case Study from an iron Ore Deposit in Northern Sweden

摘要

The prediction of source term water quality from mine waste disposal facilities is an important aspect of the design and management of mining operations. Predictive numerical calculations have been completed for the assessment of long-term leachate chemistry emanating from a proposed Waste Rock Dump (WRD) at an iron ore deposit in Northern Sweden. The prediction required the generation of source term water quality for the WRD in terms of solute concentrations and loading, in addition to assessing the effects on water quality in the receiving watercourse adjacent to the facility. A source term was developed from the results of laboratory static and kinetic testwork carried out on drillcore samples of representative waste rock lithologies. Based on static testwork results, the WRD material consists of two broad material types: (i) Potentially Acid Forming (PAF) skarn with a sulfur content >1%, which makes up 34.3Mt of the 124Mt estimated waste, and (ii) Non-Acid Forming (NAF) material types with a sulfur content <1%, making up the remaining waste. Mass balanced Humidity Cell Test (HCT) and Net Acid Generation (NAG) test results were used to develop source terms for WRD scenarios during Life of Mine (LOM) and post-closure. Model scenarios included: (i) segregated and unsegregated waste; (ii) spring and average snow melt conditions; and (iii) the application of standard soil and qualified covers post-closure. Modelling results demonstrated that loading of metals from the WRD was greatest during spring for the LOM scenarios; when seepage through the WRD is greatest and the material is uncovered. However, due to river flow being highest during the spring months, a dilution factor is experienced and predicted metal concentrations in the adjacent watercourse are generally lower than for average flow conditions. In all cases, segregation of high sulfur material was shown to give the best results in terms of elemental load release and predicted elemental concentrations in the adjacent river.