Adit Dewatering at a Proposed Gold Mine: Numerical Analysis of a Large-Scale Long-Term Pumping Test

摘要

The high-grade gold Brucejack Project, an underground mine located in northwestern British Columbia, Canada, is in the process of being developed. Surface topography has a dominant influence on the groundwater flow system at the mountainous, remote site, which is bounded by temperate glaciers. The complex hydrostratigraphy comprises thin, discontinuous unconsolidated deposits underlain by fractured and faulted sedimentary, metamorphic and volcanic bedrock; coupled with the subarctic climate and abundant precipitation, this geologic setting leads to considerable seasonal variation in groundwater elevations.As part of site exploration and evaluation, five kilometres of historical underground workings were dewatered and expanded over a three year period. The resulting field observations provided a dataset not typically available at such an early stage of project development - essentially a large-scale, long-term, variable-rate pumping test.Groundwater monitoring and numerical flow modeling were completed in support of regulatory approvals and engineering design. The numerical model was developed using MODFLOW-Surfact and was calibrated in stages to available data, including seasonal hydraulic heads, vertical hydraulic head gradients, streamflow and winter low-flow estimates, and volumetric dewatering flowrates. Two distinct approaches were used to represent the underground workings in the model; a passive approach using drain boundaries, which helped to constrain estimates of hydraulic properties, and an active approach using a fracture well. Detailed calibration to observed dewatering flowrate data and benchmarking against hydraulic head response data suggested that the bulk hydrogeological properties of this complex system were well characterized and suitable for further analyses.The calibrated groundwater flow model was subsequently used to: 1) estimate groundwater inflow rates to the proposed underground mine throughout mining operations for the calibrated, base case scenario as well as a range of sensitivity scenarios; 2) simulate groundwater discharge rates and flow paths to possible surface water receptors under pre-disturbance conditions, throughout mining operations, at closure, and in the post-closure period; and, 3) predict drawdown throughout mining operations, recovery of the groundwater system in the closure period, and steady-state water table configuration post-closure.