2016 NGWA Groundwater Summit

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

Tuesday, April 26, 2016: 1:50 p.m.
Confluence Ballroom A (The Westin Denver Downtown)
Dawn Paszkowski, M.Sc. , BGC Engineering Inc., Vancouver, BC, Canada
Carl Mendoza, Ph.D., P.Eng., P.Geo. , BGC Engineering Inc., Vancouver, BC, Canada
Trevor Crozier, M.Eng., P.Eng. , BGC Engineering Inc., Vancouver, BC, Canada
Max Holtby, B.Sc., P.Geo. , Pretium Resources Inc., Vancouver, BC, Canada

The high-grade gold Brucejack Project, a proposed underground mine located in northwestern British Columbia, Canada is in the process of being evaluated and developed. Surface topography has a pervasive 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 metamorphic and volcanic bedrock; coupled with the subarctic climate and abundant precipitation, this leads to considerable seasonal variations in groundwater elevations. Five kilometers of historical underground workings were expanded and dewatered over a 2-3 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 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, an active approach using a fracture well and a passive approach using drain boundaries, which helped constrain hydraulic property estimates. Detailed calibration to observed flowrate and 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 estimate groundwater flow rates to the proposed underground mine and surface water receptors, predict drawdown throughout operations, and simulate recovery of the groundwater system in the closure period for base case and sensitivity scenarios.

Dawn Paszkowski, M.Sc., BGC Engineering Inc., Vancouver, BC, Canada
Dawn Paszkowski is a scientist with 8 years of experience in numerical and analytical groundwater modeling, well installation, aquifer testing, mine depressurization and dewatering, seepage control, and groundwater quality assessments. She has been involved in hard rock mine developments in British Columbia, Yukon and northern Ontario, as well as in hydrogeological aspects of northern Alberta oil sands operating mines and pipeline projects.

Carl Mendoza, Ph.D., P.Eng., P.Geo., BGC Engineering Inc., Vancouver, BC, Canada
Dr. Carl Mendoza is a Principal Hydrogeological Engineer and a Professor of physical and contaminant hydrogeology who specializes in using numerical models and field investigations to understand complex hydrogeologic systems. He has degrees in Geological Engineering, Earth Sciences and Hydrogeology, and has over 30 years of experience in developing and applying conceptual and numerical models to a wide range of non-linear hydrogeological problems.

Trevor Crozier, M.Eng., P.Eng., BGC Engineering Inc., Vancouver, BC, Canada
Mr. Crozier is a geotechnical engineer specialized in hydrogeology with over 20 years of international consulting experience. He has applied his expertise to mine water management, mine dewatering, tailings and water storage dam design and construction, slope depressurization and stabilization, environmental assessment, permitting, compliance monitoring and contaminated sites investigation, remediation and regulatory closure in northern, temperate and tropical regions.

Max Holtby, B.Sc., P.Geo., Pretium Resources Inc., Vancouver, BC, Canada
Max Holtby is Director of Permitting at Pretium Resources Inc. Pretium is aggressively advancing the development opportunity at Brucejack, its advanced-stage, high-grade gold project in northern British Columbia.