Modelling groundwater flow and contaminant transport at a gold mine site in Northern Ontario

Abstract

The practices of mining and ore processing are used in many countries around the world to extract and concentrate the valuable natural resources found within the rock bodies which make up the earth’s crust. However, the solid waste disposal facilities which are used to store the large volumes of waste rock and mine tailings that are produced during these processes can pose a risk to the quality of down-gradient waters due to the slow leaching of various dissolved elements and compounds. Groundwater flow and contaminant transport modelling is an effective way of understanding and investigating a site where dissolved contaminants have been detected in the groundwater. The Northern Ontario gold mine site, which is the focus of this study, first began ore processing in 1997. Since then, the groundwater and surface water bodies surrounding the site have been continually monitored to detect any changes in water quality. Groundwater samples collected adjacent to the mine’s tailings management area (TMA) have consistently detected dissolved cobalt and iron at concentrations above the site’s self-appointed trigger level standards. In 2009 seven pumping wells, were installed in the contaminated area to restrict further spread of the plumes by capturing the contaminated groundwater and recycling it back into the TMA. A numerical groundwater flow (MODFLOW-2005) and contaminant transport (MT3DMS) model has been created which accurately simulates the flow of groundwater through the site both before and after the pumping well system was installed. The validated model was then used to simulate the fate and transport of dissolved cobalt through the subsurface of the site, and to perform a sensitivity analysis on the input parameters. The magnitude of the dispersion parameters and amount of sorption in the northern portion of the sand aquifer were determined to have the greatest effect on the evolution of the cobalt plume. An assessment of the pumping well system was performed which indicates the ability of the pumping wells to capture the dissolved cobalt plume within five years of activation. The ability of the pumping wells to continue to restrain the advancement of the cobalt plume was also confirmed for a 15 year simulation period. Additional alternatives such as an intermittent pumping schedule, a 50% reduction in pumping rates, and the decommissioning of four out of the seven pumping wells were also confirmed to successfully restrain the cobalt plume advancement for a 15 year simulation period. A preliminary investigation into the use of a permeable reactive barrier (PRB) as an alternative to the pumping wells was also performed. Two possible PRB locations were proposed, however the large width of the plume indicates that a funnel and gate system should be investigated.