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

The practices of mining and ore processing are used in many countries around the world to
\nextract and concentrate the valuable natural resources found within the rock bodies which
\nmake up the earth?s crust. However, the solid waste disposal facilities which are used to store
\nthe large volumes of waste rock and mine tailings that are produced during these processes can
\npose a risk to the quality of down-gradient waters due to the slow leaching of various dissolved
\nelements and compounds. Groundwater flow and contaminant transport modelling is an
\neffective way of understanding and investigating a site where dissolved contaminants have
\nbeen detected in the groundwater.
\nThe Northern Ontario gold mine site, which is the focus of this study, first began ore processing
\nin 1997. Since then, the groundwater and surface water bodies surrounding the site have been
\ncontinually monitored to detect any changes in water quality. Groundwater samples collected
\nadjacent to the mine?s tailings management area (TMA) have consistently detected dissolved
\ncobalt and iron at concentrations above the site?s self-appointed trigger level standards. In
\n2009 seven pumping wells, were installed in the contaminated area to restrict further spread of
\nthe plumes by capturing the contaminated groundwater and recycling it back into the TMA.
\nA numerical groundwater flow (MODFLOW-2005) and contaminant transport (MT3DMS) model
\nhas been created which accurately simulates the flow of groundwater through the site both
\nbefore and after the pumping well system was installed. The validated model was then used to
\nsimulate the fate and transport of dissolved cobalt through the subsurface of the site, and to
\nperform 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
\ndetermined to have the greatest effect on the evolution of the cobalt plume.
\nAn assessment of the pumping well system was performed which indicates the ability of the
\npumping wells to capture the dissolved cobalt plume within five years of activation. The ability
\nof the pumping wells to continue to restrain the advancement of the cobalt plume was also
\nconfirmed for a 15 year simulation period. Additional alternatives such as an intermittent
\npumping schedule, a 50% reduction in pumping rates, and the decommissioning of four out of
\nthe seven pumping wells were also confirmed to successfully restrain the cobalt plume
\nadvancement for a 15 year simulation period. A preliminary investigation into the use of a
\npermeable reactive barrier (PRB) as an alternative to the pumping wells was also performed.
\nTwo possible PRB locations were proposed, however the large width of the plume indicates
\nthat a funnel and gate system should be investigated.

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