Abstracts

310 - Assessing the Impact of a Mine Tailings Rehabilitation
Strategy on Nearby Stream and Groundwater Flows
S.D. Donald, R.G. McLaren, M. Gunsinger, F. Junqueira & B. Reiha
Golder Associates Ltd., Cambridge, Ontario, Canada
An alternatives assessment was completed to evaluate feasible rehabilitation strategies to
manage the potential risk posed by gas emissions from a historic tailings facility at a decommissioned
mine site. The preferred solution that was selected involves the construction
of a cover over the surface of the tailings, which provides a barrier of saturated material
that would significantly reduce gas migration to the surface. While the cover provides the
benefit of mitigating gas emissions, a potential concern with this strategy was that the cover
will be constructed with low-permeable soil materials that may promote surface runoff
and reduce groundwater recharge. Potential effects in turn could be a lowering of the water
table, desaturation of peat layers that underlie the tailings, and the possible oxidation and
re-mobilization of currently adsorbed waste products within the peat layer. There were
also concerns with respect to the influence of this reduced recharge on the groundwater /
surface water interactions of a stream channel immediately adjacent to the tailings facility.
In order to assess the impacts that the cover system may have on the local flow system, a
3-dimensional, integrated, fully-coupled, surface/subsurface numerical modelling approach
was utilized. An existing calibrated subsurface flow model was used as a starting point. A preliminary
base case steady-state simulation with average annual surplus water was developed
for the existing site conditions (no cover). Subsequent simulations with average monthly
surplus water were then carried out for the base case and two cover system scenarios with
reduced infiltration. Time-varying infiltration rates for the tailings facility (covered and uncovered)
where derived from a separate 1D numerical flow model and applied as a specified
flux boundary condition in the tailings on the top surface of the 3D model.
Modelling results indicate that the placement of the cover would have minimal impact on
the elevation of the water table and the saturation of the peat layer underlying the tailings,
and the surface water flow magnitude in an adjacent stream. However, the model results
did suggest that there could be significant impacts on the spatial location and temporal
variation of zones where the adjacent stream was ‘gaining’ or ‘losing’ water to and from the
subsurface. While not the case for this particular site, this could have a negative impact on
watercourses where the aquatic systems rely on these relationships. In this particular application,
the advantages of the integrated modelling approach, over traditional uncoupled or
loosely-coupled approaches, becomes evident. With the fully integrated approach, subtle
changes in the surface/subsurface flow relationships are directly visible.
IAH-CNC 2015 WATERLOO CONFERENCE
163