Abstract: The reclamation of former mining sites is a major challenge in many parts of the world. In relation to the restoration of spoil heaps (mine waste rock piles) and similar bodies of opencast backfill, key challenges include (i) the establishment of stable slopes and minimization of other geotechnical hazards (ii) developing and maintaining a healthy vegetative cover (iii) managing the hydrological behaviour of the restored ground. Significant advances have been made over the past four decades in relation to all four of these objectives. One of the most recalcitrant problems is the ongoing generation and release of acidic leachates, which typically emerge at the toes of (otherwise restored) spoil heaps in the form of springs and seepage areas. Such features are testament to the presence of a “perched” groundwater circulation system within the spoil, and their acidity reflects the continued penetration of oxygen to zones within the heaps which contain reactive pyrite (and other iron sulphide minerals). Two obvious strategies for dealing with this problem are disruption of the perched groundwater system and/or exclusion of oxygen entry. These strategies are now being pursued with considerable success where spoil is being reclaimed for the first time, by the installation of two types of physical barrier (dry covers and water covers). However, where a spoil heap has already been revegetated some decades ago, the destruction of an established sward or woodland in order to retro-fit a dry cover or water cover is rarely an attractive option for dealing with the “secondary dereliction” represented by ongoing toe seepages of acidic leachates. More attractive by far are passive treatment techniques, in which the polluted water is forced to flow through reactive media which serve to neutralize its acidity and remove toxic metals from solution. A brief historical review of the development of such systems reveals a general progression from using limestone as the key neutralizing agent, through a combined use of limestone and compost, to systems in which almost all of the neutralization is achieved by means of bacterial sulphate reduction in the saturated compost media of subsurface-flow bioreactors. In almost all cases, these passive treatment systems include an aerobic, surface flow wetland as the final “polishing” step in the treatment process. Such wetlands combine treatment functions (efficient removal of metals from the now-neutralized waters down to low residual concentrations, and re-oxygenating the water prior to discharge to receiving watercourses) with amenity value (attractive areas for recreational walking, bird-watching etc) and ecological value.

Abstract: The oxidation of sulfide minerals at an ore transfer location in Western Canada has resulted in widespread contamination of underlying soil and groundwater. The oxidation of sulfide minerals has released sulfate [SO4] and heavy metals including cadmium [Cd], copper [Cu], nickel [Ni], lead [Pb], and zinc [Zn] into the groundwater. A compost-based sulfate-reducing reactive wall was installed in the path of the plume in an attempt to reduce the potential impact of the heavy metals on a down-gradient marine inlet. Monitoring of the reactive wall over a 21-month period has shown that Cu concentrations decrease from over 4000 mug/L to less than 5 mug/L. Cadmium, Ni, Pb, and Zn concentrations also show similar decreases with treated concentrations generally being observed near or below detection limits.

Abstract: The Kidd Creek Cu-Zn sulphide mine is located near Timmins, Ontario. Mill tailings are thickened and deposited as a thickened slurry in a circular, conical-shaped pile with an area of approximately 1200 ha. Deposition of tailings as a thickened slurry results in a relatively uniform grain-size distribution and hydraulic conductivity, and a thick tension-saturated zone above the water table. The tailings are drained by numerous small, ephemeral stream channels, which have developed in a radial pattern. During storms, water from these streams collects in catchment ponds where it is held before treatment. The contribution of tailings pore water to the run off is of interest because of the potential for discharge of pore water containing high concentrations of Fe(II)-acidity, metals and SO4 to the stream. Hydraulic head measurements, measurements of water-table elevation and groundwater how modelling were conducted to determine the mechanisms responsible for tailings pore water entering the surface streams. Chemical hydrograph separation of storm run off in one of these streams, during three rainfall events, using Na and Cl as conservative tracers, indicates that the integrated tailings pore water fraction makes up between less than 1% and 20% of the total hydrograph. This range is less than the maximum fraction of tailings pore water of 22-65% reported for run off from a conventional tailings deposit. At this site, preferential flow through permeable fractures may be the dominant mechanism causing discharge of tailings pore water to storm run off. Estimates of the mass of Fe(II) that discharges to the surface run off from the pore water range up to 2800 mg s(-1) during a moderate intensity, long duration rainfall event. The greatest potential for discharge of significant masses of solutes derived from the pore water exists during long duration rainfall events, when the water table rises to the surface over large areas of the tailings impoundment.