An investigation was completed to assess the benefits of encapsulation of acid generating tailings by acid-consuming tailings as a strategy for water quality mitigation, including acid neutralization. The focus of the investigation was a column test that assessed the effects of different source materials as well as different thicknesses utilized for the bottom layer of tailings. Geochemical modelling was performed to evaluate the attenuation of arsenic that was observed in the column studies. Pyrite oxidation modelling was also completed to provide guidance on the thickness of the overlying layer that was required to prevent further oxidation of the acidic high-sulphur tailings.

The column tests were designed to simulate the placement of low pH acidic tailings sandwiched between layers of neutral tailings with excess neutralizing potential under different arrangements and layer thicknesses. Water was added to the top of tailings and allowed to migrate through the tailings. Samples were collected from the drainage outlet at the bottom of the column as well as from suction lysimeters that were placed within the individual tailings layers to sample in-situ porewater.

The results of the test confirmed that the neutral layers could consume the existing acidity in the low pH tailings and prevent acidic and metal-laden drainage on a time and spatial scale that is appropriate for field conditions. It was shown that the thickness of the high NP-containing tailings is important in improving efficiency of the technique. Better efficiency was observed with a greater thickness of and longer residence time for pore water in the underlying neutral layer and this efficiency is expected to improve at the field scale. Acid and metal loadings were effectively decreased when the acid-consuming tailings were placed under the acidic tailings.

Modelling of arsenic attenuation showed that the effective removal of arsenic was very sensitive to the amount of ferric oxyhydroxide material in the bottom neutral tailings layer. Although arsenic is readily attenuated, there are conditions for which increasing arsenic concentrations in the effluent or seepage in the field may only be delayed rather than eliminated. However, with sufficient sorption capacity on the ferric oxyhydroxide solids, the elevated arsenic concentrations may be sufficiently reduced to levels below protective environmental guidelines. The modelling results provide guidance to develop a monitoring program for the field scale trial.

The results of pyrite oxidation modelling in the top neutral tailings layer suggest that the residual sulphide content of the tailings will act as an oxygen scavenger while the neutralization potential prevents acid generation in that layer. The combination of oxygen consumption in the top layer and the resistance to oxygen transport from moist tailings act to limit further oxidation in the underlying acidic high-sulphur layer. The modelling results suggest that a 3 m thick layer of the neutral tailings with a low sulphide content would be sufficient to prevent ongoing acid generation in the high sulphur layer. After the sulphide is depleted in the cover layer of T1F material, the oxygen consumption in the top layer will cease and that layer will only act as a diffusion barrier to oxygen. Once the sulphide is depleted, and perhaps before it is entirely depleted in the top cover layer, there will be a very small flux of oxygen into the high sulphide layer that will contribute to ongoing oxidation at a rate that is limited by the oxygen flux. This oxidation will contribute to the acid generating reactions so there will be an ongoing source of weak acidic porewater infiltrating through the high sulphide layer and into the underlying neutral layer where the water will be neutralized. This will be a process that will occur over centuries or millennia. Mass balance calculations suggest that there is sufficient carbonate NP in the underlying layer to consume all of the acid produced in the overlying layer.

The pyrite oxidation model also provides a framework for a monitoring strategy to assess the effectiveness of the low-sulphur neutral tailings cover layer.