This report documents the results of the first part of a research project jointly funded by Noranda Inc. and the Mine Environment Neutral Drainage (MEND) program. The second part is covered in a separate report entitled Hydrology and Solute Transport in Oxidised Waste Rock from Stratmat Site, N.B., published concurrently with this report (MEND 2.36.2b). The overall objective of the project was to understand the geochemical and hydrological interactions between the partially oxidised waste rock and water and to improve our capabilities and techniques in the prediction of acidic drainage from waste rock piles.

Partially oxidised waste rock was sampled from the Stratmat pile at Heath Steele Division of Noranda Inc. by grabbing, trenching, and bulk excavation techniques. The samples were physically and geochemically characterised in the laboratory whereas the bulk density was measured in the field. The trenched samples were used in column dissolution tests in which 25-kg composite sub-samples were subjected to repeated washing with water to observe the water quality evolution over time. The resulting data were used to predict water quality for a hypothetical scenario where the waste rock were backfilled in the Stratmat pit. In addition, water quality profiles were measured in the Stratmat pit.

Results of the column dissolution tests suggest the following mass balance for the Stratmat pile: Approximately 7% of the original sulphide sulphur has been oxidised since deposition, releasing a total acidity of 11 800 t CaCO₃ equivalent, of which 56% has been neutralised in situ. Currently, the acidity inventory is approximately 5200 t CaCO₃ equivalent whereas the inventory of soluble zinc is about 1660 t. The mass balance appears to support preferential oxidation of sphalerite over pyrite.

The column dissolution experiments further indicate that dumping the waste rock into the flooded Stratmat pit will cause significant release of stored metals and sulphate. The long-term pore water quality in the absence of ground water movement is predicted as follows: pH 3.32, acidity 12 500 mg CaCO₃/L, SO₄^2- 19 500 mg/L, Zn 4500 mg/L, Cu 180 mg/L and Pb 2.4 mg/L. In the presence of uncontaminated ground water movement, the water quality would gradually improve as the initial pore water is displaced or diluted. It would take nine pore volumes of flushing to reduce the concentrations of most metals (except Pb) to below 0.1 mg/L. For Pb, this would take many more pore volumes.

Geochemical modelling suggests various concentration control mechanisms. Concentrations of Pb and Fe in the pore water are likely controlled by equilibria of the leachate solution with anglesite and ferric hydroxide, respectively. On the other hand, concentrations of SO₄, Zn, Ca, Mg, Mn, and Al in the pore water seem to be limited in the short term by dissolution/diffusion rate controls. The presence of gypsum is found to inhibit the dissolution of anglesite. As a result the anglesite stored in the waste rock would not dissolve appreciably until gypsum storage is exhausted by dissolution. This implies that decommissioning of the waste rock by a backfilling-flushing-treatment process would last a long time before the pore water in the backfilled waste rock becomes acceptable for discharge to the receiving groundwater.