Weathered Sulfidic Waste – Laboratory-Scale Tests for Assessing Water Quality in Backfilled Pits

Closure options under consideration at some iron ore mine sites in Western Australia include backfilling with mineralised and/or non-mineralised waste rock. Post-closure, following groundwater rebound, flow-through conditions may develop in the backfill located within the saturated zone below the water table and may release solutes accumulated from previous oxidation. The release of solutes may impact on the receiving groundwater quality. It is therefore important to quantify the potential for solute release under these conditions to understand the potential impacts that result.

Geochemical characterisation programmes are the foundation for assessing the potential solute release that may occur to water that has contacted the backfill. Such programmes generally comprise laboratory scale static and kinetic methods designed to obtain data relevant to the closure options under consideration. The tests generally are carried out under ambient atmospheric conditions which may not be representative of the conditions in the porewater of backfill after groundwater rebound. To complement standard testing techniques, SRK employ a saturated column test which is designed to generate site- specific data for solute leaching under the anoxic conditions that may occur within the inundated backfill.

This paper presents the findings of saturated column test work carried out on samples of waste rock from BHP’s Eastern Ridge mine located in the Pilbara, Western Australia. The samples were sulfidic, and had been subjected to ‘accelerated’ weathering and oxidation in free- draining columns prior to saturated column testing.