EXPERIMENTAL EVALUATION OF SUBAQUEOUS DISPOSAL OF ACID GENERATING SULPHIDE MINERALS IN THE MEGUMA SUPERGROUP, SOUTHERN NOV A SCOTIA

Abstract

Development and construction projects that require the disturbance and excavation of sulphide rich bedrock in Halifax and southern Nova Scotia produce large quantities of waste rock material. This material poses a high risk potential for acid rock drainage (ARD). Acid drainage develops due to the oxidization of iron sulphides found in the Cambro/Ordovician Meguma Supergroup, southern Nova Scotia. Variations of sulphide mineral type, texture, the presence or absence of acidophilic bacteria (Thiobacillus ferrooxidans and Thiobacillus thiooxidans) and the availability of oxygen and water determine the reactivity and consequently the rate of acid generation. Subaqueous disposal is a current potential method for waste management of excavated sulphide-bearing waste rock in Nova Scotia. The concept is based on the premise that acid generation is chemically suppressed in low oxygen conditions at depth in submerged sites. This method has yet to be evaluated with regards to the Meguma Supergroup and in particular, to disposal in marine environments. Furthermore, policies and regulations for disposal of sulphidebearing waste rock in Nova Scotia is aimed at conventional terrestrial methods, and does not adequately address disposal in a marine or freshwater environment. This project investigates in a laboratory experiment, subaqueous disposal of sulphide bearing rock from the Meguma Supergroup in seawater compared to fresh water. Polished thin sections were prepared from a drill core sample near the Halifax International Airport. The primary sulphide minerals present were pyrrhotite, marcasite, pyrite, and chalcopyrite. The experiment was conducted using distilled water, seawater and lakewater from natural environments. One thin section was submerged in 500ml of each of the three water samples at a depth of 32 em in graduated cylinders exposed to atmospheric conditions at room temperature (21 °C). One was maintained in air as a control. Dissolved oxygen and pH measurements of the water samples were obtained at initial and fmal stages. Measurements indicated a slight decrease in pH and dissolved oxygen from start to fmish in all water samples. Observations of surface features under reflected light were recorded and digitally imaged at 6, 15, and 40 days. Surface coatings (precipitate), color (tarnish), and topography of sulphide minerals were used as an indication of reaction. Most sulphide minerals experienced some degree of tarnish, residue, or slight color change and therefore suggested reaction. Reaction appeared to initiate at grain boundaries and radiate outward to cover the sulphide mineral surface. Results indicate that in general, the rate of sulphide reactivity is higher in seawater compared to freshwater. The overall relative degree of reactivity of sulphides in a submerged environment was marcasite > galena (inclusions) > pyrrhotite > pyrite > chalcopyrite.