CONTAMINANT LOAD ESTIMATES FOR THE HISTORIC MOUNT MORGAN MINE SITE, QUEENSLAND, AUSTRALIA

Abstract: The Mount Morgan Mine is a historic mine site located in Central Queensland, Australia. Between 1882 and 1981, a total of 7.6 million ounces of gold and 360,000 tonnes of copper were extracted using underground and later open cut mining methods. The mine closed in 1990 after the re-treatment of 28 Mt of tailings, which were placed into the open cut. Acid rock drainage (ARD) resulting from the exposure of sulphide-bearing waste at surface has heavily impacted the site and portions of the adjacent Dee River. The groundwater draining the mine site has low pH (2.5-3.5) and highly elevated concentrations of magnesium, sulphate, aluminium, iron, copper, zinc and various trace metals (Cd, Cr, Co and Ni). A groundwater flow model was developed to evaluate the effectiveness of the seepage interception and pump-back system (SIS) currently in place. The calibrated groundwater flow model indicates that the backfilled (and flooded) Open Cut/Sandstone Gully (OCSG) represents the largest single source of ARD seepage (8.0 L/s) on the site with tailings impoundments representing important secondary sources of seepage. An estimated 80 % of all seepage is collected by the seepage interception system (SIS). The remaining 20 % (or ~3 L/s) of ARD impacted seepage by-passes the SIS and enters the Dee River and underlying aquifer. A simplified load balance model was subsequently developed for the site to estimate the contaminant loads generated by the different mine waste units and by-passing the SIS. The flooded OCSG represents the largest single source of contaminant load. However, most of the seepage and associated load from this source is intercepted by the SIS. The results from the load balance model indicate that between 11 % and 25 % of the contaminants of concern (SO4, Al, Cu, Fe Zn) by-pass the seepage interception system and report to the Dee River aquifer system. The model identified mine waste lying within the Shepherd’s catchment as the largest contributor to this by-pass load. The results of this load balance analysis will be used to assist in developing a closure and reclamation plan for the Mount Morgan mine site