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 mining methods. The mine closed in 1990 after the re-treatment of 28 Mt of tailings, which were placed into the open cut. Most of the mine waste is acidgenerating and acidic runoff and seepage has heavily impacted portions of the adjacent Dee River.A groundwater flow model was developed for the Mount Morgan mine site to evaluate current seepage conditions and assess closure options. The calibrated groundwater flow model indicates that the backfilled (and flooded) Open Cut/Sandstone Gully 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 in a 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.The model predicts that seepage from the open cut would increase exponentially with a further increase in the water level in the open cut. The model suggests that a grout curtain or sealing the upstream side of Sandstone Gully Dam using a "blanket" of low permeability tailings would reduce seepage out of the open cut by about 40%. The amount of seepage reduction in response to placing a dry cover system onto mine waste (tailings and mine rock) is predicted to vary significantly across the site. The modeling results suggest that a combination of rehabilitation measures (including the placement of dry cover system and measures to control seepage out of the flooded Open Cut/Sandstone Gully) will be required to effectively control seepage at Mount Morgan. The calibrated groundwater flow model is currently being used to assess the effects of different closure scenarios (e.g. cover placement versus full relocation) on seepage rates and loading to the Dee River.