Mt Morgan Mine – a case study of ARD impacted groundwater

The Mount Morgan Mine is an historic mine site located in Central Queen-sland, Australia. The mine closed in 1990 after the re-treatment of 28 Mt of tailings, which were placed into the open cut pit. Mining at Mount Mor-gan has resulted in the exposure of sulphide-bearing mine waste at surface which produces acid rock drainage (ARD) and has heavily impacted por-tions of the adjacent Dee River. Historic stream channels draining the mine site (often filled-in with tailings, slag and/or waste rock) and associ-ated structures in the underlying bedrock appear to represent a preferred pathway for mine-impacted groundwater into the Dee River. The ground-water draining the minesite has low pH (2.5-3.5) and highly elevated con-centrations of magnesium, sulphate, aluminium, iron, copper, zinc and various trace metals (Cd, Cr, Co and Ni). While a seepage interception and pump-back system (SIS) is currently in place, the amount of ARD entering the groundwater system and ultimately reaching the Dee River is poten-tially substantial and requires quantification. This paper summarizes the results of a detailed hydrogeological study of the Mt Morgan minesite which included the installation of 19 monitoring bores, hydraulic testing, water level and water quality monitoring and groundwater modeling. Us-ing the results of this study, it is estimated that the amount of groundwater seepage by passing the SIS and entering the Dee River and underlying aq-uifer is about 1.8 L/s. This seepage rate is significantly smaller than the amount of seepage currently intercepted (13.8 L/s) suggesting a very high efficiency of the existing SIS