Diagnosis of a base metal mine tailings capping failure

Conrad, in northern NSW, is a derelict base metal mine that has undergone extensive rehabilitation. In 2002, the upper tailings (UT) was hydraulically isolated through clay capping; however this was unsuccessful as As, Pb, Cu, and Zn enriched water is exiting the containment structure into Borah and Maids creeks, then Copeton Reservoir, the local drinking water supply, 3 km downstream of the mine. This thesis identifies UT as the highest priority for further rehabilitation. The UT is characterised through elemental and mineralogical analysis of solid samples taken from 0-10 m depth, and leachate extraction from tailings. Metal(loid) concentrations of sediment and water exceed NEPM and ANZECC guidelines onsite and downstream of the mine. Aquatic macroinvertebrate edge sampling along the creeks shows decreased family richness and diversity onsite and downstream from the mine. These results show that capping of the UT has been ineffective, and improvements are suggested to properly rehabilitate the area to stop further offsite spread of contamination. These include a replacement capping design incorporating geofabric, and investigating the possibility that water is infiltrating through the fractured granite in the valley wall on which the UT rests.