International Mine Water Association
Faculty of Computing, Health and Science
School of Natural Sciences / Centre for Ecosystem Management
Pit lakes may form when open cut mining operations extend below groundwater level and then fill at cessation of mining and associated dewatering operations by ground and surface water influx. Pit lake hydrogeology may function as an evaporative “sink” when pit lake water evaporation rates exceed influx rates. Although not ideal closure, management of local surface and groundwaters contaminated by Acid and Metalliferous Drainage (AMD) through entrainment toward an evaporative terminal pit lake may provide a best-case scenario for protection of regional water resources required by typical mine closure time scales of hundreds to thousands of years. We present two case studies from Western Australia; the first where closure of above ground landforms such as waste dumps by covers would arguably not be successful over long terms (1,000 years or more) and another where Potentially Acid Forming waste (PAF) management is limited by current waste rock dump location and suitable cover materials. Pit lake water balance modelling indicates both case study pit lakes will function as hydraulic sinks if they are not backfilled above their equilibrium water levels. A best closure outcome for these pit lakes may be to be backfilled with PAF encapsulated with alkaline/neutral waste and then filled as rapidly as possible to minimise PAF oxidation and ensure an evaporative sink pit lake is formed.