Disposal of waste materials at the bottom of pit lakes

Abstract

Mine pits have been used as sites for disposal of wastes from mining, ore milling and refinery, oil sand processing, by-products of acid mine drainage (AMD) neutralisation, ashes of coal combustion in power plants or even industrial wastes. In several cases, pit lakes formed after disposal of the waste materials. In other cases, the disposal went on after formation of a pit lake or was even conducted in order to neutralise the pit lake. However, the deposition of waste in surface water is not allowed in many countries. The purpose of the paper is to contribute to the discussion how to handle such existing waste deposits. In order to reach that goal, the paper gives a brief overview over processes relevant for the transport of substances from the waste into the main water body of pit lakes. Examples and experiences from Germany and from international literature are presented. The presented examples and the literature show that there are advantages and disadvantages accompanying subaqueous disposal of waste. In general, the stability of the conditions inside the deposited waste and at its interface with its aqueous environment is a main prerequisite for successful long term storage of waste below a water cover. In this respect, meromixis is usually helpful. Risks such as long term change of conditions inside and around the waste deposits and the pit lakes, as groundwater contamination or as toxication of aquatic life have to be evaluated carefully and site specifically. However, there are no scientifically reasonable arguments for a general preclusion of the subaqueous disposal of waste in pit lakes.