CASE-STUDIES OF WETLAND FILTRATION OF MINE WASTE-WATER IN CONSTRUCTED AND NATURALLY-OCCURRING SYSTEMS IN NORTHERN AUSTRALIA

A problem common to mines operating in the tropics is the disposal of water, which may be alkaline, acidic, or contain toxic elements such as arsenic or cadmium. The availability of year-round water supply at many mine sites in Northern Australia, particularly from pit dewatering, together with the monsoonal climate, provide appropriate conditions for the formation of natural wetlands or establishment of artificial wetlands. Wetland species (particularly Typha spp.) flourish in the presence of flowing or shallow water from dewatering, and data collected from natural and experimental wetlands show reduction of metal concentrations by wetland filtration of mine waste water. The following case studies are considered: 1. Constructed wetlands, used to remove uranium from waste rock runoff before release to an adjacent creek provide a means of ''polishing'' runoff water prior to discharge to the creek. 2. Creek-Billabong systems with existing wetlands adjacent to mine sites adventitiously ''filter'' waters discharged from mine sites. Trace elements in dewatering water from a gold mine discharged into an oxbow show reduction of elemental concentrations downstream, compared to discharge water. 3. Naturally generated wetlands at several Northern Territory mines have developed along channels for discharge of pit water, with ingress of Typha domingensis. Such wetlands, associated with dewatering, have been examined at four mines, some with acid mine drainage. Water quality measured after wetland treatments shows reductions in concentrations of various heavy metals and sulfate. Volunteer Typha domingensis grows and spreads in shallow flowing channels, providing an inherent ''filtration'' of the water. 4. Natural swamplands are incorporated in the waste rock runoff management design of a new gold mine, to reduce potentially high arsenic levels in the waste water. Constructed and naturally occurring wetlands may be used in the treatment of most mine waste waters to achieve levels of constituents acceptable for discharge to the surrounding environment.