CASE-STUDIES OF WETLAND FILTRATION OF MINE WASTE-WATER IN CONSTRUCTEDAND 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 a s arsenic or cadmium. The availability of year-round water supply at m any 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 w etlands. Wetland species (particularly Typha spp.) flourish in the pre sence of flowing or shallow water from dewatering, and data collected from natural and experimental wetlands show reduction of metal concent rations by wetland filtration of mine waste water. The following case studies are considered: 1. Constructed wetlands, used to remove uraniu m 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 sit es 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 di scharge water. 3. Naturally generated wetlands at several Northern Ter ritory mines have developed along channels for discharge of pit water, with ingress of Typha domingensis. Such wetlands, associated with dew atering, have been examined at four mines, some with acid mine drainag e. Water quality measured after wetland treatments shows reductions in concentrations of various heavy metals and sulfate. Volunteer Typha d omingensis grows and spreads in shallow flowing channels, providing an inherent ''filtration'' of the water. 4. Natural swamplands are incor porated in the waste rock runoff management design of a new gold mine, to reduce potentially high arsenic levels in the waste water. Constru cted and naturally occurring wetlands may be used in the treatment of most mine waste waters to achieve levels of constituents acceptable fo r discharge to the surrounding environment.