SUMMARY
The annual wet season in the tropics produces excess accumulation of water on mine sites, which may become contaminated through contact with mine waste rock and mineral processing. Waste water may be discharged offsite and reach downstream aquatic ecosystems, including wetlands. In many cases constituents are diluted or ameliorated through adsorption, chelation and co-precipitation processes. Tropical climatic conditions and local mineralogy causes high incidence of acidic runoff, which may have deleterious effects on biota, making it difficult to restore the aquatic ecosystem. Wetlands are most suited to treating recessional flows associated with the cessation of the wet season, in contrast to the high dilution achieved during discharge into flood episodes. The ability of the aquatic plants, soils and sediments of wetlands to retain contaminants enables them to prevent dispersion of pollutants into the environment. Moreover, the fringe zones of large natural wetland areas can be used to protect the main area of the wetland itself.
The composition and kind of waste water (mildly alkaline, near-neutral and acid) will determine the type of wetland system to be employed. Wetlands that may be used are: existing wetlands, enhanced natural wetlands or constructed wetlands incorporating specific water treatment zones for treatment of acid mine drainage. Other factors of prime importance when using wetland systems in the tropics are mosquito control and minimisation of faunal access to contaminated flora and water in the wetlands.
Wetlands can therefore be considered as a means of cleaning up contaminated waste waters from tropical mine sites, enabling water releases; their cost effectiveness and 'green appeal' ensures they will become employed to this end at many mine sites in the future. An understanding of the physico-chemical processes involved is seen to be the key step in effectively utilising wetlands to treat mine waste water.
1. NOTATION AND UNITSpH = -log[H+], where [H+] is the concentration of hydrogen ion in moles/L. A low pH(<7) an acidic solution, while a high pH (>7) indicates an alkaline solution.
2. INTRODUCTION
2.1 Climate and Location
Northern Australia lies within the tropics, and as such, experiences a wet-dry seasonal climate (figure 1) (1). In addition, the 'Top End' (defined as the area of Australia north of 15°S) has an annual high temperature distribution (generally between 29 and 34°C in the shade, with temperatures up 77°C being measured in the sun). The monsoonal trough produces annual deluges of rain in the 'wet season', with the highest number of rain days occurring between November and April(2), reaching a maximum average of 21 days in January in Darwin (figures 2 and 3) whilst the minimum falls to an average of near zero for June-August. This region, therefore, has a high rainfall intensity during the wet season. Humidity in the Top End is also high throughout the year with wet-dry variations ranging between 70 and 100% relative humidity. Tropical cyclones are fairly rare events, particularly inland where the majority of the Northern Territory's mines are situated. However, tropical rain depressions are common events both inland and at the coast, and can produce substantial amounts of rainfall.
2.2 Modern Mining and Mineral Processing
Large scale open pit mines are a common modern mining method. Such a practice produces large volumes of waste rock, ore and tailings which require stabilisation on site as well as open voids. An example of the quantities of waste that can be produces is at the Ranger Uranium Mine, which at mining completion will have produced 175 Mt of mill tailings, waste rock and sub-economic grade material remaining for disposal, East et. al. (3). Government guidelines on stabilisation of wastes varies depending on what is mined, and the type of waste. Tailings containment structures for uranium mines, for example, require an engineered structural life of 1000 years. Dam and pond structures for smaller volumes of waste require less stringent measures.
2.3 Runoff Accumulation and Disposal
Contact of mine processing materials and waste rock with rainwater leads to incorporation of contaminants into runoff. Downstream aquatic ecosystems may be affected if overflows of waste water occur. This may be difficult to control at tropical minesites unless sufficient storage is provided. Water emanating from a waste dump may be distributed in the following ways (figure 4)(4):
Runoff water from mines in the Northern Territory is collected in dams, and only be released under licence (issued under the Water Act, 1992); such regulations are designed to minimise detriment to surrounding aquatic ecosystems. At the Ranger Uranium Mine, where the Uranium Mining (Environmental Control) Act (1978) applies, the Act introduces the concept of a restricted release zone (RRZ), which includes all areas in which material is mined, stockpiled, stored or handled, and the catchment draining rainfall from such areas. All waters in the must be kept within the RRZ, unless release is approved (6).
Generally, dams on mine sites are designed differently according to their purpose. Tailings dams are designed to contain very contaminated material, and so seepage must be minimised. Therefore, the requirements for their construction are very stringent. The performance standards for major dams are set by the Australian National Committee on Large Dams (ANCOLD) ("Guidelines on Risk Assessment 1994") and by specific state legislation. The accumulation of runoff and flood estimation is dealt with in a publication by the Institute of Engineers, Australia ("Australian rainfall and runoff", 1992).
2.4 Acid Mine Drainage
Water seeping from waste dumps is often contaminated with heavy metals, and sulfate. The common association of the mineral pyrite (FeS2) with gold, leads to a persistent problem at mine sites in the tropics: acid mine drainage (AMD). In addition, climatic conditions in Northern Australia aggravate this problem. AMD is brought about through the oxidation of mineral sulfides, particularly pyrite, by air and water to yield sulfuric acid and dissolved iron. The acid produced enhances dissolution of other sulfide and gangue minerals, and leads to the presence of a whole range of metals in solution, apart from iron, as reviewed by Salomons(7). AMD is usually accompanied by downstream coating of stream sediments by freshly precipitated hydrated ferric oxide, possessing a bright yellow-orange colour. High tropical temperatures exacerbate AMD, doubling the oxidation rate of pyrite with every 10°C increase in temperature at constant relative saturation of water (8). Coupled with the high humidity and rainfall, serious acid drainage can appear in a very short time. Maximum oxidation of pyrite under biotic conditions occurs between the pH ranges 2.4 to 3.6; bacteria amplify the rate of ferrous iron oxidation, and the oxidising bacteria Thiobacillus ferrooxidans thrives in acidic waters. T ferrooxidans has been shown to increase the rate of pyrite oxidation by a factor of more than 10.
Some mines, such as Ranger Uranium Mine do not suffer from AMD, but mobilisation of metals through erosion, acidic rains (due to weak organic acids in the atmosphere) early in the wet season, and heavy rain mid wet season ensure they too must cope with contaminated water (3). Table 1 displays some typical values for water samples taken at Ranger, and also typical AMD composition for water samples taken from a tropical gold mine. Natural metal cycles require millions of years to complete, but human intervention has altered this time scale enormously (9). It is necessary to assist natural processes to cope with abundant metal mobilisation in a short time, so as to inhibit movement of toxic metals into the areas surrounding mines which can create secondary contamination in soil, rivers and other natural locations which is then much more difficult to rectify (10).
Thus, the essential problem is how to deal with excess water accumulated during the wet season. The construction of evaporation ponds is usually impracticable, since ponds that would be capable of removing all excess water would be quite large (typically several hundred ha) and would create an environmental impact in their own right, and hence become very expensive. During periods of heavy rainfall, which may occur in the Top End from December to April (figure 2) direct water release into fast flowing creeks is a viable option, since the dilution ratio that can be achieved is often high enough to reduce the toxicity in the water to background levels, suitable for sustaining healthy aquatic ecosystems, or for some other beneficial use such as stock watering or the provision of drinking water. In the dry season, on-site spray irrigation may be used, as has been done at the Ranger Mine, to dispose of mine waters by evaporation, by infiltration into soil and through evapotranspiration by plants (6). Only water of reasonable quality (i.e. not tailings water) is disposed of by spray irrigation.
Recessional flows nearer the end of the wet season mean that toxic metals, and other contaminants become more concentrated, and with flows decreasing direct releases are no longer feasible since the required dilution ratios are impossible to achieve. Chemical treatment is possible, but problems like waste dump drainage will proceed for indefinite periods, (possibly hundreds of years) so that such treatment is impracticable, since it is not self sustaining. Wetlands, however, can provide a long-term, inexpensive and sustainable way of dealing with persistent AMD.
2.5 Aquatic PlantsSubmergent plants were found to be the greatest concentrators of heavy metals. All aquatic plants displayed greater accumulation of heavy metals during their period of rapid growth, which occurs during the wet season in the tropics, and thus corresponds with water release episodes. Also, translocation of heavy metals from the shoots to the roots in the dry season coincides with plant dieback, and hence leads to better entrapment in sediments of heavy metals from decomposing root materials. Studies by Eapaea et. al. (12) at Tom's Gully Gold Mine show that after plant senescence in a wetland, the dried sediments retain arsenic and iron oxide residues.
3. DESIGN OF WETLAND SYSTEMS FOR TREATING MINE WASTE WATER.The main sink for metals in constructed wetlands is the substratum, although they are also concentrated by colloidal material (14). The soluble metallic contaminants are the most available to biota. Metals are incorporated into sediments by adsorption, cation exchange, precipitation and chelation. Metal distribution between sediments and the water column is variously affected by the latter's acidity, reducing power, salinity (all of which correlate directly with increased dissolution of trace metals) and organic ligands (which decrease metal solubility) (14). The design of wetlands must therefore take into account all of these factors.
Free metal ions are the main source of toxicity in a wide range of plants and animals (11). Therefore, control of pH is crucial in managing the toxicity of acidic waste waters. Whilst metal uptake by plants is small compared to other removal mechanisms, temporary uptake by plants provides additional capacity for removal. Emergent plants stabilise the substratum by binding it together, and rhizosphere oxygenation aerates the substratum allowing the formation of manganese and iron hydroxides, which scavenge metals and reduce levels of cadmium, copper, nickel, lead and zinc (15). A study of rhizosphere oxygenation by Dunbabin (15) found that metal retention in wetlands was correlated strongly with pH, and only poorly with oxygen concentration and redox potential. However, oxygenation also increases the bioavailability of trace metals to aquatic plants (16). In addition to their other positive features, plants increase microbial and algal activity (which increase metal uptake) (17), provide organic matter on senescence, and act as a filter by slowing water flow, and allowing more of the water column to come into contact with the sediments and deposit particulates and metals (14). Absence of aquatic plants can contribute to erosion of sediments. Floating plants cannot achieve the filtering effect of emergents, and so are not generally used in wetlands for treatment of contaminated waters.
However, a simple wetland system is often not sufficient for the effective removal of contaminants. This is shown clearly, for example, in an experiment where a constructed wetland was used to treat coal mine drainage in Ohio (18). Limestone and mushroom compost were used as a sediment base, and Typha latifolia was planted, as the only wetland plant species. It was found that electrical conductivity, pH, manganese and sulfate were little reduced by the wetland, while iron was reduced by 50 to 60%. Iron hydroxides were precipitating from the water column, thus turbidity increased significantly, and therefore the wetland was simply acting as a sedimentation pond. The difficulty in treating metal-containing waste waters when compared to municipal and sewerage effluent, presents a significant challenge.
It is generally inadvisable to use natural wetlands in treating AMD, because they constitute valuable ecosystems, and have rarely been used successfully (13). Instead constructed wetlands can be used, often in conjunction with other treatments. In many studies, peat, hay, gravel and sawdust have been used as substratum materials in a constructed wetland (19) (peat containing 20-30% organic carbon is available in the Northern Territory from near Bynoe Harbour). Many of these prove to be an expensive substratum for use in a whole wetland but in small sections of a wetland may be viable. Instead, natural sediments are frequently used, with addition of hay at the beginning of wetland operation to provide organic matter for metal entrapment, until plants are properly grown. For AMD careful design of functional zones, including channels containing limestone for acid neutralisation, is required before water enters the wetland since plants and sediments provide little buffering capacity for strongly acidic waters. A riffle, or settling pond is often used before water enters the wetland, to remove iron as an oxide through oxidation of ferrous iron emerging from the dump, otherwise extreme turbidity can result in the system. An anaerobic zone, using peat for example, where water makes no contact with air will remove much of the sulfate, as it is reduced to insoluble metal sulfides (copper, lead, zinc, nickel and cadmium). The wetland stage follows the initial treatment zones and its purpose is to 'polish' the waste water. Sulfate reducing bacteria may also play a part, but are generally difficult to foster, since they are intolerant of acid, and require addition of organic matter
Vigorous plant growth is necessary to ensure success of a wetland. Native plants that are adapted to tropical conditions, should be employed. Moreover, a variety of species is preferred, so that the wetland is less susceptible to attack by viruses or pests. In tropical systems the grass Typha domingensis has proved useful in constructed wetlands, since it is found native in many parts of the Northern Territory's seasonally inundated flood plains (20) and is tolerant of acidic waters with a pH as low as 3. Other acid-tolerant species are Eleocharis spp. and Sclera poaliformis. Bulrushes and Typha spp. are also found in North Queensland. Stockpiling of plants gathered from natural wetlands may be necessary for wetland planting to begin. Aquatic species used in the tropics need to survive the annual cycle through to the end of the dry season and be rejuvenated or grow from seed when the wet season recommences.
3.2 Flow DesignThe use of wetlands in the tropics for AMD treatment has thus far been limited. Examples of wetlands used in treating mine waste waters of near-neutral pH in northern Australia are:
Ranger Uranium Mine, NT - a constructed wetland which is used to remove uranium, magnesium, manganese and sulfate. Magnesium and some sulfate was not effectively attenuated by the wetland (24)
Tom's Gully Gold Mine, NT - a natural ox-bow billabong wetland which effectively removed a range of transition metals, as well as arsenic from pit dewatering (24)
Woodcutter's Silver-Lead-Zinc Mine, NT - a naturally colonised wetland that removes copper, lead, cadmium and zinc; outflow water is then released into a creek(24)
Hilton base metal mine, Qld - water is degassed to reduce slight acidity, and then a wetland of reeds and algae removes iron, zinc, manganese and thallium (21)
3.5 Fundamental Processes in WetlandsWetlands are a common feature of the tropical landscape and may be adapted to treating waste water from mines. For specific control purposes, however, it is usually necessary to design the wetland system taking into account the kind of waste water (whether mildly alkaline, near-neutral and acid) and specialised treatments which may be necessary before waste water is allowed to come into contact with aquatic plants. It is therefore necessary to understand fundamental processes of contaminants in wetland systems, particularly in the case of AMD. Another environmental consideration is the minimisation of the numbers of mosquitoes which may be the vectors of debilitating tropical diseases.
Wetland system development for mine sites in the tropics is still in a state of infancy but looks very promising. Inclusion of features to ensure an even flow of water through the wetland must be incorporated, since, if the water is too deep aquatic plants will not grow.
REFERENCESGretel Parker received her B.Sc. degree with 1st Class Honours in 1995 from the Northern Territory University, majoring in computational chemistry. She has worked in Mines Division, Department of Mines and Energy, Darwin, as a graduate trainee under the supervision of Dr. Barry Noller since June 1995.