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Publications
Research Report 6
Wasson RJ (ed)
Supervising Scientist 1992
ISSN 0810-9966
ISBN 0 644 25456 4
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A sediment budget and routing model has been developed to estimate both the mass and proportion of tailings that might arrive on the Magela Plain in each average year over various time periods after erosion of the tailings has begun. The proportion of tailings in the mixture of sediment passing Mudginberri in each future year is a function of the total time taken to erode the tailings. At a rate close to the natural long-term denudation rate, the tailings would constitute less than 1% of the present-day level of suspended sediment passing Mudginberri. As the total erosion time decreases, the proportion of tailings in sediment reaching the Plain increases. If the total erosion time is £ 1800 years, then tailings will be ³ 50% of the sediment reaching the Plain. The impact of deposition in the Plain's catchment, under conditions of either catastrophic erosion of the tailings or (more likely) slow erosion, has not been assessed in this study.
A sediment budget shows that sedimentation rates on the Plain are approximately constant within large uncertainties using £ 3 years of measured suspended sediment loads. The budget further shows that the catchments adjacent to the Plain are of equal importance with Magela Creek as sources of sediment. The pattern of particle size fractions in surface sediments of the Plain supports the conclusion that sediments derived from Magela Creek do not dominate the Plain, although this analysis is confounded by large quantities of silt-size biogenic silica.
By far the most convincing demonstration of the contribution to the Plain of sediment derived from Magela Creek comes from the use of radionuclides of the natural U and Th series. Disequilibrium between 238U and 226Ra in modern stream sediments carried by Magela Creek is mirrored in the sediments of the upstream part of the Plain. The presence of disequilibrium in the surface sediments marks the area where sediment carried by Magela Creek, rather than smaller streams adjacent to the Plain, is deposited. It is clear that 90% of Magela Creek sediment is deposited upstream of Jabiluka Billabong, in an area of about 30 km2, or about 15% of the Plain. The rest of the Plain's surface sediment comes from other catchments. This conclusion is consistent with the major disjunction between the upstream and downstream parts of the Plain, defined by biogenic silica and organic content of surface sediments, soil chemistry and morphology.
A sediment routing model is derived from £ 3 years of measurements. The radionuclide disequilibrium, upon which is based the identification of the pattern of deposition of Magela Creek sediments, has been found in sediments no more than ~140 years old. The predictions are likely to apply only to future conditions like those of today and the last two centuries, unless confidence in extrapolation can be gained by showing that the sedimentary system of the Plain has been stable for many centuries to millennia. The sedimentation rate over the entire Plain has been shown to be approximately constant at about 0.2 mm y-1. This figure holds for time periods from 0.003 to 3.3 ka, implying considerable stability within the relative uncertainties of 6 to 19%.
Extrapolation of sedimentation rates on the Plain, and application of the predicted tailings dispersal, may be appropriate for the next 1000 years, but only if sea level, climate and the sediment delivery system do not change markedly.
The most likely futures for the Plain, against which it might be possible to judge the applicability of the predicted patterns of tailings dispersal are:
1. If the Earth warms with rising atmospheric greenhouse gases the ocean will expand and some glacial ice could melt, raising sea level by between 20 and 140 cm by about the middle of next century. At the same time, the Australian monsoon is likely to intensify, and the Wet season duration and rainfall will increase. Although by no means certain, ENSO may behave as it has during previous warm periods, and so the frequency and intensity of tropical cyclones is likely to increase.
Sedimentation rates on the Plain are not very sensitive to climate change, as shown by their near constancy over a wide range of time periods during which climate change has been detected. If large cyclones are more frequent, however, the tailings may be eroded more rapidly than would be the case under current conditions.
But the Plain is sensitive to changes of sea level, as shown by detailed stratigraphic studies. Absolute sea level is not the only factor that controls the distribution of saline vegetation and sedimentary environments. Connection to tidal channels is just as important, at least for small shifts in the level of the sea. Although not precise, the best prediction is that a sea level rise would reconnect the old tidal channels of the Downstream Plain and possibly the Central High. But it seems highly unlikely that the Upstream Plain would be connected to tidal flows. The Central High, first established between 3 and 2 ka, may offer some resistance to tidal inundation of the Upstream Basin. However, a rise of sea level of several metres, brought about by melting of ice sheets, would inundate the entire Plain.
The Holocene sequence of change from mangrove forest sediments to transition conditions to freshwater sediments is not the inevitable sequence that is to be expected in reverse if sea level rises in the future. The sequence recorded in the sediments is the combined result of a sea level rise and climate change. Estimates of future sea level rise indicate a rate between 0.06 cm y-1 and 2.3 cm y-1. The range 0.06 to 0.15 cm y-1 seems most likely, a rate 13 to 5 times slower than the post-glacial sea level rise of 0.8 cm y-1 during the period 8-6.5 ka. Although the sedimentary sequence deposited on the Plain in the future will not necessarily mirror the sediments of the last 6 ka, because of differences of climate, the relatively slow rate of sea level rise will almost certainly allow time for the establishment of transition-like vegetation and sedimentary environments.
2. Within the next 5 ka the Earth should begin to feel the effects of the next glacial as small insolation variations begin the process of cooling that has repeated itself every ~100 ka for the last 700 ka or so. Within the next 5 ka sea level lowering, as glaciers grow, will probably be a few metres at most. It is possible that some shallow incision of the Plain could occur under these conditions, although this is not likely given the very low gradients of the area. A fall of sea level is likely to follow the super interglacial which a doubling of atmospheric C02 will bring about.
Within the next 1 ka, the period during which any tailings impoundment at Ranger must retain its integrity and protect the tailings from erosion of any kind, the most likely climatic change will be the result of the Greenhouse Effect. It seems highly unlikely that the first effects of the next glacial will be felt in this period.
If global emissions of C02 are reduced so that the atmospheric concentration stabilises at twice its pre-industrial value, sea level will probably rise but by less than 1 m. The area of Magela Plain most likely to receive tailings, or other Mine Site detritus, namely the Upstream Plain and Mudginberri Corridor, will probably not be connected to the sea. Furthermore, this area of the Plain will be some 20 ± 13 cm higher in 1000 years as a result of continuing freshwater sedimentation.
It can be concluded that tailings dispersal on the Plain probably will not be affected by a mild Greenhouse Effect, either by a change of sea level or by climate change directly, other than by the effect of increase cyclone activity on the total time of tailings erosion.
At the natural denundation rate for the tailings dam, there is little evidence that the Plain's biota would be affected by tailings deposition. This is a strong argument for building the tailings impoundment to mimic the natural landscape.
Climatic changes over millennia are relatively slow and seem to be driven by variations of solar insolation, caused by changes in orbital geometry of the solar system, and modulated by C02 and the oceans. At shorter time scales, variations of solar output are important, along with volcanic eruptions and changes to oceanic circulation. While these relationships are becoming clear as research proceeds, none of them is strictly periodic and therefore predictable over the next 1000 years.
it is likely, however, that the next 1000 years will include another warm period like the Medieval Warm Period, superimposed on the warming resulting from the Greenhouse Effect. Warm periods are likely to be associated with a more positive Southern Oscillation Index and so are likely to be accompanied by more intense and frequent tropical cyclones. The calculation of the return period of intense rainstorms, caused by tropical cyclones, for the next 1000 years based upon the observations of this century is obviously a hazardous exercise in the face of the almost certain changes just predicted.
The sediments of the Plain do not hold a record of extreme events.
Following massive erosion, deposition of tailings would cause serious problems in the Mine Site catchments for plants and animals, and would have aesthetic implications. Under the more likely conditions of slow erosion of the tailings, the most important effect on the biota will be by mobilisation of metals from tailings.
The heavy metals left in the tailings, after the extraction processes of the mill, occur as inclusions of minerals in silicate fragments and as minerals of the kind found in or near the ore bodies. These metals are in relatively stable forms and are unlikely to be readily mobilised if mixed with soil materials. Sequential extraction of metals from the tailings showed that sulfides of Pb, Cu and Fe were mobilised in the oxidisable fraction, suggesting that, in the long term, the metals occurring as sulfides could be mobilised under oxidising conditions. The reducing agent extracted some Mn, Cu and Pb, suggesting that these elements would be mobilised if added to saturated soils.
Experiments to determine metal transformations in mixtures of tailings and soils were carried out using the major soil material types of the Plain: two varieties of Dark Brown/Black Clay, and one sample each of the jarositic horizon, and the pyritic mud. These soil material types were chosen because they represent the materials most likely to be exposed to tailings. The Dark Brown/Black Clay is the most likely recipient of tailings during the next 1000 years. If the level of the sea rises substantially, however, there is no material currently on the Plain that can represent the pedologically unaltered transition sediments that are likely to accumulate and also receive tailings. In the unlikely circumstance that sea level falls sufficiently within the next 1000 years to allow incision of the Plain, the jarositic and pyritic materials may receive tailings and so have been included in the experiments.
The tailings have concentrations of Mn, Pb, U and 226Ra higher than any of the soil materials chosen for the experiments. These metals are therefore the most likely to increase in concentration if tailings reach the Plain. Assuming that the tailings will erode at about the natural long-term denudation rate, then 0.4% of tailings will occur in each average year's load of sediment reaching the Plain. This proportion of tailings in any of the soil materials increases only the Pb and 226Ra concentration in mixtures of soil and tailings.
Assuming a total erosion time of the tailings of 5 x 104 years, a rate 3.5 times faster than the long-term natural rate, then the Plain's surface sediments should consist of on average 4% tailings. All metals, apart from Zn, increased in their concentrations in the mixtures in all soil materials when 4% tailings was added. The pollution potential of the tailings is highest for Pb and 226Ra, and lowest for Zn.
Pyrite oxidation appears to produce more changes to the forms of metals than the addition of tailings. There is greater potential for metal mobilisation from native sources of metals within the pyritic mud than from those added as tailings. It is concluded that, up to a concentration of 4% tailings, the effects on the forms of metals is small, and less than that found during pyrite oxidation.
226Ra in mixtures of tailings and soil materials is either mobilised or immobilised according to the organic content of the soil material. In highly organic material, 226Ra is mobilised and remains in an exchangeable and hence a bioavailable form. Radium is most likely to be available in the first 10 km of the Plain, that is as far as Mine Valley Billabong. Further downstream, availability should fall rapidly.
If sea level rises sufficiently to re-establish tidal connections on the Downstream Plain and Central High, then it is likely that the Upstream Basin will be more poorly drained. The period of inundation of the Basin will increase, aided by a longer Wet season, and aquatic macrophytes will probably become more common. The acid sulfate process will only operate if the water table falls far enough to allow oxygen to penetrate to the pyritic substratum. This oxidation will be less frequent if the length of the period of inundation increases, and so the rate of chemical and mineralogic differentiation, both vertically and horizontally, will decrease.
Given that most changes to the forms and hence bio availability of metals in soil material of the Plain occurs as a result of oxidation of pyrite, a decrease in the rate of this process will slow the release of metals to the interstitial waters. The Al content of the future part of the Dark Brown/Black Clay, will probably be lower than it is in existing sediment of this unit.
It is probable that at the natural long-term rate of denundation the tailings would make an undetectable impact on turbidity on the Plain.
The most important conclusions of this report relevant to the general understanding of the natural and human history of the ARR, but not strictly relevant to the problem of sediment management, are:
Wasson RJ (ed) 1992. Modern sedimentation and late quaternary evolution of the Magela Creek Plain. Research report 6, Supervising Scientist for the Alligator Rivers Region, AGPS, Canberra.
