Characterisation and impact assessment of mine tailings in the King River system and delta, western Tasmania
Supervising Scientist Report 105
Taylor JR, Weaver TR, McPhail DC and Murphy NC
Supervising Scientist, 1996
ISBN 0 642 24304 2
- Title and publishing information (PDF - 41 KB)
- Table of contents (PDF - 167 KB)
- Chapter 1-2 (PDF - 794 KB)
- Chapter 3 (PDF - 1903 KB)
- Chapter 4 (PDF - 1864 KB)
- Chapter 5 (PDF - 6074 KB)
- Chapter 6 (PDF - 949 KB)
- Chapter 7-9 (PDF - 756 KB)
- Appendices and references (PDF - 555 KB)
About the report
Ninety-seven million tonnes of mine tailings and 1.4 Mt of slag have been deposited into the Queen and King River systems over the past 78 years. The sediments are rich in sulphide minerals, mainly pyrite (iron-sulphide), and are derived from the Mount Lyell copper mine. This material is currently residing in overbank, river bottom and delta deposits associated with the King River. The sediments and their pore waters contain concentrations of metals and acid that are potentially toxic to aquatic life in the King River and Macquarie Harbour. In order to quantify the impact of the sediments on water quality in the river and harbour, an integrated program of field, analytical and computer modelling work is used to estimate the fluxes of elements and acid between the sediments and the water.
Two types of sediment bank were identified: high mounded banks upstream of Teepookana and relatively low flat-topped banks downstream. Two examples of each type of sediment bank, as well as specific locations on the north and south lobes of the King River delta, were targeted for study. Field work consisted of installing mini-piezometers (groundwater monitoring devices), measuring hydraulic head and water chemistry, and sampling sediment, groundwater and surface water. Sediment and water samples were analysed and their compositions were measured using a combination of techniques. Fluxes of fluid, elements and acid are calculated using computer modelling of fluid flow, the hydraulic parameters measured in the field and water compositions measured in the laboratory. Computer modelling also indicates which minerals and processes are controlling the composition of the groundwater in the sediments.
The tailings contain rock, crystal and slag fragments, generally ranging in size from 0.01 to 0.2 mm, as well as variable proportions of organic debris from natural sources in the King River catchment. Minerals in the rock and crystal fragments are dominated by common silicate, oxide and sulphide minerals. Sulphide minerals are the source of many of the heavy metals. The slag is also enriched in trace metals such as zinc, cobalt, nickel and lead, and may be a significant source of some metals because of its high reactivity with water. Tailings in the delta contain 5 to 7% by weight of pyrite and about 0.16% by weight of copper, while the sediment banks are estimated to contain 2 to 3% by weight of pyrite and 0.085% by weight of copper.
The upper layers of the tailings sediment are not saturated with water, and this permits infiltration by air. Acid production in the tailings is initiated by the reaction of sulphide minerals with atmospheric oxygen. The upper 1.5 m of the delta containing about 4.4 Mt of tailings is undersaturated with water, and is the most significant source of acid and metals, at least from the delta sediments. Interaction between the products of sulphide oxidation and water produces sulphuric acid, and a range of soluble heavy metals. The oxidation of aqueous iron compounds at and above the water table, results in further acidity and widespread formation of iron-oxide precipitates which coat most of the sediment grains. Preliminary estimates indicate that almost complete oxidation of pyrite in permanently unsaturated tailings takes place in 1 to 4 years.
The composition of groundwater in the tailings is highly variable, and steep chemical gradients are present at the water table in the delta. Groundwater varies from highly acid to near-neutral (pH 2.54 to 7.1) and is enriched in copper, iron, aluminium, manganese, silicon and arsenic, with some samples also showing elevated concentrations of nickel, zinc, cobalt, lead, selenium and mercury. Groundwater in the banks is generally more acid and oxidised than groundwater in the delta. The north lobe of the delta contains the most reduced and near-neutral pH groundwater. Much of the variation in groundwater chemistry is attributed to differences in the sulphide and organic content of the tailings, and other local controls on redox conditions. The activity of sulphate-reducing, and possibly methane generating, bacteria at the tidal interface on the delta appears to be important in lowering metal and acid concentrations in delta groundwater discharged to the harbour.
Groundwater chemistry is controlled by interaction between groundwater and sediment. Some elements appear to be controlled by mineral solubility and equilibrium processes, for example, silicon by amorphous or microcrystalline silica, and iron by the iron-oxide coatings on mineral grains. Other elements are present in the groundwater in concentrations much higher than mineral solubility suggests. For example, the concentrations of aluminium are orders-of-magnitude higher than calculated solubilities for aluminium-bearing minerals. One possible explanation is that aluminium is out of equilibrium with the minerals in the sediments. The sources for many other elements in the water are identified, although the processes which control their aqueous concentrations are not always clear.
Element and acid fluxes from the tailings to surface water are calculated using a combination of groundwater modelling, measured water compositions and estimated pollutant discharge areas. The hydraulic conductivity of tailings deposits in the sediment banks and delta is high. Groundwater fluxes are approximately 20, 40 and 50 L/d/m2 for the high mounded sediment banks, relatively low flat-topped banks and delta, respectively. Estimates of copper and acid fluxes into the King River and Macquarie Harbour are 4.5 kg copper/d and 155 kg sulphuric acid equivalent/d. Additional fluxes into the river and harbour water result from episodic rain and flood events, where water flushes through and over the sediments. Although these fluxes are difficult to estimate accurately, they are thought to be broadly similar in magnitude to those from groundwater sources. The contribution of metals and acid from the river bottom sediments that include slag is unknown. Using the estimates outlined above, it is concluded that the King River and Macquarie Harbour currently receive an average daily addition of approximately 10 kg of copper and about 300 kg of sulphuric acid equivalent from the sediment banks and delta. These levels alone are likely to have significant ecological consequences; however, in the King River system this represents only 1 to 5% by weight of the total quantity of metal and acid entering the Queen and King River systems from the Mount Lyell lease site. These figures are considered to be reliable unless the estimated contribution from episodic rainfall events is much higher. Furthermore, if high concentrations of metals and acid are produced from short-lived flushing events, their environmental impact may be quite significant. Priority should be given to the remediation of acid drainage from the Mount Lyell lease site and understanding the effects of periodic rainfall events.
Based on current hydrogeological parameters and groundwater chemistry, the mass loadings recorded from groundwater discharge and surface-water runoff are predicted to continue for thousands to tens of thousands of years.
Any physical disturbance of the tailings which involves oxidation will have the potential to significantly lower the pH and raise the metal content of the associated leachate. It is evident that high concentrations of copper, iron, aluminium, silicon, manganese, zinc, cobalt and nickel can be readily mobilised from oxidised tailings material by acidic fluids, and that such fluids are routinely generated by natural infiltration processes. Under some circumstances, however, it may be possible to relocate a portion of the tailings from one subaqueous site to another without exacerbating metal or acid release.
The installation of low permeability, reactive substrates (clay+calcium/magnesium carbonate+organic matter) on the sediment banks prior to revegetation is predicted to assist with decreasing groundwater discharges, decreasing surface-water/tailings interaction, and developing sustainable revegetation programs. Enhancing and extending naturally occurring bioremediation processes in the delta is considered to be one of the most cost-effective methods for improving the quality of groundwater discharges from the delta. This may be achieved by inundating dry sediment with water and providing organic matter to promote the growth of sulphate reducing bacteria.