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PROJECT No. 5 of the Mount Lyell Remediation
Research and Demonstration Program
Characterisation and impact assessment of mine tailings
in the King River system and delta, Western Tasmania.
Jeff R. Taylor*, Tamie R. Weaver**, D. C. “Bear” McPhail*** and Nigel C. Murphy*
*Earth Systems Pty. Ltd., P.O. Box 57, Armadale, Victoria, Australia, 3143.
**School of Earth Sciences, VIEPS, The University of Melbourne,
Parkville, Victoria, Australia, 3052.
***The Department of Earth Sciences, VIEPS, Monash University,
Clayton, Victoria, Australia, 3168.
THE TASMANIAN DEPARTMENT OF
ENVIRONMENT AND LAND MANAGEMENT
THE OFFICE OF THE SUPERVISING SCIENTIST
Ninety-seven million tonnes (Mt) 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 potentially dangerous concentrations of metals and acid that are 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.
Field work consisted of installing mini-piezometers (groundwater monitoring devices), measuring hydraulic head and water chemistry, and sampling sediment, groundwater and surface water for later chemical analysis. Two types of sediment bank were identified: high mounded banks upstream and relatively low flat-topped banks downstream. Two examples of each type of bank, as well as specific locations on the north and south lobes of the King River delta were targeted for study. Sediment and water samples were examined 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 compositions of the groundwater in the sediments.
The tailings contain rock, crystal and slag fragments, generally ranging in size from 0.01 mm 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 5-7 % by weight of pyrite and about 0.16 % by weight of copper, while the sediment banks are estimated to contain 2-3 % 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. For example, 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. Preliminary estimates indicate that almost complete oxidation of pyrite in permanently unsaturated tailings takes place in one to four years. 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.
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.
The 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 discharge areas. The hydraulic conductivity of tailings deposits in the sediment banks and delta is high. Groundwater fluxes are approximately 20, 40 and 50 litres/day/square metre for the high mounded sediment banks, relatively low flat-topped banks and delta, respectively. Examples of copper and acid fluxes into the King River and Macquarie Harbour are 4.5 kg copper/day and 155 kg sulphuric acid equivalent/day. 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 these estimates, 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. Such a release is likely to have significant ecological consequences in a pristine river system; however, in the King River system this represents only 1-5 % 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 programmes. 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.
EXECUTIVE SUMMARY ii
PREVIOUS WORK 2
FIELD WORK 6
Sediment Sampling 7
Banks / Delta 7
River Bottom 8
Auger and Grab Samples 8
Groundwater Monitoring Network 8
Piezometer Design, Construction and Installation 9
Piezometer Monitoring 9
Groundwater and Surface Water Sampling and Field Analysis 10
Field Analytical Procedures 11
LABORATORY ANALYTICAL WORK 12
Water Chemistry 12
Sediment Chemistry AND MINERALOGY 12
Preparation Procedures 12
Bulk Chemical Analysis 12
Electron Microprobe Analysis 13
In Lens Field Emission SEM 13
Leach Tests 14
Deionised Water and Dilute Sulphuric Acid 14
Ammonium Acetate 14
Morphology and Geology of the Delta 16
Morphology and Geology of River Banks 17
King River - Bottom Sediments 18
Microbial Activity 18
Pyrite Oxidation Rate 19
DIMENSIONS OF TAILINGS DEPOSITS 20
Analytical Data 21
Acid Generation 23
Sediment Mineralogy and Mineral Chemistry 24
Pre-Mine Sediments 24
Mine Tailings 24
Saturated Tailings 28
Unsaturated Tailings 29
Secondary Phases 30
Leach Tests 31
Distilled Water 31
Dilute Sulphuric Acid 32
Ammonium Acetate 33
Sources of Metals in Groundwater 33
Parameters for Groundwater Flow Modelling 34
Approaches to Modelling 36
Darcy Flow Modelling 36
Two-Dimensional Steady-State Groundwater Flow and Solute Transport Modelling 37
Bank H 38
South Lobe of Delta - Perpendicular to the King River 39
Conclusions from Hydrogeological Modelling 39
Quality of the Analytical Data for Groundwater Samples 41
Predominant Aqueous Species 42
Mineral Saturation States 44
North Lobe of the Delta 44
South Lobe of the Delta 45
Bank D 45
Bank H 45
Bank N 45
Bank R 45
Controls on Groundwater Composition 46
Acid Production 46
Current Impact of Tailings on Water Quality 48
Metal and Acid Fluxes from Groundwater Discharge 48
Metal and Acid Fluxes from Surface Runoff 49
Conclusions on Metal and Acid Fluxes 50
Predicted Impact of Tailings on Water Quality 50
Predicted Impact of Severe Drought 51
Predicted Impact of Physical Disturbance 51
Acid Neutralisation 52
Implications of Study for Revegetation 52
POTENTIAL REMEDIAL MEASURES 53
Sediment Banks 53
Considerations for Future Work 54
APPENDIX 1 - Sediment Sample Descriptions and Locations 62
APPENDIX 2 - Geological sections of drillholes 67
APPENDIX 3 - Spectra from XRD Analysis 69
APPENDIX 4 - Analytical data from Microprobe Analysis 76
APPENDIX 5 - Spectra from IFESEM Analysis 113
APPENDIX 6 - Hydrogeological Modelling 116
APPENDIX 7 - Geochemical Modelling 124
APPENDIX 8 - Bibliography 150
LIST OF FIGURES
Figure 1: Location Plan showing the King River Catchment below Lake Burbury.
Figure 2: Location of surface sampling traverse lines and drillhole on the King River Delta.
Figure 3: Location and designation of the King River sediment banks.
Figure 4: Piezometer locations on the King River sediment banks.
Figure 5: Location of drillholes by Helen Locher (CRC-Catchment Hydrology).
Figure 6: Piezometer locations on the King River Delta.
Figure 7: Design of Piezometers.
Figure 8: Cross section of Bank R showing piezometers.
Figure 9: Cross section of Bank N showing piezometers.
Figure 10: Cross section of Bank N showing piezometers.
Figure 11: Cross section of Bank H showing piezometers.
Figure 12: Cross section of Bank D showing piezometers.
Figure 13: Longitudinal section of Bank D showing piezometers.
Figure 14: Cross section of South Delta Lobe showing piezometers perpendicular to river.
Figure 15: Cross section of South Delta Lobe showing piezometers perpendicular to harbour.
Figure 16: Miscellaneous piezometers from the South Delta Lobe.
Figure 17: Cross section of North Delta Lobe showing piezometers perpendicular to harbour.
Figure 18: Cross section of North Delta Lobe showing piezometers near harbour.
Figure 19: pH of the upper layer of groundwater in the King River Delta.
Figure 20: Electrical conductivity of the upper layer of groundwater in the King River Delta.
Figure 21: Redox potential of the upper layer of groundwater in the King River Delta.
Figure 22: Regional annual rainfall for the study area.
Figure 23: Location of regional water monitoring stations.
Figure 24: Surface characteristics of the King River Bed.
Figure 25: Graphical analysis of hydraulic head for the delta and banks.
Figure 26: Bank R - hydraulic gradient and fluxes.
Figure 27: Bank N -hydraulic gradient and fluxes.
Figure 28: Bank H - hydraulic gradient and fluxes.
Figure 29: Bank D - hydraulic gradient and fluxes.
Figure 30: South lobe of delta - hydraulic gradient and fluxes perpendicular to river.
Figure 31: South lobe of delta - hydraulic gradient and fluxes perpendicular to harbour.
Figure 32: North lobe of delta - hydraulic gradient and fluxes perpendicular to harbour.
LIST OF TABLES
Table 1: Field data from piezometers.
Table 2: Groundwater and surface water chemistry - Field and laboratory parameters.
Table 3: Groundwater and surface water chemistry - Analytical results.
Table 4: Sediment chemistry.
Table 5: Analytical results from distilled water leach.
Table 6: pH and EC results from the distilled water leach.
Table 7: Analytical results from dilute sulphuric acid leach.
Table 8: pH and EC results from the dilute sulphuric acid leach.
Table 9: Analytical results from ammonium acetate leach.
Table 10: pH, EC and Eh results from the ammonium acetate leach.
Table 11: Dimensions of tailings deposits.
Table 12: Hydraulic conductivity values.
Table 13: Slug test calculations.
Table 14: Mass transfer calculations.
LIST OF PLATES
Plate 1: View from the north-west margin of the south lobe of the delta.
Plate 2: View of the downstream end of Bank N.
Plate 3: View of Bank H looking downstream.
Plate 4: View of Bank D looking upstream.
Plate 5: View of the installation of DEL-C1 on the north lobe of the delta.
Plate 6: Vertical profile through part of the unsaturated zone in Bank H.
Plate 7: Typical surface expression of foresets on the south lobe of the delta.
Plate 8: View looking up the King River on Bank H.
Plate 9: View of fresh sulphidic tailings on the surface of Bank D.
Plate 10: View of the downstream end of Bank H during a period of heavy rainfall.
Plate 11: View of the discrete occurrence of a gaseous emission from the delta.
Plate 12: View of groundwater seepage from the delta.
Plate 13: Reflected light photomicrograph of sample DT-1.
Plate 14: Reflected light photomicrograph of sample DT-1.
Plate 15: Reflected light photomicrograph of sample DEL-WS3-S.
Plate 16: Reflected light photomicrograph of sample DEL-WS3-S.
Plate 17: IFESEM photomicrograph of sample DT-1.
Plate 18: IFESEM photomicrograph of sample DEL-WS12-S.
Plate 19: IFESEM photomicrograph of sample DT-1.
Plate 20: IFESEM photomicrograph of sample DT-1.
Plate 21: IFESEM photomicrograph of sample DEL-S9.
Plate 22: IFESEM photomicrograph of sample DEL-WS5-S.
Plate 23: IFESEM photomicrograph of sample DEL-WS12-S.
Plate 24: IFESEM photomicrograph of sample DEL-WS12-S.
Plate 25: IFESEM photomicrograph of sample D-S-8.
Plate 26: IFESEM photomicrograph of sample DEL-WS5-S.
Plate 27: IFESEM photomicrograph of sample DEL-WS5-S.
Plate 28: IFESEM photomicrograph of sample DEL-WS12-S.
Plate 29: IFESEM photomicrograph of sample DEL-WS12-S.
Plate 30: IFESEM photomicrograph of sample DEL-WS12-S.
Plate 31: IFESEM photomicrograph of sample DEL-WS12-S.
Plate 32: Back Scattered Electron and X-Ray mapping image from microprobe analysis.
Plate 33: X-Ray mapping image from microprobe analysis.
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