AHL Tenth Legion Project: Water Quality
V2 18 May 2021
Report prepared for Australian Hualong Pty Ltd L Koehnken Pty Ltd AHL Tenth Legion Mining Project – Water Quality Component
DOCUMENT TYPE: Report
TITLE: Tenth Legion Project: Water Quality
VERSION: V2
CLIENT: Australian Hualong Ltd
PREPARED BY: Lois Koehnken V1 05/02/2021 V2 9/06/2021
DISTRIBUTED TO: JMP & Partners Electronic: .doc
L Koehnken Pty Ltd 2 18 May 2021 AHL Tenth Legion Mining Project – Water Quality Component
Contents 1 Water quality...... 7 1.1 Overview of river catchments and water quality ...... 7 1.1.1 Setting...... 7 1.2 Water quality characteristics – Kynance and Piney Creeks ...... 9 1.3 Water quality in Dent Creek ...... 13 1.4 Biological monitoring in the creeks ...... 17 1.5 Comstock Creek...... 19 1.5.1 Upper and Middle Comstock Creek...... 21 1.5.2 Allisons Pit ...... 21 1.5.3 Main Adit ...... 23 1.5.4 Other inflows to TSF ...... 26 1.5.5 Discharge from TSF via Polishing Pond...... 27 1.5.6 Fluxes from Polishing Pond to Comstock Creek ...... 31 1.5.7 Lower Comstock Creek ...... 32 1.6 Composition of sludge in TSF ...... 35 1.7 Groundwater ...... 38 1.7.1 Groundwater quality at Comstock mine site...... 40 2 Water quality management on site...... 42 2.1 Phase 1 water balance...... 43 2.2 Discharge limits for Dent Creek...... 44 2.3 Proposed Discharge Criteria for Comstock Polishing Pond in Phase 2...... 47 3 Proposed monitoring...... 49 References...... 56
Figures
Figure 1-1. Waterways in the area of the Tenth Legion Iron Ore Mine...... 8 Figure 1-2. Map showing Kynance, Dent and Comstock Creeks, and wetland area at confluence of Dent and Kynance Creeks...... 9 Figure 1-3. Geology of the Tenth Legion area...... 10 Figure 1-4. Continuous (15-minute) flow in Kynance Creek at the Trial Harbour Road...... 10 Figure 1-5. Mineral occurrences and known adits in the Dent Creek sub-catchment...... 13 Figure 1-6. Mercury news article about vandalism at the Kynance mine site noting ‘one of the tunnels’...... 14
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Figure 1-7. Acid drainage emanating from historic mine workings near the Kynance Mine (see Figure 1-5 for location)...... 14 Figure 1-8. (left) Total zinc and (right) sulphate and flow in Dent Creek upstream of the highway. ...15 Figure 1-9. Monitoring sites in Dent Creek...... 16 Figure 1-10. Measured discharge in Kynance Creek and the Dent sub-catchment on monitoring days...... 17 Figure 1-11. Summary of biological monitoring results near the Tenth Legion project site...... 18 Figure 1-12. SIGNAL results for Dent Creek at Highway. From Kanunnah Pty Ltd (2021)...... 19 Figure 1-13. Features on the AHL mining lease. The redlines indicate known underground workings...... 20 Figure 1-14. Water quality monitoring locations in Comstock Creek...... 20 Figure 1-15. Schematic of relationship between Allisons Open Pit, the underground workings and the location of the Main Adit...... 22 Figure 1-16. (top left) Allisons opencut showing surface runoff (top right) discharge of water into shaft tunnel (bottom left) discharge of Allisons Pit and underground water from Main Adit...... 23 Figure 1-17. Daily discharge from Main Adit July 2020 to Jan 2021 based on water height in V-notch. Orange squares indicate water quality sampling dates in July 2020 to January 2021 ...... 23 Figure 1-18. pH and EC in Main Adit discharge...... 24 Figure 1-19. (left) Concentrations of total and dissolved zinc, lead and sulphate, and (right ) Total suspended solids at Main Adit, June 2020 to January 2021...... 25 Figure 1-20. Zinc, lead and sulphate fluxes in Main Adit...... 26 Figure 1-21. Flow in Main Adit and zinc, lead and sulphate loads. Equation and R2 for linear fit trend lines are shown for each data set...... 26 Figure 1-22. Google Earth image of Polishing Pond, Swansea WRD and original Comstock Creek channel and holding pond...... 27 Figure 1-23. Seepage from the Swansea WRD flowing into the TSF...... 27 Figure 1-24. (left) Discharge from Polishing Pond at V-notch weirs (right) calculated discharge from Polishing Pond compared to flow at Main Adit...... 28 Figure 1-25. Time-series of discharge from the TSF (via the Polishing Pond) with (left) daily pH and (right) EC values...... 28 Figure 1-26. Total and filtered zinc and pH in the discharge from the Polishing Pond...... 30 Figure 1-27. Photo of shoreline of Polishing Pond showing high level of turbidity due to poor settling of metal hydroxides...... 30 Figure 1-28. (left) Depth (m) to surface of sludges in TSF and Polishing Pond as determined by Aquatic Science in July 2020 (right) disturbance of sludge by dog...... 31 Figure 1-29. Comparison of zinc, sulphate and lead fluxes in Main Adit and in the discharge from the Polishing Pond...... 32 Figure 1-30. Lower Comstock Creek showing extensive deposits of iron-rich oxy/hydroxides. Photo by Aquatic Science...... 33 Figure 1-31. Discharge from the Polishing Pond and at Lower Comstock Creek. Highest flow in Comstock Creek is calculated based on preliminary rating curve. Note log scale...... 33 Figure 1-32. (left) Sulphate and (right) zinc in the discharge from the Polishing Pond and in the Lower Comstock. Note scales are different for each site in each graph...... 34 Figure 1-33. Sampling locations in the TSF and Polishing Pond for sludge investigation completed by Aquatic Science...... 36 Figure 1-34. Distribution of zinc (mg/kg DMB) sludge in the Tailings Dam and Polishing Pond...... 37 Figure 1-35. Distribution of calcium (mg/kg DMB) in sludge in the Tailings Dam and Polishing Pond.37
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Figure 1-36. Vertical drill holes and groundwater ores near Tenth Legion. Red dots shows drill holes where groundwater samples were collected in May 2020...... 39 Figure 1-37. Locations of groundwater bores on the Comstock site...... 41 Figure 1-38. Time-series of groundwater quality in bores ZZ1 - ZZ4...... 41 Figure 3-1. Surface water monitoring locations at Tenth Legion mine site. ADD Groundwater sites? ...... 53 Figure 3-2. Ambient monitoring sites in the Piney, Kynance and Dent Creek catchments...... 54 Figure 3-3. Surface water monitoring locations on the Comstock lease site...... 55 Figure 3-4. Groundwater bore monitoring locations at the Comstock mine site.Error! Bookmark not defined.
Tables
Table 1. Summary of water quality results from upper Piney Creek, March 2020 to January 2021....11 Table 2. Summary of water quality results from Kynance Creek March 2020 to January 2021...... 11 Table 3. Water quality results from Dent Creek (unnamed tributary of Kynance Creek) March 2020 to Mar 2021...... 12 Table 4. Comparison of median values for select water quality parameters in the Piney, Kynance and Dent Creeks...... 13 Table 5. Summary of water quality monitoring at historic Kynance mine site, Dent Creek at Highway, and Upper Dent Creek (See...... 16 Table 6. Summary of biological monitoring completed at Tenth Legion...... 17 Table 7. Water quality monitoring results from Comstock Creek upstream of mine site. Sample collected March 2020, river flow was 5 L/s. Results are for total metal...... 21 Table 8. Median water quality results from Middle Comstock Creek collected March 2020 - Jan 2021. Results are for total metal, n=9...... 21 Table 9. Historic (2001 – 2006) water quality results from Allisons Pit. Results are for total metal, n= 19 or 20...... 23 Table 10. (Top rows) Median and maximum water quality results from monthly sampling (June 2020 to Jan 2021) at Main Adit. For EC and pH n = 187, for other parameters n = 7 (bottom rows) Maximum dissolved metal concentrations for 3 samples collected in June and October 2020 and January 2021...... 25 Table 11. Median concentrations of parameters in Swansea WRD seepage that episodically enters the Polishing Pond. n=5...... 27 Table 12. Median and maximum concentrations for parameters in Polishing Pond discharge. Top rows summarise total metal and metalloid results, bottom rows summarise dissolved (0.45µm) results. Based on 9 samples collected between March 2020 and January 2021...... 29 Table 13. Median and maximum concentrations of major ions, total nutrients and total suspended solids in the discharge from the TSF. n=9 for all parameters except fluoride (n=3)...... 29 Table 14. Median and maximum concentrations for parameters in Lower Comstock Creek. Top rows summarise total metal and metalloid results, bottom rows summarise dissolved (0.45µm) results. .35 Table 15. Median and maximum concentrations of major ions, nutrients and TSS. n=9 for all parameters except fluoride (n=3)...... 35 Table 16. Summary of sludge and supernatant water from sludge samples collected from the Tailings Dam and Polishing Pond. Metal concentrations are for dissolved metals in the supernatant...... 36 Table 17. Summary of water quality results from groundwater samples collected on 18 May 2020 by W.C Cromer Pty Ltd...... 39
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Table 18. Stages of development by AHL at Tenth Legion and Comstock mine sites...... 43 Table 19. Summary of flows from AHL mine site and in the receiving environment...... 44 Table 20. Proposed investigation and regulation levels for discharge from the AHL Tenth Legion mine site...... 45 Table 21. Summary of water quality in Dent Creek...... 46 Table 22. Proposed investigation and regulation levels for discharge from Comstock Polishing Pond for Phase 2 of the operation...... 48 Table 23. Proposed monitoring at the AHL and Comstock mine sites following commencement of operations at AHL...... 50
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1 Water quality This report forms part of the body of information related to the development of the Tenth Legion iron ore deposit near Zeehan Tasmania. It is a stand alone document, but prepared with the intention of being incorporated as a chapter in the EIS. It addresses the key issues of water quality, and many of the requirements listed under section 6.2 of the EIS guidelines. 1.1 Overview of river catchments and water quality 1.1.1 Setting The proposed AHL Tenth Legion Iron Ore Mine is located on the divide between Piney and Comstock Creeks (Figure 1-1). The Piney River flows northward and enters Lake Pieman, an artificial lake managed for the production of hydropower. There is no development in the Piney catchment except for the Heemskirk Road that crosses the lower catchment near Lake Pieman. Comstock Creek flows towards the south, and joins the Little Henty River. The Comstock catchment has been a mining centre for over a century, with historic working within the footprint of the proposed project, and throughout the catchment.
The Tenth Legion lease includes the headwaters of Comstock Creek, which flows eastward from the proposed project site, and is not affected by the development, before flowing south and entering the existing Comstock mining lease (Figure 1-2). Two other small tributaries drain the western side of the Tenth Legion site. The larger tributary, Kynance Creek skirts the western edge of the mining lease before joining Comstock Creek south of the Trial Harbour Road within the mining lease for the Avebury nickel project. The second tributary, an eastern, branch of Kynance which has been named Dent Creek by the project team, drains the project site and joins Kynance Creek a short distance downstream (south) of the Trial Harbour Road, also within the Avebury lease. This tributary has been named Dent Creek by the AHL project team. The lower reaches of Dent Creek are characterised by a broad marshy area, that may have been enhanced by the development of the Trial Harbour Road.
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Piney Ck
Kynance Dent Ck Ck AHL Mine Lease
Comstock Ck
Avebury Mine Leases
Little Henty
Figure 1-1. Waterways in the area of the Tenth Legion Iron Ore Mine. Red arrows indicate waterways that will be directly affected by the proposed development. Blue arrows show waterways near the development. Red dot indicates water quality monitoring location in Piney Creek, and green triangle shows approximate location of historic adit (see next map for monitoring locations in Kynance and Dent.
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Figure 1-2. Map showing Kynance, Dent and Comstock Creeks, and wetland area at confluence of Dent and Kynance Creeks. Red dots indicate ambient monitoring sites in Kynance and Dent Creeks upstream of highway bridge, and in Piney Creek.
The proposed pit and waste rock dump will be located predominantly within Dent Creek as shown by the footprint in Figure 1-2. The project will include a small area within the upper Piney Creek catchment. Upper Piney Creek will be diverted if possible around the development to minimise any impact on the northward flowing creek, with all diffuse drainage retained within the Dent catchment. The water pumped from the pit, along with stormwater runoff from the site and the waste rock dump will be collected via drains and directed to settling ponds. If the water quality of the pit water is sufficiently good for direct discharge, it will be discharged to Dent Creek. If the water is not suitable for direct discharge, it will be piped back to the existing Comstock mine for treatment and discharge via the existing tailings dam and polishing pond (See Section 1.5). 1.2 Water quality characteristics – Kynance and Piney Creeks The Kynance Creek catchment is underlain by granitic rocks and the Oonah formation, which is characterised by silica-rich quartzwacke turbidite sequences (Figure 1-3). The upper Piney Creek also drains a large area of the Oonah formation, and the crest of the divide between the two catchments is composed of Pleistocene alluvial and lacustrine sand, gravels and mud. These units support very thin organic rich soils, and produce river waters typical of the west coast – low pH, naturally elevated aluminium and iron, organic acid rich and low in other metals and sulphate (Table 1, Table 2). Flows in the two creeks are similar at the monitoring locations, with median flows of 80 – 90 L/s in each waterway, and maximum measured flows of 385 L/s in Piney and 587 L/s in Kynance on monitoring days.
Continuous (15-minute) flow results from Kynance Creek at the Trial Harbour Road recorded between September 2017 to September 2019 show that flow is characterised by short duration high flow events, with a median flow of 108 L/s and an 90th percentile flow of 602 L/s (Figure 1-4).
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Comparing these values with the recorded flowrates during sampling suggesting that the water quality monitoring is capturing the typical range of flow conditions in the creeks.
Flow in Kynance Creek increases rapidly below the gauging site due to the inflow of Dent Creek (discussed below), and Contiguous Creek, which joins Kynance less than 100 m downstream of the confluence of Dent and Kynance Creeks.
Piney
Dent
Kynance
Figure 1-3. Geology of the Tenth Legion area. Dga = granites, Lo=quartzwackes, Qh = quaternary sand and gravel deposits, Cb & Cbtg = mafic volcanics. From TheList Map.
Figure 1-4. Continuous (15-minute) flow in Kynance Creek at the Trial Harbour Road.
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Table 1. Summary of water quality results from upper Piney Creek, March 2020 to January 2021. Results reported as less than the laboratory reporting limited have been included as 50% of the LoR value.
ot
tot tot tot
tot
8.3
tot
tot
tot tot
tot
pH tot tot tot
tot ganese
nesium tot mium omium. g n enic balt r d lcium s ckel a a uminium. t itrogen l r a hloride tot h opper hosphorus luoride Statistic Acidity Alkalinity A A C Ca C C Co EC Field C DOC Flow F Iron Lead M M Ni N pH field P Sodium Sulphate TSS (0.45um) Zinc tot
3 Unit mg/l mg/l µg/l µg/l µg/l mg/l mg/l µg/l µg/l µS/cm µg/l mg/l m /s mg/l µg/l µg/l mg/l µg/l µg/l mg/l pH mg/l mg/l mg/l mg/l µg/l Maximum 9 4 525 10 1 0.90 13.1 1 1.5 159 1 15 0.385 0.03 409 2.5 1.4 13 5 0.4 6.52 0.005 7.3 1.4 7 3 90th Percentile 8 3.3 452 10 1 0.82 12.2 1 1.5 84 1 14 0.245 0.03 409 2.5 1.4 12.2 5 0.4 6.43 0.005 6.8 1.4 4 2.6 80th Percentile 7 2 434 10 1 0.72 11.8 1 1.5 62 1 14 0.105 0.03 398 3 1.3 11.4 5 0.4 6.09 0.005 6.6 1.3 3 2.2 Median 5.5 0.5 410 10 1 0.45 10.1 1 1.5 53 1 13 0.087 0.03 268 3 0.9 8 5 0.3 5.69 0.005 5.9 1 3 1 20th Percentile 4 0.5 352 10 1 0.34 9.7 1 1.5 46 1 11 0.074 0.03 231 3 0.8 6.6 5 0.3 5.18 0.005 5.8 1 2 1 10th Percentile 3.5 0.5 348 8 0.8 0.34 9.2 1 1.3 43 0.9 11 0.064 0.03 208 2.1 0.8 5.8 4.1 0.3 4.91 0.005 5.6 0.9 0.5 1 Minimum 3 0.5 345 0.5 0.05 0.34 8.4 0.5 0.5 41 0.5 10 0.055 0.03 198 0.25 0.8 5 0.5 0.3 4.6 0.005 5.2 0.5 0.5 1 Count 6 8 9 9 9 8 9 9 9 9 9 4 6 3 9 9 9 9 9 4 9 4 8 9 9 9 Limit or 2 2 20 2 2 3 1 2 0.05 10 10 0.1 0.01 1 1 2 Reporting*
Table 2. Summary of water quality results from Kynance Creek March 2020 to January 2021
8.3
tot
pH
luminium. tot Statistic Acidity Alkalinity A Arsenic tot Cadmium tot Calcium tot Chloride tot Chromium. tot Cobalt tot EC Field Copper tot DOC Flow Fluoride Iron tot Lead tot Magnesium tot Manganese tot Nickel tot Nitrogen tot pH field Phosphorus tot Sodium Sulphate TSS (0.45um) Zinc tot
Unit mg/l mg/l µg/l µg/l µg/l mg/l mg/l µg/l µg/l µS/cm µg/l mg/l m3/s mg/l µg/l µg/l mg/l µg/l µg/l mg/l pH mg/l mg/l mg/l mg/l µg/l Maximum 7 10 532 10 1 2.21 16.4 1 1.5 92.6 1 170* 0.547 0.03 398 5 2.7 25 5 0.4 7 0.03 9.5 3.0 21 14 90th Percentile 6.5 6.5 521 10 1 1.713 15.12 1 1.5 90.52 1 123.5 0.531 0.03 356 5 2.1 20.2 5 0.4 6.97 0.023 8.9 2.3 11.4 12.4 80th Percentile 6 4.6 478 10 1 1.336 14.32 1 1.5 79.44 1 77 0.497 0.03 336 5 1.7 17.2 5 0.3 6.86 0.015 8.2 2.0 6 9.6
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8.3
tot
pH
luminium. tot Statistic Acidity Alkalinity A Arsenic tot Cadmium tot Calcium tot Chloride tot Chromium. tot Cobalt tot EC Field Copper tot DOC Flow Fluoride Iron tot Lead tot Magnesium tot Manganese tot Nickel tot Nitrogen tot pH field Phosphorus tot Sodium Sulphate TSS (0.45um) Zinc tot
Median 6 3 425 10 1 0.94 12.1 1 1.5 62.6 1 13.5 0.081 0.03 294 5 1.3 14 5 0.3 6.27 0.005 7.0 1.7 2 7 20th Percentile 5 1 399 10 1 0.772 11.46 1 1.5 53.68 1 9.24 0.049 0.03 278 5 1.2 10.8 5 0.3 5.95 0.005 6.6 1.5 1.4 6 10th Percentile 4 1 360 8.1 0.81 0.751 11.1 0.9 1.3 52.26 0.9 7.17 0.032 0.03 271 4.1 1.2 9 4.1 0.3 5.93 0.005 6.4 1.2 0.5 5.3 Minimum 3 1 235 0.5 0.05 0.73 11.1 0.5 0.5 49.7 0.5 5.1 0.020 0.03 249 0.5 1.1 9 0.5 0.3 5.75 0.005 6.1 0.5 0.5 2.5 Count 6 8 9 9 9 8 9 9 9 9 9 4 8 3 9 9 8 9 9 4 10 4 8 9 9 9 *This concentration is higher than naturally occurring concentrations in West Coast waters. The laboratory confirms the results but it is suspected to be an error. The next highest value is 15mg/L.
Table 3. Water quality results from Dent Creek (unnamed tributary of Kynance Creek) March 2020 to Mar 2021.
tot
8.3
tot
pH
luminium. Statistic Acidity Alkalinity A Arsenic tot Cadmium tot Calcium tot Chloride tot Chromium. tot Cobalt tot EC Field Copper tot DOC Flow Fluoride Iron tot Lead tot Magnesium tot Manganese tot Nickel tot Nitrogen tot pH field Phosphorus tot Sodium Sulphate TSS (0.45um) Zinc tot
Unit mg/l mg/l µg/l µg/l µg/l mg/l mg/l µg/l µg/l µS/cm µg/l mg/l m3/s mg/l µg/l µg/l mg/l µg/l µg/l mg/l pH mg/l mg/l mg/l mg/l µg/l Maximum 16 16 220 10 1 4.0 21.4 2 1.5 140 4 4.2 0.248 0.03 276 5 4.5 11 5 0.6 6.94 0.03 12.2 9.2 5.0 181 90th Percentile 9.4 15.2 166 10 1 3.82 19.2 2 1.5 124 3.1 4.0 0.166 0.03 195 5 4.4 9.4 5 0.4 6.84 0.023 11.7 9.0 5.0 164 80th Percentile 5 15 141.6 10 1 3.44 18.9 2 1.5 121 3 3.8 0.086 0.03 176 5 3.9 9.0 5 0.3 6.78 0.015 11.5 8.5 3.4 148 Median 5 12 115.5 10 1 2.80 17.6 2 1.5 113 3 3.4 0.026 0.03 139 5 3.3 6.0 5 0.1 6.71 0.005 10.1 7.5 2.0 129 20th Percentile 3.2 8.2 74.6 10 1 2.10 13.8 2 1.5 86 3 3.0 0.023 0.03 108 5 2.5 4.0 5 0.1 6.63 0.005 8.5 5.6 1.8 112 10th Percentile 3 7 68.9 8.1 0.9 2.04 12.9 1.8 1.3 81 3 2.9 0.016 0.03 105 4.1 2.4 2.5 4.4 0.1 6.59 0.005 8.4 4.6 1.0 106 Minimum 3 7 68 0.5 0.3 1.91 10.0 1 0.5 79 3 2.7 0.015 0.03 97 0.5 2.3 2.5 2 0.1 6.35 0.005 8.2 1.1 1.0 105
Count 7 9 10 10 10 9 10 10 10 11 10 4 10 3 10 10 9 10 10 4 11 5 9 10 10 10 *Note on water quality results: The first samples collected were analysed to very low levels to provide an understanding of all potential parameters of concern. Subsequent analyses have been completed by methods with higher laboratory limits of reporting. This accounts for many of the minimum values being lower than 50% of the laboratory limit of reporting.
L Koehnken Pty Ltd 12 18 May 2021 1.3 Water quality in Dent Creek Dent Creek will be the receiving waters for the discharge from the Tenth Legion project site. The geology of Dent Creek differs considerably from Kynance and Piney Creek, with the Dent sub- catchment underlain by mafic volcanic rocks (Figure 1-3) which is the source of the Tenth Legion ore body. These differences affect the fundamental water quality of Dent Creek, and result in it having different properties as compared to the Piney or Kynance Creeks. Median values for parameters that show substantial differences between the waterways are summarised in Table 4, with Dent at the highway having higher EC and concentrations of alkalinity, calcium, magnesium, and sulphate and lower concentrations of aluminium and DOC.
The catchment contains a high density of known iron ore deposits, and abandoned historic mining sites and adits (Figure 1-5). Investigations by AHL has identified that seepage from the historic Kynance Mine continues to discharge to Dent Creek, resulting in elevated levels of zinc and sulphate in the waterway (Table 4).
Table 4. Comparison of median values for select water quality parameters in the Piney, Kynance and Dent Creeks.
EC pH Calcium Magnesium Alum. Zinc Site Flow Alkalinity DOC Sulphate Field field tot tot tot tot pH Units m3/s µS/cm mg/L mg/L mg/L mg/L mg/L µg/L µg/L units Piney 0.087 53.0 5.69 0.5 0.45 0.9 13.0 1.0 410 1 Kynance 0.081 62.6 6.27 3.0 0.94 1.3 13.5 1.7 425 7 Dent 0.026 113 6.75 11 2.50 3.0 3.6 7.3 134 124
Dent
Figure 1-5. Mineral occurrences and known adits in the Dent Creek sub-catchment. All mineral occurrences are associated with the presence of magnetite. Map from theList Tasmania.
The extent of the workings at the Kynance mine site are undocumented, and the workings have collapsed preventing detailed investigation. The area is also overgrown which makes identifying potential water inflows to the workings difficult. A Mercury news article from 10 Feb 1923 stated that steel was stolen from one of the tunnels, suggesting there was more than one (Figure 1-6). Table 5. Summary of water quality monitoring at historic Kynance mine site, Dent Creek at Highway, and Upper Dent Creek (See
Statistic pH EC (uS/cm) SO4 (mg/L) TSS (mg/L) Acid. (mg/L) Alk Tot (mg/L) Ca tot (mg/L) Mg tot (mg/L) Al tot (ug/L) As tot (ug/L) Cd tot (ug/L) Cr tot (ug/L) Co tot (ug/L) Cu tot (ug/L) Fe tot (ug/L) Pb tot (ug/L) Mn tot (ug/L) Ni tot (ug/L) Zn tot (ug/L) Kynance Mine 9/02/21 6.23 349 70 6 6 23 15 13 291 <20 2 <2 15 17 6570 70 1210 22 1440 Upper. Dent 21/03/21 24 1 6 7 5 5 84 <20 <2 <2 <3 5 210 <10 33 <10 393 Dent @ HW 9/02/21 6.63 121 8 NA 4 13 3 4 76 <20 <2 <2 <3 4 112 <10 8 <10 159 21/03/21 6.35 121 7 2 6 15 4 4 72 <20 <2 <2 <3 4 102 <10 7 <10 152 Kynance @ HW 9/02/21 5.80 73 <2 <1 7 2 1 2 471 <20 <2 <2 <3 <2 356 <10 16 <10 6 21/03/21 6.23 81 2 2 6 5 2 2 382 <20 <2 <2 <3 <2 390 <10 14 <10 8
Kynance Mine
Upper Dent
Dent @ HW
Kynance @ HW
Figure 1-9. Monitoring sites in Dent Creek. AHL Tenth Legion Mining Project – Water Quality Component
Photos of seepage from the site taken in February 2021 show diffuse flow and extensive iron precipitation (Figure 1-7).
Figure 1-6. Mercury news article about vandalism at the Kynance mine site noting ‘one of the tunnels’..
Figure 1-7. Acid drainage emanating from historic mine workings near the Kynance Mine (see Figure 1-5 for location).
A time series of total zinc, sulphate and flow in Dent Creek (Figure 1-8) suggests that zinc and sulphate concentrations decrease during periods of high flow, resulting in minimum values of about 100 µg/L. During periods of low flow, zinc levels increase, with a maximum recorded value of 180 µg/L. Sulphate shows a similar trend, but concentrations remain low, with all values <10 mg/L. This behaviour likely reflects a relatively constant input of groundwater from the historic Kynance mine, with variable rates of dilution from surface runoff.
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Figure 1-8. (left) Total zinc and (right) sulphate and flow in Dent Creek upstream of the highway. No flow results are available in October due to flow conditions being too high to safely gauge. September gap due to equipment malfunction.
Investigative monitoring in Dent Creek was completed to gain a better understanding of how the mining inputs affect water quality along the length of the creek. Samples were collected from the sites shown in Figure 1-9 in February and March 2021. The results for Dent at Highway are very similar for the two monitoring dates, suggesting that conditions were similar in February and March. Based on this, it is reasonable to assume that the water quality results at the upper two sites can be compared, even though the Kynance mine site was collected in February, and the Upper Dent site in March.
Comparing the concentrations between the sites shows the following:
• Total arsenic, cadmium and chromium concentrations are low at all of the Dent sites • Total iron, manganese and lead concentrations decrease substantially between the Kynance mine site and the Upper Dent site. This suggests the precipitation and deposition of iron oxy/hydroxides as observed downstream of the Kynance workings are also removing other metals • Sulphate, total aluminium, copper, nickel and zinc show a 3 to 4 fold reduction between the Kynance mine site and the Upper Dent • Sulphate, zinc and manganese, show additional ~3-fold reduction between the Upper Dent and Dent at Highway sites • In total, there is about a 10-fold reduction in concentrations of sulphate and zinc between the Kynance mine and Dent at Highway monitoring sites.
The mine plan for Tenth Legion includes the development of the low-grade stockpile over the historic Kynance mine site. Development of the pad for this stockpile will include the partial excavation and covering of the historic workings with acid consuming waste rock. During excavation, any drainage that is found to be entering the historic workings will be diverted, if feasible. AHL aims to decrease the pollutant load from the historic workings through reducing oxygen and water ingress into the underground, whilst increasing the local input of alkalinity to the catchment. These measures are consistent with clause 37.4 of the State Policy on Water Quality Management (1997) which states that where reasonable and practical the current operators of mines should also take steps to reduce the volume or environmental effects of acid drainage from historic workings which they have not carried out, but which are on their mining lease. Such action should be voluntary and not mandated by regulatory authorities.
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The water balance derived for the site (Cromer, 2021) concludes that the input from the Tenth Legion site will contribute approximately 25% of the flow in Dent Creek upstream of the Highway, and the total flow in Dent Creek (about 110 L/s median) contributes about one-third of flow in Kynance Creek downstream of the confluence (Figure 1-10). The highest contribution from Dent occurs during periods of very low flow. The flow contribution is consistent with the relative catchment areas, with Dent comprising about one-third of the combined Dent – Kynance catchment area (Dent ~200 Ha, Kynance above Dent ~400 Ha). The water balance at Tenth Legion with respect to discharge and mixing is discussed in more detail in Section 2, Water quality management on site.
Figure 1-10. Measured discharge in Kynance Creek and the Dent sub-catchment on monitoring days.
1.4 Biological monitoring in the creeks Biological monitoring of the waterways near the Tenth Legion project site were conducted by Walsh (2020, 2021) on the dates and using the survey methods listed in Table 6. The SIGNAL method was used in the Dent because the habitat characteristics were outside of the limits of AusRiv modelling.
Table 6. Summary of biological monitoring completed at Tenth Legion.
Waterway Date Survey Method Result Kynance Creek at 24/03/2020 AusRivs - Autumn O/E = 1.17 Highway Richer than reference Piney Creek 19/03/2021 AusRivs - Autumn O/E = 0.68 Band B= Significantly impaired Dent Creek at Highway 24/03/2020 SIGNAL SIGNAL = 4.9 Mild pollution
The biological monitoring found that Kynance Creek supported more macroinvertebrate families than expected, making it equivalent to a reference site. In contrast, some of the expected macro- invertebrate families were not present in Piney Creek resulting in a score of ‘B’, significantly impaired. This score does not reflect the water quality results collected from the creek, which showed very low metal and sulphate concentrations. It is plausible that the very dilute nature of the water combined with the natural acidity in the creek attributable to the high concentration of AHL Tenth Legion Mining Project – Water Quality Component organic acids is limiting the number of macro-invertebrate communities. It is also possible that due to the sampling site being located near the headwaters of the creek, that flow variations are inhibiting establishment of some of the expected communities.
Figure 1-11. Summary of biological monitoring results near the Tenth Legion project site. Kynance (green) has an AUSRIVs score equivalent to reference condition; Piney has an AUSRIVs score of ‘B’ moderately impacted. Dent Creek was assessed using SIGNAL and shows moderate impacts from pollution. Comstock Creek is considered biologically dead.
The biological condition of Dent Creek at the highway was also investigated in autumn 2020. The habitat characteristics at the site prevented AusRivAS modelling. SIGNAL scoring, a less complex method of assessing the biological condition of a river, was subsequently used and resulted in a score of 4.9, indicative of mild pollution. The combination of the SIGNAL score and the number of macroinvertebrate families found (13) in Dent Creek classified the site within SIGNAL Quadrant 4, near the boundary with Quandrant 3 (Figure 1-12), indicating an upstream pollution source. Walsh interpreted the results as being consistent with known upstream historic mining inputs.
During the 2020 surveys, Walsh visited the lower Comstock monitoring site and found it to be highly degraded, with no evidence of invertebrate life. Based on this observation no additional biological monitoring was completed at the site.
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Figure 1-12. SIGNAL results for Dent Creek at Highway. From Kanunnah Pty Ltd (2021).
1.5 Comstock Creek The headwaters of Comstock Creek drain the eastern slopes of McIvor Hill and the Tenth Legion project site. The historic lead and silver Comstock Mine site is located within the catchment on AHL’s mining lease and the existing AHL EPN requires remediation of the historic workings. Water quality in Comstock Creek is affected by runoff and discharge from Allisons open cut, discharge from an historic adit (Main adit, which drains the open cut and extensive underground workings), the Swansea Waste Rock dump, a tailings pond and a polishing pond (Figure 1-13). The water quality characteristics of each of these inputs is discussed in the following section, along with background water quality in Comstock Creek. Monitoring locations within the Comstock catchment are indicated in Figure 1-14.
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Allisons Pit
Main Adit
Swansea WRD
TSF Polishing Pond
Figure 1-13. Features on the AHL mining lease. The redlines indicate known underground workings.
Upper Comstock
Adit (approx.) Comstock Middle
Outflow Polishing Pond
Comstock Lower
Figure 1-14. Water quality monitoring locations in Comstock Creek. Arrows show the diversion of upper Comstock Creek and discharge pathway from Polishing Pond. Pink areas denote mining leases. 2068 is AHL lease.
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1.5.1 Upper and Middle Comstock Creek Comstock Creek upstream of the mine site includes a network of small tributaries that drain the south eastern slopes of McIvor Hill, which has a similar geology as Dent Creek, and the historic mine workings east of the Trial Harbour Road. At the entrance to the mine site, Comstock Creek is ephemeral. The results of a water quality sample collected near the entrance to the mine site are shown in Table 7. The water is acidic, with low concentrations of acidity and sulphate, and contains elevated concentrations of lead and zinc. These results are similar to water quality results collected from Comstock Creek upstream of the mine site between 2000 and 2006, at historic monitoring site W1. Median pH in the historic data set is 5.0 (n=33), and median lead and zinc values are 119 µ/L and 534 µg/L, respectively (n=27 for both parameters).
The historic W1 site is located upstream of the Comstock site, suggesting that the source of the metals is the upper catchment. Similar to Dent Creek, the water quality likely reflects the natural inputs from the mineralised area combined with discharge from historic workings and adits. The low concentration of sulphate in the 2020 Upper Comstock sample (12 mg/L) suggests that the lead and zinc may not be sourced from active sulphide oxidation, but rather the dissolution of secondary minerals. No sulphate results are available in the 2000 to 2007 data set.
Table 7. Water quality monitoring results from Comstock Creek upstream of mine site. Sample collected March 2020, river flow was 5 L/s. Results are for total metal. pH EC Alk Acid SO4 Al Cd Cu Fe Pb Mn Ni Zn pH µS/cm mg/L mg/L mg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L unit 4.66 100 2 10 12 450 1.5 4 870 129 160 7 615
The Middle Comstock Creek monitoring site is located downstream of the mine site, but upstream of where the discharge from the polishing pond mixes with the creek. The site has been monitored 9 times between March 2020 and January 2021. Comparing the results in Table 8 with Table 7 suggests there is little change between the Upper and Middle sites in the Comstock. This is consistent with AHL’s water management strategy that diverts the creek around sources of contamination. Mine site runoff and discharge from Main Adit (which includes the runoff from Allisons opencut) are neutralised and directed into the tailings dam.
During periods of very high flow, when the discharge from Main Adit exceeds the capacity of the channel transporting the discharge to the tailings dam, the diversion pond at the weir overflows, and water is discharged into the Comstock upstream of the middle monitoring point. This has been observed by project personnel but has not coincided with monitoring.
Table 8. Median water quality results from Middle Comstock Creek collected March 2020 - Jan 2021. Results are for total metal, n=9. pH EC Alk Acid SO4 Al Cd Cu Fe Pb Mn Ni Zn pH µS/cm mg/L mg/L mg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L unit 6.58 107 3.5 4.0 12 299 <2 4 723 58 11 <10 403
1.5.2 Allisons Pit Allisons Pit is located at a high level on the mine site, and is hydraulically connected to the underground workings and Main Adit (Figure 1-15). Allisons Pit is a major source of acid drainage on the site, but the source of the AD is limited, as the host rock around the pit is rich in silica and carbonate. During periods of rainfall, surface runoff flows through the pit and is directed into the
L Koehnken Pty Ltd 21 18 May 2021 AHL Tenth Legion Mining Project – Water Quality Component shaft and is discharged from Main Adit after mixing with flow from the underground workings (Figure 1-16). Historic water quality results (2001 – 2006) are summarised in Table 9 and shows the water is acidic and contains very high concentrations of metals. The pit is not currently being monitored due to the flow being episodic and diffuse, making the collection of representative samples difficult. The water is included in the samples collected at Main Adit (Section 1.5.3).
Once mining commences at the Tenth Legion site, NAF waste rock will be carted to Allisons Pit and used to fill and encapsulate the walls of the pit. This should reduce acid drainage generation through reducing oxygen and water infiltration.
Figure 1-15. Schematic of relationship between Allisons Open Pit, the underground workings and the location of the Main Adit. Source: Groundwater monitoring and Management Plan, Resource & Environmental Management Pty Ltd, 2007.
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Figure 1-16. (top left) Allisons opencut showing surface runoff (top right) discharge of water into shaft tunnel (bottom left) discharge of Allisons Pit and underground water from Main Adit.
Table 9. Historic (2001 – 2006) water quality results from Allisons Pit. Results are for total metal, n= 19 or 20.
pH EC Alk Acid Al As Cd Cu Fe Pb Mn Ni Zn pH µS/cm mg/L mg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L unit 2.6 1340 <1 1340 64300 362 498 1110 267000 473 6300 613 51800
1.5.3 Main Adit Main Adit is the main contaminated water source on site. Flow from the adit is captured and directed to the TSF via a diversion weir. Water level in the weir has been recorded at a V-notch on a daily basis since July 2020 with the results used to determine flowrates (Figure 1-17). Discharge from the adit has ranged from 16 to 60 L/s over the July 20 to January 21 period with the highest discharge associated with 69 mm of rainfall in 2 days in October. Considering the variability of rainfall through the year, discharge from the Adit is very uniform, with 20th to 80th percentile flows of 19 and 24 L/s respectively. This range coincides with the change in seasonal baseflow and suggests that groundwater input dominates the flow, rather than the surface inputs from Allisons Pit.
Figure 1-17. Daily discharge from Main Adit July 2020 to Jan 2021 based on water height in V-notch. Orange squares indicate water quality sampling dates in July 2020 to January 2021
Daily pH values recorded at the Adit range from about 5.8 to 6.4. pH values decrease during short term rainfall events and show seasonal trends with higher values recorded during the drier summer months. (Figure 1-18). This is consistent with surface inflows transporting acidic water, and with groundwater draining carbonate rich bedrock. The EC time-series also show decreased EC values during rainfall events, suggesting that whilst surface inflow is acidic, it does not contain higher solute
L Koehnken Pty Ltd 23 18 May 2021 AHL Tenth Legion Mining Project – Water Quality Component loads as compared to the groundwater. EC values are typically in the range of 600 µS/cm to 800 µS/cm, with the highest values recorded during the summer.
Figure 1-18. pH and EC in Main Adit discharge. Some of the low EC and pH values in July and August 2020 maybe be due to errors in the automatic recording probes.
Median and maximum water quality monitoring results based on samples collected between June 2020 and January 2021 are shown in Table 10 for total metals and other parameters. Compared to the historic results from Allisons Pit, the water quality in Main Adit has considerably higher pH values, reflecting the input of groundwater derived from the silicate/carbonate host rock. Cadmium and copper are present at low levels, but the other metals are elevated, with zinc having very high concentrations. Table 10 also shows maximum dissolved metal concentrations based on 3 samples. Comparing the total and dissolved results shows that aluminium and lead are predominantly present in the particulate phase, with dissolved values being much lower than total. The other parameters show only small differences between the total and filtered results, with is consistent with the pH of the discharge being <6.5, which is insufficient to promote the precipitation of metals as oxy/hydroxides.
These trends are shown in Figure 1-19. The time-series also show that zinc and sulphate concentrations decreased during the wet winter period, consistent with dilution from stormwater runoff, however concentrations of total lead increased in winter. TSS concentrations showed little change during winter, suggesting that particulates higher in lead are being transported during winter. This may reflect surface runoff from Allisons Pit transporting lead rich particulates.
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Table 10. (Top rows) Median and maximum water quality results from monthly sampling (June 2020 to Jan 2021) at Main Adit. For EC and pH n = 187, for other parameters n = 7 (bottom rows) Maximum dissolved metal concentrations for 3 samples collected in June and October 2020 and January 2021.
Median/Max total metal concentrations and other parameters in Main Adit pH EC Alk Acid SO4 Al As Cd Co Cu Fe Pb Mn Ni Zn tot tot tot tot tot tot tot tot tot tot pH µS/cm mg/L mg/L mg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L unit 6.22 707 14 118 297 1820 178 6 51 2 48600 75 7510 97 17100 5.66 1196 44 132 331 2040 230 8 53 11 58000 140 7950 101 18500 (min) Maximum dissolved metal Al As Cd Co Cu Fe Pb Mn Ni Zn concentrations (n=3) dis dis dis dis dis dis dis dis dis dis µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L 406 156 8 51 10 49500 31 8000 97 18300
Figure 1-19. (left) Concentrations of total and dissolved zinc, lead and sulphate, and (right ) Total suspended solids at Main Adit, June 2020 to January 2021.
Lead, zinc and sulphate fluxes for the seven month period have been derived using the water quality and flow results (Figure 1-20). Lead loads range from 1.2 – 2.4 kg/day (median = 2.1 kg/day), zinc ranged from 26 to 63 kg/day (median = 32) and sulphate ranged from 457 – 1150 kg/day (median 569 kg/day). The calculated loads are plotted against flow at Main Adit in Figure 1-21, with each parameter showing a different trend. Zinc loads increase linearly as flow increases, suggesting the availability of zinc does not diminish as flow increases. Sulphate loads decrease as flow increases, consistent with a reservoir of sulphate being generated through oxidation and exhausted over the duration of the high flow. Both zinc and sulphate show good linear trends, although there is a lack of intermediate flow events. In contrast, lead results do not show any trend with flow, and the sample collected during the high flow event did not yield the highest flux. As previously discussed this is likely attributable to total lead being associated with particulates.
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Figure 1-20. Zinc, lead and sulphate fluxes in Main Adit.
Figure 1-21. Flow in Main Adit and zinc, lead and sulphate loads. Equation and R2 for linear fit trend lines are shown for each data set.
1.5.4 Other inflows to TSF In addition to Main Adit, diffuse seepage from the Swansea WRD, and runoff from the site enter the TSF. Some of the drainage from the Swansea WRD enters the abandoned Comstock Creek channel (Comstock Creek has been diverted upstream of this area), and is collected in a small settlement pond (sediment pond in Figure 1-22) north of the Polishing Pond. During periods of high flow, this water can overflow into the Polishing Pond near the discharge point.
Monitoring of the pond has been completed on five occasions, although only once was water flowing into the Polishing Pond, with the flow rate estimated at 1 L/s. The quality of water captured in the pond is summarised in Table 11. Compared to the water in Main Adit, the seepage has lower pH and alkalinity, reflecting the lack of carbonate input in the WRD as compared to the groundwater in the Adit. The seepage contains similar concentrations of copper, lead and nickel, but substantially lower concentrations of the other metals. At 1 L/s the seepage would contribute 0.3 kg/day of zinc to the Polishing Pond whereas a similar volume of water from the Adit would transport 1.5 kg/day of zinc. Other seeps from the Swansea WRD have been observed but have not been regularly monitored for flow or composition (Figure 1-23).
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Swansea WRD Old Comstock Ck
Polishing Pond Outlet Sediment Pond
Inflow from TSF
Figure 1-22. Google Earth image of Polishing Pond, Swansea WRD and original Comstock Creek channel and holding pond.
Table 11. Median concentrations of parameters in Swansea WRD seepage that episodically enters the Polishing Pond. n=5.
Median of total metal concentrations and other parameters in seepage from the Swansea WRD
pH EC Alk Acid SO4 Al As Cd Co Cu Fe Pb Mn Ni Zn tot tot tot tot tot tot tot tot tot tot pH µS/cm mg/L mg/L mg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L unit 3.79 466 <2 30.5 185 1120 <20 5 28 4 2290 84 2830 64 3780
Figure 1-23. Seepage from the Swansea WRD flowing into the TSF.
1.5.5 Discharge from TSF via Polishing Pond The water discharged from Main Adit is diverted out of the Comstock Creek channel, neutralised and directed into the tailings storage facility (TSF). Runoff from the Swansea waste rock dump is also
L Koehnken Pty Ltd 27 18 May 2021 AHL Tenth Legion Mining Project – Water Quality Component directed into the TSF along with other site runoff. Discharge from the TSF enters the Polishing Pond and is ultimately discharged into Comstock Creek. The discharge from the Polishing Pond is regulated under EPN 7797/2, which requires the discharge to be maintained at a minimum pH value of 7.5.
Discharge from the polishing pond is measured via three v-notch weirs at the outflow (Figure 1-24). Flow rates range from about 20 L/s to 110 L/s, with high flow events characterised by short duration and a rapid increase followed by a slower decrease in flow. The base flow of about 20 L/s is consistent with the input from Main Adit, with the higher flows reflecting storm water runoff from the Swansea Dump and remaining mining lease.
Figure 1-24. (left) Discharge from Polishing Pond at V-notch weirs (right) calculated discharge from Polishing Pond compared to flow at Main Adit.
pH in the discharge (Figure 1-25) is dominated by high frequency fluctuations ranging between pH 6 and pH 11. These are caused by the batch dosing of lime to the tailings dam inflow to maintain an elevated pH. In addition to these rapid fluctuations, pH tends to decrease during periods of high inflow / outflow. EC in the discharge ranges from about 500 µS/cm to 800 µS/cm, and is lower during periods of high flow.
Figure 1-25. Time-series of discharge from the TSF (via the Polishing Pond) with (left) daily pH and (right) EC values.
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Median and maximum results from water quality samples collected between March 2020 and January 2021 are summarised in Table 12 and Table 13. The results show that water quality is generally good with the exception of elevated iron, manganese, nickel and zinc, with zinc having the most elevated values. Sulphate is relatively low for a water so intensely affected by mining, and total ions, total N, total P and TSS are all low. Metal concentrations are substantially lower as compared to the inflow from Main Adit. The reduction is predominantly attributable to removal in the higher pH environment of the TSF, with dilution contributing during storm events.
Table 12. Median and maximum concentrations for parameters in Polishing Pond discharge. Top rows summarise total metal and metalloid results, bottom rows summarise dissolved (0.45µm) results. Based on 9 samples collected between March 2020 and January 2021.
Median/Max total metal concentrations and other parameters in discharge from TSF
pH EC Alk Acid SO4 Al As Cd Co Cu Fe Pb Mn Ni Zn tot tot tot tot tot tot tot tot tot tot
pH µS/cm mg/L mg/L mg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L unit
7.64 619 9.5 5 258 103 <20 5 22 <2 1660 <5 2780 52 5080 Med
4.93 903 28 23 318 969 <20 18 27 13 4920 27 4340 60 8010 Max (min) Median / Max dissolved metal Al dis As dis Cd Co Cu Fe Pb Mn Ni dis Zn
concentrations (n=9) dis dis dis dis dis dis dis
µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L
10 <20 3 18 <2 183 <5 2680 35 3750 Med
503 <20 17 25 11 2700 11 3900 57 7260 Max
Table 13. Median and maximum concentrations of major ions, total nutrients and total suspended solids in the discharge from the TSF. n=9 for all parameters except fluoride (n=3).
Ca tot Mg tot K tot Na tot Cl F Tot N Tot P TSS Unit mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L Med 76 15 0.9 9.5 15 <0.15 0.05 <0.01 8 Max 122 24 1.1 10.6 36 <0.15 0.13 <0.01 16
Time-series of total and dissolved zinc concentrations are graphed with pH values in Figure 1-26 and show that zinc concentrations show a range of behaviours; during some samplings both total and filtered values are elevated with little difference between the values, whilst during other sampling runs the dissolved values are considerably lower. In December 2020 both total and filtered values were relatively low (<1 mg/L). These results suggest that the pH of the TSF is not the only control on the discharge of zinc, as it would be expected that dissolved zinc would be very low at these pH levels.
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Figure 1-26. Total and filtered zinc and pH in the discharge from the Polishing Pond.
Investigations have identified several factors that are contributing and combining to sustain elevated zinc concentrations in the discharge, even when pH values exceed 7.5. These include:
• Poor mixing of alkali with influent waters in the TSF. The batch dosing of lime into the TSF has not been achieving efficient and uniform mixing of water with the lime. This accounts for the relatively low concentrations of alkalinity in the dam, in spite of high volumes of lime being added. Large volumes of unreacted lime are stored within the TSF and Polishing Pond. • Poor settlement of metal hydroxides in the TSF and Polishing Pond (Figure 1-27). Some of the metal hydroxides being created are remaining in suspension. This is linked to the short residence time in the impoundments due to large volumes of sludge and the shallow water cover, as discussed in the next dot point.
Figure 1-27. Photo of shoreline of Polishing Pond showing high level of turbidity due to poor settling of metal hydroxides.
• Build-up of sludges in the Polishing Pond. The Polishing Pond has only a shallow water cover over the large volume of metal rich sludge in the dam. Investigations documented <0.2 m of water over the sludge. Wind induced waves can resuspend the metalliferous sludge, increasing the concentration of metal hydroxides in the Polishing Pond discharge (Figure 1-28).
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Figure 1-28. (left) Depth (m) to surface of sludges in TSF and Polishing Pond as determined by Aquatic Science in July 2020 (right) disturbance of sludge by dog.
• Episodic inflow of acid drainage from the old Comstock Creek channel causing short- circuiting in the Polishing Pond. When seepage and runoff from the Swansea WRD overflows the sediment pond, metal rich water enters the Polishing Pond near the discharge point. When this occurs, there is insufficient mixing time for neutralisation and settlement of precipitated solids.
Recent actions implemented by AHL to address these issues include:
• Upgrading the lime-dosing system so mixing between the lime slurry and Adit water occurs within a pipe and open channel prior to entering the TSF rather than within the TSF; • Implementing a sludge recycling system that pumps lime-rich sludge from the polishing pond and uses it to neutralise Adit water, and for application on the Swansea WRD. This should improve conditions in the TSF in a number of ways: o Removing and reusing sludge from the Polishing Pond will increase the available storage volume in the dam thus reducing the resuspension of sludge. o The recycled sludge will provide increased surface area and nucleation sites for additional neutralisation reactions, leading to more efficient metal removal and larger particles that will settle more rapidly. o Reduce the volume of new lime added to the system, leading to a more efficient use of the available volume of the impoundment for metal sludges. • Implementing earth works to allow capture of a greater volume of the seeps and runoff entering the original Comstock Creek channel. • Increasing the height of the tailings dam wall to provide additional volume for the disposal of tailings and sludges. This action is scheduled to occur during Phase 1 of the project, before any new waste material from AHL is discharged to the TSF.
In addition, capping the Swansea WRD and Allisons Pit will reduce the discharge of acid drainage into the TSF. 1.5.6 Fluxes from Polishing Pond to Comstock Creek Zinc, lead and sulphate fluxes in the discharge from the Polishing Pond were calculated using the water quality and flow results, and are compared to the discharge from Main Adit in Figure 1-29. The results show that a maximum of 40 kg/day of zinc, <0.5 kg/day of lead and 1,400 kg/day of sulphate was discharged from the Polishing Pond during the very high flow event. Compared to the discharge
L Koehnken Pty Ltd 31 18 May 2021 AHL Tenth Legion Mining Project – Water Quality Component from Main Adit, zinc fluxes decreased by between 45-50%, lead decreased 92 -99% and sulphate increased by about 20%.
These results are minimum capture estimates for lead and zinc, as they do not reflect the additional inputs from the Swansea Waste Rock Dump or other stormwater sources. The increase in sulphate, which is not removed through neutralisation, reflects the inflow of these other sources. If it is assumed that an additional 20% of lead and zinc are also entering the pond, then the retention rates of metals in the dam are considerably higher.
The first upgrades to the neutralisation system, enhancing the mixing between acid drainage and lime in a pipe and open channel, were implemented prior to summer 2020. The zinc results show a greater removal from the Polishing Pond in December and January, suggesting these upgrades have enhanced neutralisation. The similarity in sulphate fluxes between Main Adit and the Polishing Pond in January 2021 is due to dry conditions limiting stormwater inflows to the TSF system.
Figure 1-29. Comparison of zinc, sulphate and lead fluxes in Main Adit and in the discharge from the Polishing Pond.
1.5.7 Lower Comstock Creek The original Comstock Creek channel flows through the Polishing Pond in a southerly direction (Figure 1-14, with the discharge from the Polishing Pond entering a tributary to the west (the same tributary into which the upper Comstock Creek is diverted). This western tributary joins the original creek channel about 800 m downstream of the Polishing Pond, in a low lying moorland area characterised by rushes and sedges. This marshy area contains extensive iron oxide/hydroxide deposits due to the long history of mining and discharge of poor water quality into the creek (Figure 1-30).
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Figure 1-30. Lower Comstock Creek showing extensive deposits of iron-rich oxy/hydroxides. Photo by Aquatic Science.
Measured flow at the Lower Comstock monitoring site has ranged from 0.03 to 0.80 m3/s (30 – 800 L/s). These rates are compared with the discharge from the Polishing Pond on the same day in Figure 1-31. During periods of low flow, the discharge from the Polishing Pond can account for between 30% and 50% of the flow at the lower site. During periods of high flow, the Lower Comstock transports about 8-times greater flow. This is consistent with the large catchment area that is diverted around the mine site flowing into the lower Comstock site.
Based on sulphate results, the concentrations in the Lower Comstock can be accounted for by a 4- fold dilution of the Polishing Pond discharge. This is generally consistent with the flow rates. Total zinc concentrations decrease show a different trend, with concentrations decreasing considerably more than can be accounted for by dilution between the Polishing Pond and Lower Comstock site. Zinc levels decrease by at least 10-fold between the sites (Figure 1-32), strongly suggesting that zinc hydroxides are being captured and stored at least temporarily in the marshy area near the confluence of the waterways.
Figure 1-31. Discharge from the Polishing Pond and at Lower Comstock Creek. Highest flow in Comstock Creek is calculated based on preliminary rating curve. Note log scale.
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Figure 1-32. (left) Sulphate and (right) zinc in the discharge from the Polishing Pond and in the Lower Comstock. Note scales are different for each site in each graph.
The water quality results for the Lower Comstock Creek (Table 14 and Table 15) show the pH is consistently above pH 6. Total and filtered metal results indicate that arsenic, cadmium, cobalt, copper, lead and nickel are all below the laboratory reporting limits, recognising the reporting limits are in the 2 to 20 µg/L range. The elevated levels of total and dissolved iron in this pH range suggest that the iron is present as ferrous, which accounts for the clear nature of the water. A portion of this iron likely precipitates within the river channel as it flows towards the Little Henty. Total ions, nutrients and TSS are all relatively low and within typical west coast values (e.g. Na and Cl are elevated due to proximity to the coast).
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Table 14. Median and maximum concentrations for parameters in Lower Comstock Creek. Top rows summarise total metal and metalloid results, bottom rows summarise dissolved (0.45µm) results.
Median/Max total metal concentrations and other parameters in Lower Comstock Creek (n=9)
pH EC Alk Acid SO4 Al As Cd Co Cu Fe Pb Mn Ni Zn tot tot tot tot tot tot tot tot tot tot
pH µS/cm mg/L mg/L mg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L unit
6.31 228 4.5 8 63 98 <20 <2 <3 <2 3520 <10 208 <10 108 Med
6.15 320 22 35 85.2 200 <20 <2 <3 <2 15500 <10 345 <10 484 Max (min) Median / Max dissolved metal Al dis As dis Cd Co Cu Fe Pb Mn Ni dis Zn
concentrations (n=9) dis dis dis dis dis dis dis
µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L
75 <20 <2 <3 <2 2130 <10 205 <10 112 Med
130 <20 <2 <3 <2 15600 <10 349 <10 435 Max
Table 15. Median and maximum concentrations of major ions, nutrients and TSS. n=9 for all parameters except fluoride (n=3).
Ca tot Mg tot K tot Na tot Cl F Tot N Tot P TSS Unit mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L Med 18.6 5.8 0.54 9.3 15.4 <0.06 0.19 <0.01 8 Max 28.5 8.8 1.79 10.4 17.1 <0.06 0.42 0.01 42
1.6 Composition of sludge in TSF The composition of sludges and pore water within the Tailings Dam and the Polishing Pond was investigated by Aquatic Science in August 2020. Samples were collected from the sites shown in Figure 1-33. A 30 mm pipe and syringe was used to draw 2 vertical metres of sludge from each site. A 50 ml aliquot of sludge was mixed with 1 L of deionised water and allowed to settle for 20 hours. The TSS for each of the samples was determined to estimate the settled density of the sludge. Three samples each from the Tailings Dam (TD1, 3, 6)and Polishing Pond (PP1, 2, 4) were submitted for detailed analysis of the supernatant and the sludge solids.
Results of the water and sludge analyses are summarised in Table 16, and show the following characteristics:
• The pH values in the sludges are consistent with the target pH in the dam, with all values exceeding 7.5. • The sludges have a very low settled density, with only one sample (PP1) having a value greater than 7%.
• Alkalinity is in excess within the sludges, with site PP1 containing 1,680 mg/L CaCO3 alkalinity at a dilution of 1:20. • Dissolved metals are generally low in all of the supernatant samples. Dissolved Al is elevated in a few samples due to the increased solubility of Al at high pH values. Dissolved zinc is
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recorded as elevated in sample PP1, but it is likely the zinc is associated with very fine particulates that passed through the filter. • Total metal concentrations are elevated in the sludge, with zinc concentrations ranging up to 9.8 weight percent on a dry meta-basis. Sites located near inflows (TD3, PP1) tended to have lower metal concentrations as compared to sites in more quiescent areas. An example of this for zinc is shown in Figure 1-34. • Calcium is very high in sludge, reflecting the large volume of lime added to the impoundments. The highest concentration is at PP1 reflecting direct lime addition to the Polishing Pond and / or the transport of lime through the Tailings Dam, and subsequent settlement in the Polishing Pond (Figure 1-35).
The abundance of lime and associated alkalinity in the stored sludges is being utilised by AHL to neutralise inflowing acid drainage, and increase alkalinity inputs to the Swansea WRD. The aim is to increase the available volume of storage in the impoundments and increase the density of the sludges through recycling of the material.
Figure 1-33. Sampling locations in the TSF and Polishing Pond for sludge investigation completed by Aquatic Science.
Table 16. Summary of sludge and supernatant water from sludge samples collected from the Tailings Dam and Polishing Pond. Metal concentrations are for dissolved metals in the supernatant.
Parameter Tailings Dam Polishing Pond Tailings Dam Polishing Pond Supernatant Supernatant Solids Solids Diluted 1:20 Diluted 1:20 mg/kg DMB mg/kg DMB n=6 n=4 pH 8.8 – 12.7 10.8 - 12.85 EC (mS/cm) 112 - 3570 129 - 6890 Percent Solids 1.7 – 6.7 1.3 – 9.1 Alkalinity 15 – 694 mg/L 23 – 1680 mg/L Al <20 – 597 mg/L <20 – 724 mg/L As <20 mg/L <20 mg/L 444 - 1320 31 - 148 Ca 20-47 mg/L 17 – 758 mg/L 8470 – 72,600 60,700-295,000 Cd <2 mg/L <2 mg/L 14 – 91 38 - 128 Co <3 mg/L <3 mg/L 37 - 97 50 – 262 Cr <2 mg/L <2 mg/L 14 - 51 11 – 116 Cu <2 mg/L <2- 2 mg/L 62 - 155 46 - 165 Fe <20 mg/L <20 mg/L
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Parameter Tailings Dam Polishing Pond Tailings Dam Polishing Pond Supernatant Supernatant Solids Solids Diluted 1:20 Diluted 1:20 mg/kg DMB mg/kg DMB n=6 n=4 K 0.2 – 0.3 mg/L 0.2 – 1.3 mg/L 150 - 944 98 - 370 Mg 0.5 – 10.0 mg/L <0.05 – 0.16 mg/L 4,730 – 39,000 44,300 – 71,200 Mn 5 – 291 mg/L <5 mg/L 2200 – 10,200 6920 – 29,200 Mo <5 mg/L <5 mg/L Na 1.2 mg/L 1.2 – 1.3 mg/L 78 - 169 51 – 211 Ni <10 mg/L <10 mg/L 99 - 198 105 – 542 Pb <10 mg/L <10 – 237 mg/L 691 - 1400 102 - 573 Se <30 mg/L <30 mg/L Zn 4 – 25 mg/L 3 – 888 mg/L 18,100 – 39,800 19,200 – 98,000
18,100 98,000
97,900 39,800 39,500
19,200
Figure 1-34. Distribution of zinc (mg/kg DMB) sludge in the Tailings Dam and Polishing Pond.
8470 64,200
60,700 72,600 35,000
295,000 200
Figure 1-35. Distribution of calcium (mg/kg DMB) in sludge in the Tailings Dam and Polishing Pond.
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1.7 Groundwater A comprehensive groundwater investigation and associated report has been completed by Cromer (2021) for AHL, and should be consulted for details. This section provides a brief summary of ground water quality and provides comments about how it will contribute to the expected discharge quality from the AHL site.
Figure 1-36 shows the distribution of bore holes from which water quality samples have been collected. The results (Table 17) shows that the groundwater quality reflects the surface water quality in the Dent, with most bores having low concentrations of metals and sulphate. Maximum zinc concentrations, 117 µg/L in bore TLR10, are considerably lower than the seepage emanating from the Kynance mine workings, but are similar to the concentrations in lower Dent Creek. This same bore has the highest sulphate concentrations and the lowest pH, suggesting it is intercepting some acid forming material. The elevated concentrations of iron and manganese in the bores is consistent with the low dissolved oxygen concentrations in groundwater. Precipitation of these metals would be expected to occur following oxidation upon exposure to air.
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Figure 1-36. Vertical drill holes and groundwater ores near Tenth Legion. Red dots shows drill holes where groundwater samples were collected in May 2020. Map provided by Laurie Veska, March 2020.
Table 17. Summary of water quality results from groundwater samples collected on 18 May 2020 by W.C Cromer Pty Ltd. All units are mg/L, except pH (pH units) and electrical conductivity (µS/cm).
Parameter GW2 GW3 TLR10 TLR15 TLR37 TLC39 TLR40 pH 6.7 6.01 3.34 5.88 5.40 5.38 5.18 EC 156 133 314 113 73 74 99 Tot Alk 62 14 <1 15 4 1 2
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Parameter GW2 GW3 TLR10 TLR15 TLR37 TLC39 TLR40 SO4 <1 2 47 14 6 5 5
Arsenic <0.001 <0.001 <0.001 <0.001 <0.001 0.002 0.005 Cadmium <0.000 <0.0001 <0.0001 0.0007 0.0002 0.0001 <0.0001 1 Chromium <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 Copper <0.001 <0.001 0.001 0.005 0.007 0.021 0.004 Iron 7.56 1.6 7.35 <0.05 2.21 1.38 0.57 Lead <0.001 <0.001 0.005 <0.001 <0.001 0.002 <0.001 Manganese 0.267 0.045 0.365 0.868 0.059 0.141 0.215 Mercury <0.000 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 1 Nickel <0.001 0.002 0.003 0.008 0.002 0.001 0.001 Zinc <0.005 0.008 0.117 0.093 0.033 0.023 0.018
Calcium 6 <1 1 1 <1 1 1 Magnesium 5 2 2 6 2 2 2 Sodium 16 18 11 9 7 9 12 Potassium 2 2 <1 <1 <1 <1 <1 Total CN <0.004 <0.004 <0.004 <0.004 <0.004 <0.004 <0.004 Fluoride 0.3 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 NOx <0.01 <0.01 0.02 <0.01 0.04 0.07 <0.01
The input of groundwater to the main mining pit at the Tenth Legion site is estimated to range from 1 L/s in the early stages of mining to a maximum of about 10 L/s in the final stages. Based on the available groundwater results, the final water quality in the pit water is expected to be good, with low metal bearing / higher pH water dominating inflows. 1.7.1 Groundwater quality at Comstock mine site Groundwater monitoring on the Comstock mine site at the sites shown in Figure 1-37 is ongoing, with the results submitted to the EPA on a regular basis. The last two years of results are summarised in Figure 1-38, and show that bore ZZ1, at the toe of the polishing pond generally has the highest concentrations of metals, and lowest pH values.
The proposed Tenth Legion development will not impact groundwater at Comstock in the short term. In the longer term, once tailings required storage in the existing TSF, the raising of the dam wall and disposal of NAF and acid consuming tailings in the TSF is likely to improve groundwater quality over the long-term.
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Figure 1-37. Locations of groundwater bores on the Comstock site.
Figure 1-38. Time-series of groundwater quality in bores ZZ1 - ZZ4.
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2 Water quality management on site The Tenth Legion proposal includes activities related to water quality at both the new Tenth Legion site, and the existing historic Comstock mine site. At Tenth Legion, the first phase of development includes development of the NW small open cut, and the Main Pit with DSO and oxide ore extracted. The development is predicted to generate predominantly good water quality, with only a small volume of PAF present, estimated to be <5% of the total WRD. The PAF will be placed in the NW pit and covered. Much of the remaining waste, predominantly NAF and acid consuming waste rock, will be transported to site and used in the remediation of the Comstock Mine in Allisons Pit and the Swansea Waste Rock dump, and raising of the Comstock TSF. Any remaining waste will be placed in the WRD in the Dent catchment. Phase 2 of the mine plan will include benefaction of ore that will generate tailings that will be stored in the enlarged TSF.
Remediation of the site and discharge from the Comstock TSF is presently regulated under EPN 7977/2. The development of the Tenth Legion project will require a new or revised licence for discharge from the pit into Dent Creek, construction of a new waste rock dump, new haul roads and other ancillary equipment and installations.
It is proposed that water discharge from the site be managed differently for each stage of mining as outlined in Table 18. During Phase 1, discharge limits will be established for the discharge of water from the Tenth Legion site to Dent Creek. If the limits are not achieved, the Pit water will be piped to the Comstock site, mixed with the inflowing Adit water, neutralised and discharged via the TSF and polishing pond. During Phase 1 it is proposed that the existing discharge requirements for the Polishing Pond be maintained.
From an environmental perspective, discharging the pit water and site runoff to Dent Creek is the preferred approach, as increasing the inflow to the TSF will dilute the Main Adit water, but will greatly decrease the residence time of water within the impoundment and polishing pond. Higher concentrations of zinc have been recorded in the discharge from the TSF during periods of high inflows as compared to dry periods, so permanently increasing the inflow to the TSF is likely to reduce the metal loads captured in the TSF. Comparing the highest recorded groundwater concentrations of zinc at Tenth Legion, ~120 µg/L, with the range of zinc concentrations being discharged from the TSF, 700 µg/L to 8,000 µg/L suggests the Tenth Legion runoff would dilute concentrations in the TSF, but decreasing residence time could also increase zinc in the discharge relative to present conditions.
Discharging water from the Tenth Legion site to Dent Creek would prevent increased stress on the TSF, and maximise the benefit of the present management actions being implemented on site. These management actions include:
• Improving the aeration and mixing of lime with the Adit water through the development of open channels and improved lime-dosing procedures • Pumping alkali rich sludges from the polishing pond to mix treat the Adit water and to increase alkalinity inputs to the Swansea WRD. One benefit of this approach is that as the neutralised water re-enters the TSF, the metal hydroxides should be larger in size, settle more rapidly and produce a denser sludge. An associated benefit is an increase in available volume in the TSF for additional sludge deposition.
During Phase 1, as capping in Allisons Pit and on the Swansea WRD progresses, the quality of acid drainage entering the TSF and requiring treatment should improve. During this Phase, the wall of the
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TSF will also be raised, such that by the time tailings are produced (Phase 2) the available storage volume and residence time in the TSF will be substantially increased.
Table 18. Stages of development by AHL at Tenth Legion and Comstock mine sites.
Phase Tenth Legion Comstock Mine site Regulation 1 Development of pit & WRD for Continue remediation • New permit DSO • Recycle alkali-rich conditions for • All activities restricted to sludge from PP Tenth Legion Dent Creek catchment • Initiate filling of includes discharge Allisons Pit & covering criteria for direct of Swansea WRD discharge of pit • Capture & divert water and site Swansea seepage from runoff to Dent western flank into TSF Creek • If not met, pit water & runoff piped to Comstock site for treatment • pH criteria for TSF discharge remains Phase 2 Tenth Legion mining progresses • Remediation well • Discharge from to ore requiring benefaction on advanced and Tenth Legion to Comstock Mine site continues as required Dent Creek or • Tailings dam wall piped to Comstock increased in height to as required under increase storage permit capacity and hydraulic • Revised discharge retention time criteria from TSF • Tailings from Tenth implemented to Legion Ore discharged reflect storage of to TSF tailings from Tenth Legion Phase 3 Tenth Legion ore body • Additional mining and • TSF continues to exhausted processes initiated or receive tailings or • PAF stored in pit(s) & site site closure undergoes closure rehabilitated • Runoff to Dent Creek
2.1 Phase 1 water balance As shown in Figure 1-2, the Tenth Legion pits and waste rock dump are located in the upper Dent Creek catchment. A small area of the project may extend into the Piney Creek catchment, but the project will be developed to ensure that all runoff during operations is directed into the Dent. The water balance for the site has identified the volume of discharge expected from the site from both mine pit water, and surface runoff (Table 1). The values in the table include the maximum estimated pit water input, 10 L/s, reflecting the latter stages of mining. During most of the project, groundwater inflow to the pit is expected to be less, beginning at about 1 L/s during the first mining stage (See Groundwater Report for more information).
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Table 19. Summary of flows from AHL mine site and in the receiving environment.
Source 50th Percentile 95th Percentile Pit water (including NW pit 15 L/s 17 L/s when operating) (up to 10 L/s attributable (up to 10 L/s attributable to to groundwater) groundwater) Run off from site including WRD 10 L/s 16 L/s Total from AHL mine site 25 L/s 33 L/s Dent upstream of confluence 110 L/s 130 L/s with Kynance Kynance upstream of confluence 220 L/s 290 L/s with Dent Kynance upstream of confluence 460 L/s 550 L/s with Comstock
2.2 Discharge limits for Dent Creek Kynance Creek and Dent Creek are tributaries of Comstock Creek. The Protected Environmental Values for Comstock Creek include the Protection of modified (not pristine) ecosystems from which edible fish are not harvested and Recreational water quality and aesthetics (DPIPWE, 2000).
These values recognise the existing degraded condition of Dent Creek, and extremely impacted Comstock Creek. Proposed water quality discharge limits and investigative limits have been derived based on the water flows in Dent and Kyanace. Within the mine site, the pit water discharge will mix with clean stormwater runoff from the site, in rations of at least 1:1 based on maximum modelled groundwater inflow rates to the pit (10 L/s) and median runoff from the site (15 L/s). Between the proposed discharge point on the Tenth Legion Mine site, and the Dent at Highway monitoring site, an approximate 4-fold increase in flow occurs, with the influent water including the seepage from the historic Kynance mine site. The mixing of Dent with Kynance Creek provides an almost 10-fold dilution of the discharge from the mine site.
Proposed discharge limits for the Tenth Legion site are based on ANZG (2018) trigger values for the protection of modified ecosystems, Default Guideline Values for Tasmania, and site-specific toxicity testing that has been completed using organic-rich west coast water, which is applicable to waterways such as Kynance Creek, and the lower Comstock. The discharge from the site will be monitored at the final settling pond at Tenth Legion, with ambient monitoring completed at the Dent at Highway, Kynance at Highway and Piney downstream of the mine sites.
Most of the Investigative levels are based on a minimum 4-fold mixing of the discharge with ambient waters, with regulatory limits based on a 6-fold mixing, reflecting the available flows in the lower Dent and Kynance Creeks. Background water quality in the Dent is provided in Table 21 (same table as presented in Dent water quality section). Zinc is the parameter of most concern, and the proposed limits have the potential to increase levels up to 0.025 mg/L at the Highway, and about 0.01 mg/L at the confluence of Kynance and Comstock Creeks. There is potential for the impact of dissolved metals to be reduced through adsorption onto iron or manganese oxy/hydroxides formed as the pit water is pumped and stored.
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Table 20. Proposed investigation and regulation levels for discharge from the AHL Tenth Legion mine site.
Parameter Unit ANZECC/ Tas DGV Proposed Proposed ANZG King/Henty Investigation Regulation Triggers & site- Level Level specific toxicity testing Electrical µS/cm 116-140 550 1000 Conductivity pH 6.2 – 7.4 <5 >6 Turbidity NTU 4.6 – 8.3 30 75 Total mg/L 7-15 25 50 Suspended Solids Sulphate mg/L >1400* 100 300 Dissolved Al mg/L 0.05 NA# NA# Dissolved As mg/L 0.013 0.060 0.080 0.023 Dissolved Cd mg/L 0.0002 0.001 0.002 Dissolved Cr mg/L 0.001 0.012 0.020 0.0033 Dissolved Co mg/L 0.0014 0.006 0.085 Dissolved Cu mg/L 0.0014 0.005** 0.020 0.030 Dissolved Pb mg/L 0.0034 0.014 0.020 Dissolved Ni mg/L 0.011 0.045 0.066 Dissolved Zn mg/L 0.008 0.024* 0.100 0.140 *West Coast specific sulphate toxicity test showed no toxicity at >1600 mg/L; **West coast specific copper toxicity test showed NOEC of 48 µg/L ***West coast specific zinc test had NOEC of 239 µg/L #Natural concentrations of aluminium exceed these guideline values in west coast waters
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Table 21. Summary of water quality in Dent Creek.
8.3
tot
pH
luminium. tot Statistic Acidity Alkalinity A Arsenic tot Cadmium tot Calcium tot Chloride tot Chromium. tot Cobalt tot EC Field Copper tot DOC Flow Fluoride Iron tot Lead tot Magnesium tot Manganese tot Nickel tot Nitrogen tot pH field Phosphorus tot Sodium Sulphate TSS (0.45um) Zinc tot
Unit mg/l mg/l µg/l µg/l µg/l mg/l mg/l µg/l µg/l µS/cm µg/l mg/l m3/s mg/l µg/l µg/l mg/l µg/l µg/l mg/l pH mg/l mg/l mg/l mg/l µg/l <20 Maximum 16 16 220 1 4.0 21.4 2 1.5 140 4 4.2 0.248 0.03 276 5 4.5 11 5 0.6 6.94 0.03 12.2 9.2 5.0 181 <20 90th Percentile 9.4 15.2 166 1 3.82 19.2 2 1.5 124 3.1 4.0 0.166 0.03 195 5 4.4 9.4 5 0.4 6.84 0.023 11.7 9.0 5.0 164 <20 80th Percentile 5 15 141.6 1 3.44 18.9 2 1.5 121 3 3.8 0.086 0.03 176 5 3.9 9.0 5 0.3 6.78 0.015 11.5 8.5 3.4 148 <20 Median 5 12 115.5 1 2.80 17.6 2 1.5 113 3 3.4 0.026 0.03 139 5 3.3 6.0 5 0.1 6.71 0.005 10.1 7.5 2.0 129 <20 20th Percentile 3.2 8.2 74.6 1 2.10 13.8 2 1.5 86 3 3.0 0.023 0.03 108 5 2.5 4.0 5 0.1 6.63 0.005 8.5 5.6 1.8 112 10th Percentile 3 7 68.9 8.1 0.9 2.04 12.9 1.8 1.3 81 3 2.9 0.016 0.03 105 4.1 2.4 2.5 4.4 0.1 6.59 0.005 8.4 4.6 1.0 106 Minimum 3 7 68 0.5 0.3 1.91 10.0 1 0.5 79 3 2.7 0.015 0.03 97 0.5 2.3 2.5 2 0.1 6.35 0.005 8.2 1.1 1.0 105 Count 7 9 10 10 10 9 10 10 10 11 10 4 10 3 10 10 9 10 10 4 11 5 9 10 10 10 *Note on water quality results: The first samples collected were analysed to very low levels to provide an understanding of all potential parameters of concern. Subsequent analyses have been completed by methods with higher laboratory limits of reporting. This accounts for many of the minimum values being lower than 50% of the laboratory limit of reporting.
It is proposed that if the discharge from the Tenth Legion site achieves the proposed limits, it can be discharged directly to Dent Creek. If the limits are exceeded, the Pit water will be piped to the Comstock site, mixed with the Adit water and treated through the TSF and Polishing Pond.
Compliance with the proposed limits will be demonstrated through:
• Continuous pH, turbidity and EC monitoring at the outlet. The EC values can provide an indication of sulphate in the discharge; • Weekly monitoring of the discharge for total and dissolved metals and sulphate • Quarterly monitoring of major cations and anions other than sulphate (Na, K, Ca, Mg, Cl, F)
This and other monitoring are described in Section 3. 2.3 Proposed Discharge Criteria for Comstock Polishing Pond in Phase 2 During Phase 1 of mining at AHL, when the TSF and Polishing Pond are being used to treat historic acid drainage from the Comstock site, it is proposed that the existing discharge criteria for the TSF be maintained (discharge from the Polishing pond have a pH value > 7.5). This recognises that during Phase 1 no new mining waste will be stored in the TSF, with the emphasis on remediation of the acid drainage sources on site.
Activities on the Comstock site during Phase 1 will including increasing the height of the TSF wall, which will increase the volume and retention time of the impoundment. NAF waste rock will be used to complete the lift.
Phase 2 of the project will commence when Tenth Legion processes ore on site, and tailings are discharged into the TSF. During dry processing, very little water is used and there isn’t expected to be a change in the discharge volume from the Polishing Pond to Comstock Creek.
During wet processing, about 75 L/s of water will be required, with up to 75% of the water recycled back into the process from the Polishing Pond. Additional water will be sourced from Main Adit (up to 20 L/s) and from a new input (up to 10 L/s). During wet processing the average discharge from the Polishing Pond is projected to increase from about 23 L/s to ~33 L/s. The ‘new’ 10 L/s of water added to the process is anticipated to be of substantially better quality as compared to the water in Main Adit, so there should be a reduction in metal concentrations entering the TSF system. The quality of inflow from the Swansea WRD should also improve as capping and remediation progresses.
The concentrations of parameters not removed through neutralisation, such as sulphate, will increase in the recycled water, but should not exceed the maximum concentration in the Adit water (~350 mg/L) unless added during processing. Calcium concentrations will also increase due to lime addition to the system. Based on the present levels of sulphate and calcium in the system, there is a low risk of scaling, but should it occur additional clean water may need to be introduced into the circuit.
Measured flow rates in Comstock Creek at the downstream monitoring site (Figure 1-14) have ranged from 40 L/s to 430 L/s. These results are considered under-estimates due to the diffuse nature of flow through the iron-hydroxide rich, marshy area. Sulphate concentrations collected at the lower Comstock site suggest a 4-fold mixing occurs between the discharge from the Polishing Pond and the Creek within the first one-kilometre downstream of the discharge point (Figure 1-14). Many tributaries enter Comstock Creek below this monitoring point, and the median and 95th percentile flow in the Comstock upstream of the confluence with Kynance Creek are estimated at AHL Tenth Legion Mining Project – Water Quality Component
460 L/s and 550L/s, respectively, based on the modelling for the site water balance. This suggests that the initial 4-fold mixing increases to 20-fold mixing over the next 2 km downstream.
The present water quality at the Polishing Pond discharge is summarised in Table 12 and Table 13. pH values greater than 7.5 have not been consistently maintained in the Polishing Pond, and it is reflected in the water quality results, with cobalt, nickel and zinc having elevated concentrations relative to ambient guideline values. Once the wall of the TSF has been increased in size, the maintenance of pH is expected to improve.
The PEVs for Comstock Creek include the Protection of modified ecosystems from which edible fish is not harvested, and recreational water quality and aesthetics. The following discharge criteria for discharge from the Polishing Pond are proposed to be adopted at the commencement of tailings discharge into the TSF, and be applicable throughout the life of the TSF. The criteria are based on achieving a large improvement compared to the present water quality, and ultimately supporting the establishment of a modified but healthy ecosystem in Comstock Creek. It must be recognised that decades of poor quality discharge from the Comstock site have greatly impacted Comstock Creek downstream of the mine-site, which is presently devoid of aquatic life. The proposed values do not consistently achieve ANZG (2018) 95th percentile protection limits with a 4-fold mixing, but will achieve them with further distance downstream, and above the confluence with Kynance Creek.
The proposed discharge criteria are based on the present understanding of water quality on site, and the likely water quality that can be achieved using the existing neutralisation system. Over time, the conditions on site may change, and it is proposed that the discharge criteria be reviewed after 1-year of tailings discharge into the TSF.
Table 22. Proposed investigation and regulation levels for discharge from Comstock Polishing Pond for Phase 2 of the operation..
Parameter Unit ANZECC/ Tas DGV Proposed Proposed ANZG King/Henty Investigation Regulation Triggers & site- Level Level specific toxicity testing Electrical µS/cm 116-140 800 1,200 Conductivity pH 6.2 – 7.4 <6 >7 Turbidity NTU 4.6 – 8.3 30 75 Total mg/L 7-15 25 50 Suspended Solids Sulphate mg/L >1400* 600 Dissolved Al mg/L 0.05 NA# NA# Dissolved As mg/L 0.013 0.040 0.060 0.023 Dissolved Cd mg/L 0.0002 0.002 0.005 Dissolved Cr mg/L 0.001 0.004 0.008 0.0033 Dissolved Co mg/L 0.0014 0.020 0.040 Dissolved Cu mg/L 0.0014 0.005** 0.020 0.030 Dissolved Pb mg/L 0.0034 0.014 0.020 Dissolved Ni mg/L 0.011 0.045 0.066
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Parameter Unit ANZECC/ Tas DGV Proposed Proposed ANZG King/Henty Investigation Regulation Triggers & site- Level Level specific toxicity testing Dissolved Zn mg/L 0.008 0.024* 0.200 0.340 Total Petroleum mg/L 0.050 Hydrocarbons *West Coast specific sulphate toxicity test showed no toxicity at >1600 mg/L; **West coast specific copper toxicity test showed NOEC of 48 µg/L ***West coast specific zinc test had NOEC of 239 µg/L #Natural concentrations of aluminium exceed these guideline values in west coast waters
3 Proposed monitoring Proposed monitoring at Tenth Legion and Comstock is summarised in Table 23 and Figure 3-1 to Figure 3-4. In the table, the waterways listed in column A are proposed to be monitored at the sites listed in column B for the parameters shown in column C at the frequency shown in column E. The parameter units and reporting schedule are shown in columns D and F, respectively. All ambient receiving environment sites are established with over 1-year of baseline information collected.
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Table 23. Proposed monitoring at the AHL and Comstock mine sites following commencement of operations at AHL.
A B C D E F Waterway Location Parameter Units Frequency Reporting Ambient Monitoring Piney Creek Downstream of Flow, EC, pH, alk, acidity EC=µS/cm Monthly Annual Env. Review mine site TSS, SO4, total and pH=pH units filtered metals, Alk, acidity, SO4 = mg/L Metals = µg/L Ca, Mg, K, Na, TN, TP mg/L Quarterly Annual Env. Review Kynance Creek at Trial Harbour Flow, EC, pH, alk, acidity EC=µS/cm Monthly Annual Env. Review Road TSS, SO4, total and pH=pH units filtered metals, Alk, acidity, SO4 = mg/L Metals = µg/L Ca, Mg, K, Na, TN, TP mg/L Quarterly Annual Env. Review Dent Creek At Trial Harbour Flow, EC, pH, alk, acidity EC=µS/cm Monthly Annual Env. Review Rd TSS, SO4, total and pH=pH units filtered metals, Alk, acidity, SO4 = mg/L Metals = µg/L Ca, Mg, K, Na, TN, TP mg/L Quarterly Annual Env. Review Comstock Creek Downstream of Flow, EC, pH, alk, acidity EC=µS/cm Monthly Annual Env. Review Polishing Pond TSS, SO4, total and pH=pH units filtered metals, Alk, acidity, SO4 = mg/L Metals = µg/L Ca, Mg, K, Na, TN, TP mg/L Quarterly Tenth Legion Monitoring Pit Water & site Discharge from Flow, pH, EC, turbidity Flow: L/s Continuous Results available to stormwater settling pond Turbidity: NTU recording if EPA upon request, EC: uS/cm discharged to Dent summarised in pH: pH units Monthly if piped to quarterly report site and annual review AHL Tenth Legion Mining Project – Water Quality Component
A B C D E F Waterway Location Parameter Units Frequency Reporting pH, EC, temperature EC=uS/cm; T=°C Weekly if Results available to Total & dissolved metals Metals=mg/L discharged to Dent EPA upon request, (Al, As, Cd Cr, Co, Cu, Fe, Sulphate=mg/L Monthly if summarised in
Pb, Mn, Ni Zn), sulphate, Acid/alk = mg CaCO3 equivalents transported to site annual review Alkalinity, acidity, Frequency Exceedances reviewed after 12 notified to EPA as months soon as practical calcium, magnesium, mg/L Monthly, reviewed Results available to sodium, potassium, after 12 months EPA upon request, chloride, Total N, Total P summarised in annual review Pit water Inflow to pipeline pH, EC, temperature EC=uS/cm; T=°C Weekly if Results available to if piped to site Total & dissolved metals Metals=mg/L discharged to Dent EPA upon request, (Al, As, Cd Cr, Co, Cu, Fe, Sulphate=mg/L Monthly if summarised in
Pb, Mn, Ni Zn), sulphate, Acid/alk = mg CaCO3 equivalents transported to site annual review Alkalinity, acidity, Frequency Exceedances reviewed after 12 notified to EPA as months soon as practical Seepage from WRD Toe of dump pH, EC, total metals, (Al, EC=uS/cm; T=°C Weekly if Results available to if present As, Cd Cr, Co, Cu, Fe, Pb, Metals=mg/L discharged to Dent EPA upon request, Mn, Ni Zn), sulphate, Sulphate=mg/L Monthly if piped to summarised in
Alkalinity, acidity, Acid/alk = mg CaCO3 equivalents site annual review
Groundwater bores GW1, GW4 (to be pH, Turb, EC, dis metals pH=pH unites 6-monthly with Results available to installed) (As, Cd, Co, Cr, Cu, Hg, Ni, EC=µS/cm parameters EPA upon request, GW2, GW3 Pb, Zn) Tub = NTU reviewed after 24 summarised in Dis metals = µg/L months annual review
Groundwater WL recorder in Level m Continuous Results available to TLR15 EPA upon request, summarised in
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A B C D E F Waterway Location Parameter Units Frequency Reporting annual review
Comstock Mine site Main Adit At v-notch pH, EC, total metals, (Al, EC=uS/cm; T=°C Monthly with Results available to As, Cd Cr, Co, Cu, Fe, Pb, Metals=µg/L parameters EPA upon request, Mn, Ni Zn), sulphate, Sulphate=mg/L reviewed after 12 summarised in
Alkalinity, acidity, Acid/alk = mg CaCO3 equivalents months annual review Middle Comstock Upstream of pH, EC, total metals, (Al, EC=uS/cm; T=°C Monthly with Results available to discharge from As, Cd Cr, Co, Cu, Fe, Pb, Metals=µg/L parameters EPA upon request, Polishing Pond Mn, Ni Zn), sulphate, Sulphate=mg/L reviewed after 12 summarised in
Alkalinity, acidity, Acid/alk = mg CaCO3 equivalents months annual review Discharge from At v-notches pH, EC, total & dissolved EC=uS/cm; T=°C Monthly during Results available to Polishing Pond metals, (Al, As, Cd Cr, Co, Metals=µg/L Phase 1 EPA upon request, Cu, Fe, Pb, Mn, Ni Zn), Sulphate=mg/L Weekly after Tenth summarised in
sulphate, Alkalinity, Acid/alk = mg CaCO3 equivalents Legion tailings annual review acidity, discharged to TSF Groundwater bores ZZ1, ZZ2, ZZ3, ZZ4 Water level, pH, EC, turb, Level=m; Quarterly Results available to sulphate, dissolved EC=uS/cm; T=°C EPA upon request, metals (Al, As, Cd, Cr, Co, Metals=µg/L summarised in Cu, Pb, Ni, Zn) Sulphate=mg/L annual review
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Discharge to Dent
Figure 3-1. Surface water monitoring locations at Tenth Legion mine site.
AHL Tenth Legion Mining Project – Water Quality Component
Figure 3-2. Location of Tenth Legion ore bodies and location of existing (red dots) and proposed (orange dots) ground water bores.
Figure 3-3. Surface and ground water monitoring sites in the Piney, Kynance, Dent and Comstock Creek catchments. Groundwater sites indicated by ZZ.
L Koehnken Pty Ltd 54 18 May 2021 AHL Tenth Legion Mining Project – Water Quality Component
Figure 3-4. Surface and groundwater monitoring locations on the Comstock lease site.
L Koehnken Pty Ltd 55 18 May 2021 AHL Tenth Legion Mining Project – Water Quality Component
References ANZG 2018. Australian and New Zealand Guidelines for Fresh and Marine Water Quality. Australian and New Zealand Governments and Australian state and territory governments, Canberra ACT, Australia. Available at www.waterquality.gov.au/anz-guidelines.
Department of Primary Industries, Water and Environment (DPIPWE), 2000. Environmental Management Goals for Tasmanian Surface Waters, West Coast Municipal Area (Excluding the Gordon and Pieman River Catchments).
Tasmanian Government (1997) State Policy on Water Quality Management.
Walsh, T (2020). AusRivAS autumn Survey of Zeehan sites. Report by Todd Walsh of Kanunnah Pty Ltd for John Miedecke 2020.
Walsh, T (2021). AusRivAS autumn Survey of Piney Creek. Report by Todd Walsh for John Miedecke 2021.
L Koehnken Pty Ltd 56 18 May 2021