Current remediation plan

The Days Creek Domain remediation is the first stage in the complete remediation of the Brukunga Mine Site. It in an area in the northwest corner of the site that is approximately 123 hectares (1.23 km²) in size stretching approximately 800 m to the south of Days Creek to the South Waste Rock Dump and 300 m from the highwall in the west towards Dawesley Creek.

The ultimate goal of the remediation is to remove the sources of acid mine drainage (AMD) and thus the need to provide ongoing active water treatment. The main features of the current design are as follows:

Co-disposal of mines waste

• Waste rock and tailings are to be excavated from the current storage areas and “co-disposed” by mixing and compacting
• Crushed limestone (4-6% by dry mass) will be added to the co-disposed wastes to neutralise stored oxidation products like jarosite
• Co-disposed mixture is to be kept saturated with water to prevent contact with air and minimise AMD generation

Diversion of Days Creek

• Most of the year, Days Creek is dry. From time to time in the summer months, Days Creek flows through the mine and picks up AMD water.
• To keep Days Creek water from being contaminated, Days Creek will be diverted flow to the existing Dawesley Creek Diversion by a rock channel and an underground pipe.

Low permeability perimeter

• The Days Creek Domain is already bordered by practically impermeable highwalls to the north and west.
• To complete the “bowl” to contain the saturated co-disposed waste, embankments will be constructed to the east and south.

Robust cover system

The cover system will consist of three components as follows:

• A layer of top soil suitable to grow mainly grasses. This layer will have rocky areas that allow water to soak down into the co-disposed waste to help keep it saturated from the expected yearly rainfall.
• A drainage layer consisting of crushed rock that will help distribute water evenly to the co-disposed waste or help it spill out into the Days Creek Diversion when there is too much.
• Sandy filter layers that sandwich the drainage layer. They prevent the drainage layer from getting clogged with clay and silt from the top soil and co-disposed waste.

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Technical Advisory Group (TAG)

The South Australian Government implemented a process for determining the most effective long-term solution for the third stage of the remediation of the Brukunga Mine Site. The Brukunga Remediation Project – Technical Advisory Group (TAG) was formed to assist in this process. The purpose of the TAG is to advise and recommend an appropriate technical remediation option for the Brukunga Mine Site.

The goal of the TAG was to develop a “walkaway” solution to return the land in a suitable state for release of ownership by the government. Ideally, the site would require no further intervention, responsibility or cost to the government or the community.

The work of the TAG is organised in 3 phases.

• Phase 1 – Identification of a preferred remediation option for the Brukunga Mine Site (completed 2008)
• Phase 2 – Attending to the technical issues for the implementation of the preferred solution (completed 2009)
• Phase 3 – Define the detailed design, specification and costing (in progress)

After the completion of Phase 2, it was decided to remediate in stages. In the first stage, the remediation of the Days Creek Domain (northwest corner of the site) would serve as a pilot project on the way to the full site remediation.

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Brukunga Mine Site Remediation Board (BMSRB)

Field trials

When the remediation is complete, all the wastes generated from the mining activities must be saturated and capped to prevent any further AMD generation. As there is approximately twice as much waste rock as tailings, any co-disposed mix must be designed in a roughly 2:1 ratio of waste rock to tailings.

Waste Rock Pile Testwork (2007, 2008)

The trial involved the construction of seven one-thousand tonne waste rock test piles with various acid management methods applied. These methods were:

• Alkaline substances (limestone) covering the waste
• Blending alkaline substances into the waste
• Impermeable cover on the waste
• No method applied as a control.

It was determined that alkaline generating covers were very effective in managing acid drainage. Blending waste with alkalinity (limestone) also proved to be beneficial only for short term AMD minimisation.

The current remediation design is blending 4–6% alkalinity into the co-disposed waste rock and tailings.

Co-Disposal Project (2009)

The results of the testing carried out under the Co-Disposal Project provided confidence that the current option will ensure that the compaction of co-disposed waste rock and tailings will achieve a high density and a high degree of saturation, with a low potential for further oxidation, which will be further enhanced by full saturation. Further, the compacted waste rock/tailings mixture will have a high capability to hold water and remain saturated between rainfall events.

Field Mixing and Compaction (10,000 Tonne) Trial (2013)

The 10,000 Tonne Trial was conducted to determine and demonstrate the optimal methodology for mixing, placement and compaction of waste rock, tailings and limestone (co-disposed material) at full scale and establish an appropriate quality assurance/quality control method for the main construction phase of Days Creek Domain Rehabilitation.

Mixing the co-disposed occurred in roughly two phases.

• Phase 1 – At the tailings dam, limestone was spread on the surface of tailings. Tailings and the placed limestone were then excavated and transported to the co-disposal site.
• Phase 2 – At the co-disposal site, excavators placed the tailings-limestone mix on a layer of waste rock. The resulting mixture was then irrigated with water.

Waste rock particle sizes, water contents and compaction methods were varied to help establish the optimum construction specifications. The findings of this trial have been valuable in the determination of the construction plan for the current remediation plan for the Days Creek Domain.

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Bench and lab trials

Geochemical bench scale tests were done to characterise the saturated co-disposed waste model. As the waste rock and tailings contains sulfide and acid-sulfate minerals, the design must allow for sufficient limestone to neutralise any acidic seepage produced.

Manual of geochemical leaching trials 2010–2014

Column leach trials – Stage 1 (2011)

A series of cylinders with varying mixtures of tailings and waste rock were amended with 1 wt.% limestone, saturated and covered with water.

Water quality analysis of the column leachate and the simulated overlying water cover still showed elevated levels of acidity. Although, it was believed that the water cover prevented further sulfide oxidation, it was determined that jarosite in the waste was to acidify both the leachate and water cover.

The key outcome was acidity additions from the dissolution soluble jarosite were a greater issue than anticipated and needed to be resolved with a larger limestone addition.

Bulk leach test (2011)

Bottle roll tests provided ideal mixing conditions for co-disposed waste and various limestone additions. The tests accelerated the dissolution of jarosite and limestone, in order to estimate the minimum limestone addition requirements for achieving complete jarosite neutralisation.

An optimum ratio of 60 wt.% waste rock, 36 wt.% tailings material and 4 wt.% CaCO₃ (limestone) to ensure acidity neutralisation under fully mixed conditions was identified.

Column leach trial – Stage 2 (2012)

Following the bottle roll test work a series of column leach tests were conducted to investigate whether the ideal co-disposed material ratio would neutralise leachate acidity and prevent acidification of a water cover. It was observed that 4 wt.% CaCO₃ in co-disposed waste quickly and effectively neutralised the acidity and produced acceptable water quality.

Oxygen consumption testing (2014)

These tests were similar to the column leach trials. In this case, the objective was to determine if the proposed saturated drainage layer sand above the co-disposed waste would be sufficient to control oxygen diffusion. Results showed that an oxidised tailings (or otherwise inert waste) layer is needed to cap the co-disposed sulfidic waste. This added barrier would reduce the oxygen migration into the waste and limit the acidity in the seepage.

For inquiries relating to the Brukunga Mine Site please contact:

Email: DEM.MiningRegRehab@sa.gov.au

or address correspondence to:

Principal Project Manager Remediation Strategy
Mining Regulation Branch
GPO Box 320
ADELAIDE SA 5001

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