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Tailings are water-based waste products generated through both the open pit mining operations and the processing plant.

Waste water and solids are taken by pipeline to the nearby Tailings Storage Facility (TSF) on the west side of the Carpentaria Highway, south of Surprise Creek.

The TSF is divided into three cells. Tailings are being deposited into cell two with water being decanted back to the plant. Cell one underwent its first stage of rehabilitation in 2008 with 61% being capped with a 500mm layer of clay. Total Ground Control was applied to portions of the remaining area in 2009 in order to reduce dust generation. The third area is a water management dam, which recycles water for use within the processing plant. The TSF plays an important role in reducing water use on-site.

We considered relocating the TSF as part of the open pit development. However, all alternative sites within the region have similar soils and therefore provide no benefit in terms of seepage control. The current site was found to offer the best environmental performance based on technical performance, seepage control and lowest long-term risks.

We also considered a number of options for the design of the TSF but found the best is one which combines six 'lines of defence' or strategies to minimise seepage and protect the environment. This strategy is being used for the design, operation, and ultimate closure and rehabilitation of the TSF. It aims to:

• improve the way the tailings facility works
• ensure there are no uncontrolled emissions to the environment
• minimise the area disturbed.

Six lines of environmental protection

1. The way the tailings are deposited in the cells

Spreading the tailings in thin layers against the cell wall provides a ‘beach’ of tailings between the water and the wall which will help block water from seeping. This also offers a number of benefits in the management of the tailings as it:

• supports the system of raising the embankments to increase the tailings capacity; and 
• limits the potential for oxidation of tailings which is what generates sulphates (like salt deposits).

2. The way the outside wall is built

The high embankment around the outside of the TSF is made of compacted clay and topped by rocks which water will find hard to pass through. This two metre thick, four-layered cover system is designed to be safe and stable in both wet and dry seasons. Only non-acid forming material is used for this outer surface to ensure there are no contaminants in the water run-off.

3. The continued use of geopolymer barriers

In 2005, we invested around $2 million dollars to construct a barrier system to provide an additional safeguard to eliminate seepage into Surprise and Barney Creeks. Under this system, a glue-like polymer is injected every two meters around the base of the TSF wall. This forms a hard ground-level barrier that helps stop seepage outside the wall. We are continuing to upgrade this facility as new technology and practices emerge.

4. A recovery bore system around the perimeter of the TSF wall

Seepage from the TSF is recovered by a bore system around the entire perimeter of the TSF collecting it before it reaches freshwater systems. Any seepage would be collected and pumped back to the dam for recycling water. The bores are between 10 metres and 30 metres deep. Plans show these bores will be successful in trapping a majority of seepage within the footprint of the TSF and before it reaches the natural groundwater system.

5. Flood protection for a 1 in 500 year event

The height of the TSF embankment is greater than the 1 in 500 year flood levels, however, an emergency spillway will be located in the north-west corner of the tailings dam to allow for overflows in extreme weather conditions. This is based on the Australian National Committee on Large Dams (ANCOLD) guidelines for the worst possible flood or rainfall experience. An Emergency Tailings Dam Water Release Procedure has been developed and the risks assessed. This has shown the site’s water can be managed without any reliance on controlled releases.

6. Thorough monthly monitoring program

Every day, the water levels within each TSF cell are checked. Surface water is checked every week and groundwater is checked every two months through monitoring bores. Water samples are taken from the sediment ponds as well as Surprise and Barney Creeks. The condition of the embankment is also inspected.

The TSF design is in line with standards set by the Australian National Committee on Large Dams (ANCOLD) Guidelines (1999). That means the facility is built to the same safety standard as all major water storage dams.

ANCOLD has specific guidelines for Tailings Dam Design, Construction and Operation. These are regarded as the best practice standards under Australian conditions and are recognised as the industry standard by State and Territory Governments.

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