 Commonwealth of Australia 2010

70.1Risks to aquatic life

Environmental discharges of solutions containing cyanides are typically treated prior to effluent discharge to Australian natural surface water bodies. Discharges are managed and monitored in accordance with statutory requirements and environmental permits, as noted in Section 22.2. Discharge limits are generally based on ensuring the ANZECC/ARMCANZ (2000a) trigger values or site-specific limits are not exceeded in the mixing zone downstream of the discharge points. Various forms of accidental discharge could also lead to aquatic exposure from which potentially harmful effects can be assumed, hence there are mechanisms in place to minimise these risks and respond to such events if they should arise (Refer Chapter 71).

Concentrations of cyanide in managed, artificially constructed facilities associated with ore processing (e.g. TSFs, decant ponds, leach pad drains and collection ponds, etc) contain elevated concentrations of cyanide. While concentrations in discharge to TSFs have typically been in the range 10-100 mg WAD CN/L, they may be as high as ~600 mg WAD CN/L in exceptional cases. These residues may potentially be very harmful to aquatic life if they were to reach natural waters, but are not a concern when collected in specific structures, except for potential localised impacts on amphibians moving in from other nearby aquatic areas (Section 27.3.1).

Controlled release of treated solutions is necessary in situations such as high rainfall areas or where site assessment shows that heavy rainfall may lead to overflow of TSFs into downstream areas. The concentration of initial cyanide discharge to TSFs is then reduced to low levels by cyanide destruction techniques, eg to 10 mg WAD CN/L in Queensland, or as low as 1-2 mg WAD CN/L in Tasmania. Water may then flow through further ponds to allow natural degradation, with protective limits placed on concentrations of cyanide and its breakdown products (e.g. thiocyanate and ammonia) ultimately released to streams or natural water bodies (refer Sections 23.6.4, 26.1.1, 27.3.3 and 29.1.1).

More generally, exposure of and risk to aquatic organisms in downstream areas could occur through accidental or uncontrolled release of contaminated solutions from mine sites to natural water bodies (e.g. from overflow of the storage facility, run-off from the processing site or TSF areas, or through contaminated seepage and groundwater returning to the surface). Harmful effects on aquatic organisms could follow, depending on the amount and concentration released, and on dilution effects. As well as releasing freshly deposited material which may still contain free cyanide, release to aquatic areas following failure of a storage facility or heap leach ore pile could expose tailings containing cyanide products to the air and to sunlight, potentially mobilising cyanide and HCN that would otherwise have remained safely immobilised in forms such as metal cyanide complexes. Harmful effects on aquatic organisms and other wildlife could be expected from cyanide, other toxic components, and from physical effects from the large volume of material potentially released. Hence it is important that TSF and other facilities are properly designed, operated, monitored and maintained. Measures in place to ensure this are discussed in Chapter 71.

70.2Risks to vegetation

Trees and other vegetation in the immediate vicinity of TSFs may be deliberately removed to avoid the habitat being attractive to birds or other wildlife, additional to clearance for structural and access reasons. Appropriate vegetation is re-established as part of mine closure management requirements, hence it is important that residues in soil on and around TSFs or heap leach ore piles do not inhibit this process. Modern environmental plans for mining operations address this from the start, with removal and stockpiling of topsoil, and subsequent restoration of a suitable layer of topsoil for replanting (EPA, 1995b; MPD, 2004; DITR, 2007).

Consequently, long term impacts from cyanide on vegetation in the vicinity of TSFs or heap leach piles, or when these areas are restored and revegetated after use has ceased, are not expected and existing controls (refer Section 71) sufficiently mitigate risks to vegetation. However, maintenance