Risks to wildlife with use for flotation

70.5Summary of risk characterisation

Cyanide releases from sodium cyanide manufacturing facilities are unlikely to pose an adverse risk to the environment because residues in air from the manufacturing facility are scrubbed to minimise release of HCN to the atmosphere. Cyanide in this or other water from the plant is recovered or destroyed, and residues remaining in water are treated to destroy cyanide and passed through subsequent effluent treatment processes before release. Discharges to waters and sewers of cyanide and its breakdown products are managed under environmental protection license agreements with state and territory environment protection agencies.

In the case of transport of cyanide around the country, the large total quantities in circulation for the mining industry are of concern in the event of accident or misadventure. Transport incidents in Australia and overseas indicate the nature of such events and their possible consequences, particularly where spilt material cannot adequately be recovered or contained and destroyed, e.g. major spills into flowing water (Section 22.3.2). However, the Australian Tanami Desert incident listed also indicates that terrestrial/avian wildlife may be exposed under some circumstances. In that case, a spill of liquid occurred along a road, from which wildlife drank and were harmed before measures were taken to remove the hazard. Measures are necessary to minimise the risks of accidents, leakage or other incidents occurring and to respond promptly with appropriate cleanup and recovery procedures in the event that an incident occurs.

Risks to terrestrial animals and birds at mines

In general, without mechanisms to control wildlife habitation or controls on cyanide concentrations in solutions in areas that are accessible to wildlife, the concentrations of cyanide reportedly contained in gold ore processing infrastructures (e.g. TSFs, decant ponds, heap leach pads and drainage channels) in Australian mining facilities have the potential to adversely affect wildlife health, particularly through the oral (drinking water) route. The gold mining industry appears to be trending towards the adoption of the International Cyanide Management Code, which advises various strategies for risk management including a threshold guideline value of 50 mg WAD CN/L in tailings to minimise the potential impact of TSF and related facilities on wildlife.

However, consideration of the available data in a risk quotient-based assessment approach using acute oral avian toxicity data from laboratory studies indicates that this level is not adequately protective of most bird species, with some mortality possible at concentrations well below 50 mg/L. To assure protection of sensitive avian species from acute mortality and from harmful sublethal effects that might lead to delayed mortality, such as greater susceptibility to predators or reduced flying ability of migratory birds, assessment based on the available data indicates that the concentration of WAD CN in water available to birds would need to be 1 mg/L.

This is a highly conservative figure and is likely to be impracticable to implement in many situations. It is recognised that the assessment has been based on assumptions that may not always be the case, particularly that the birds consume their entire day’s drinking water within a short period (i.e. comparably to acute oral dosing) and that the toxicity of TSF and associated waters is similar to that of NaCN solutions. However, this approach has been necessary to protect individual species that might be heavily exposed due to their drinking behaviour and due to the poor quality of the data available. It should also protect against potentially greater availability of total cyanide to raptors, or contributions through other routes of exposure (e.g. dermal) which were assumed to be relatively minor and were not considered. Laboratory data more directly related to the nature of exposure (drinking water studies, and use of mining effluent rather than simply NaCN solutions) were not considered acceptable for the purposes of risk assessment, and they only related to mallard ducks, which being waterbirds take several drinks of water per day.

Field observation data from a range of sources are available, ranging from anecdotal or unpublished information to major scientific studies. International and Australian evaluations show that significant wildlife mortalities were observed at WAD CN concentrations >50 mg/L, but that relatively few or no mortalities were observed at lower WAD CN concentrations. Observations have also been reported for some sites which indicate that serious mortality incidents were alleviated as WAD CN concentrations were treated to reduce them below 50 mg/L. There is very little information regarding the extent of wildlife mortalities or sublethal effects occurring in association with WAD CN concentrations below 50 mg/L. Some reports of wildlife mortality incidents of varying scales do not indicate the WAD CN concentration to which the animals were exposed. There is one brief report of bat deaths occurring at a time when the mine reported cyanide concentrations <20 mg/L in the TSF, but the report lacks details to confirm the actual form and concentration of cyanide to which the affected bats were exposed. Another published paper noted individual animals had been found dead where WAD CN concentrations were 16 and 26 mg/L, but again limited information was available. Not all wildlife deaths occurring at goldmine operations are due to cyanide toxicity, and it may be difficult to reliably determine the cause of death. In areas of Western Australia where groundwater used for ore processing is hypersaline (>50 000 mg/L TDS), field evaluations have confirmed that no mortalities occurred despite WAD CN concentrations exceeding 50 mg/L, as minimal exposure via drinking water or other sources occurred.

It is concluded that significant wildlife mortalities are likely if wildlife are exposed to WAD CN concentrations exceeding 50 mg/L, with the exception of sites where water is hypersaline and animals do not consume the cyanide-containing waters. Based on available field data, significant wildlife mortalities are unlikely to occur at WAD CN concentrations below 50 mg/L. However, some caution is needed in the extent to which this is considered a safe value because field observations to determine the extent of sublethal effects are lacking. Lower concentration control levels may therefore be appropriate for species which need high protection.

Detailed consideration of other risk mitigation strategies to protect birds from harm to exposure to cyanide at these facilities is therefore essential. In particular, these include measures to reduce or prevent access by birds, bats and terrestrial vertebrates to the contaminated water, and measures to keep the habitat unattractive to wildlife in areas where contamination may be present. A combination of such measures could be applied, together with a concentration-based approach. As part of such an approach, it would also be important to monitor cyanide concentrations in water accessible to wildlife as well as monitoring for the presence of wildlife, impacts on wildlife, and presence of habitat attractive to wildlife. Active measures could then be applied, including hazing to deter birds while a risk is present, and correction of conditions making harmful concentrations of cyanide present in areas attractive and/or accessible to wildlife.

Risks to wildlife during use for flotation is low as cyanide concentrations arising in TSFs from this use are very low.

For the natural aquatic environment, reported cyanide concentrations in some ore processing facility solutions and tailings generally exceed water quality guidelines for cyanide for the protection of aquatic life and pose a risk should an environmental release to natural waters occur. However, these facilities are designed and constructed specifically to contain cyanide tailings and discharges to natural waters are not anticipated within operational designs, unless the water is first treated to destroy the cyanide present. Where seeps or leaks do occur, corrective measures are typically implemented.

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 sufficiently mitigate risks to vegetation.

Industrial users

Industrial users of sodium cyanide, excluding mining operations, mostly discharge waste from processes using it to landfill and/or to wastewater treatment facilities and/or sewage systems that are managed under licenses based on aquatic ecosystem protection and administered by state and territory environment protection agencies. In many cases, cyanide wastes are treated on site before they are discharged from the site. As such, discharges of cyanide solutions from these facilities are unlikely to pose an adverse risk to the environment under these regulatory arrangements.