 Commonwealth of Australia 2010

90.2Environmental exposure

Large amounts of sodium cyanide are transported from the manufacturing sites to mines by rail and road transport. It may be argued that transportation of bulk liquefied sodium cyanide by road and rail poses a significantly greater risk to the environment and human health than the solid form due to the potential for spillage and release in the event of a transportation incident. The liquefied form would be more mobile and difficult to contain before impacts occur, and there are likely to be greater difficulties for emergency response organisations undertaking clean-up procedures. However, there are also advantages in using the liquid form, particularly in Western Australia, due to the proximity of many mines to the Kwinana factory, and difficulties where hypersaline groundwater is used. In Western Australia, extensive community consultation has been undertaken in planning transport of sodium cyanide by rail and road, together with preparation and training with emergency response agencies along transport routes. Relatively little liquid transport occurs from the Gladstone factory in Queensland.

Available data on incidents where unplanned release of material containing cyanide has occurred are often lacking in detail, and reports are not necessarily reliable. Consideration of such data for incidents that have occurred in Australia and around the world indicates the nature of incidents that can occur, the potential for serious impacts on downstream aquatic areas or other wildlife, and the necessity for measures to prevent them occurring. These include spillage or leaks during transport, or leaks, spills, seepage, overflows and failure of storage structures and associated facilities.

Information on the relatively few transport incidents with NaCN which have occurred during transport of the substance in Australia indicate that where solid NaCN has been spilt, it has been recovered and/or contaminated soil and water collected and disposed of appropriately. Intensive clean-up operations have been required for two accidents, both of which involved transport in CIBCs. Rehabilitation of the site was conducted at the recent Northern Territory (NT) truck accident site, where material spilt into a pond and contaminated soil and water were removed. Release of NaCN in solution occurred in an incident in the Tanami Desert in the NT, but this involved release of liquid remaining in the StoL container used on the return trip after delivery, and was not as a result of overturn or fracture of the StoL container used. Information for occasions where isotainers containing NaCN in solution have been involved in accidents indicates that the vessels have retained their integrity, e.g. with only a minor amount of release occurring through a pressure release valve when a truck overturned. Thus the existing legislative and voluntary control measures in Australia have generally assisted in preventing significant adverse effects on the environment from transport incidents. Additional measures have been taken to reduce the risk of an incident such as that which occurred in the Tanami Desert from recurring.

However, concerns arising from a NaCN spill resulting from a truck accident in Northern Territory in 2007 led to the Nothern Territory Government initiating a review of the regulatory regime applying to dangerous goods transport in the Northern Territory. Key recommendations from that review included the adoption of the 7th Edition of the Australian Dangerous Goods Code (ADG7) and improvements to coordination across Government agencies, both for compliance monitoring arrangements and for emergency response procedures. All jurisdictions, including the NT, have either legislated already or aim to legislate by the end of 2009 for the adoption of ADG7. Improvements in the transport configurations used by the principal transporters of sodium cyanide in Australia have also been made. However, the incident indicates a need for the relevant authorities to monitor the adequacy of legislation and voluntary measures used in the transport of sodium cyanide.

90.3Environmental risk assessment

However, for birds, the available drinking water studies were not considered to be of an acceptable standard for determining endpoints to be used in risk assessment, and suitable dietary studies with birds were also lacking. For this reason, Toxicity Reference Values (TRVs) for assessment of the risk to birds were based on acute toxicity studies. Endpoints (LD50s) for seven bird species were available and that for the most sensitive species was used (mallard ducks, 1.4 mg CN/kg bodyweight), with an assessment factor of 10. There is some support for this value in studies of sublethal toxicity and effects on biochemistry in birds. The resulting TRV of 0.14 was related to allometric drinking water consumption estimates for a range of bird bodyweights, at a range of possible WAD CN concentrations in TSF discharge. This risk assessment indicated that to assure protection of sensitive avian species from acute mortality and from potentially harmful sublethal effects that might lead to delayed mortality (such as greater susceptibility to predators or reduced flying ability of migratory birds), the concentration of WAD CN in water available to birds would need to be 1 mg/L.

This assessment is highly conservative, as it is based on the assumption that birds consume all their day’s water in a single dose. However, differences in drinking behaviour between species make it difficult to extrapolate toxicity results from one species to another. Thus, while waterbirds take several drinks per day and may be able to detoxify a dose of cyanide they have taken in before their next drink, birds with different drinking behaviour may consume a toxic dose within a short period. This may also be the case for bats.

Information on wildlife impacts confirms that major events involving large numbers of bird deaths in a short period - or significant numbers of ongoing deaths - have occurred at heap leach and tailings storage facilities in the USA, as well as at tailing storage facilities in Australia. Information from these reports and from very comprehensive and detailed observations of bird visitation, behaviour and impacts at Australian TSFs gives a good indication of the species of birds and other animals that visit, the types of habitat that are attractive, possible measures to minimise exposure through minimising water areas or making them inaccessible, and shows the relative inadequacies and benefits of active deterrent measures such as scare guns and flashing lights. The Australian evaluations also indicated the difficulties involved in monitoring for wildlife impacts and inadequacies in wildlife monitoring procedures which were common practice, and guidance has been developed to enable more reliable monitoring in the future.

It is concluded from field observation data from a range of sources including comprehensive scientific observations in TSFs in the Northern Territory and Western Australia 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, but that significant wildlife mortalities are unlikely to occur at WAD CN concentrations below 50 mg/L. At concentrations below 50 mg WD CN/L laboratory data indicate that some mortalities may still occur, but few deaths conclusively due to cyanide have been observed at <50 mg WAD CN/L in field studies. Laboratory studies also indicate that sublethal effects may develop, potentially leading to mortality due to other causes (e.g. greater predator susceptibility), but there are no data to confirm this in the field.

Consideration of these possibilities is greatly complicated by differences in bird behaviour, site differences and the actual toxic effects exhibited. Observations indicate that birds do not become averse to drinking cyanide-contaminated mine waste water and may take further drinks even after awakening from cyanide stupefaction. It has also been proven that aversion to drinking hypersaline water does help protect birds from exposure to toxic cyanide concentrations.

The 1 mg WAD CN/L target based on a risk quotient approach is much lower than the protective level of 50 mg WAD CN/L used by the ICMC and is unlikely to be considered generally practicable with present technology, or justified based on other available evidence and the difficulty of extrapolating from the acute toxicity studies. Available field data does indicate that above 50 mg WAD CN/L significant wildlife mortalities are likely to be observed if animals (in particular birds and bats) are exposed. While available data indicate that few mortalities are likely if concentrations remain below 50 mg WAD CN/L, it cannot be concluded that this level is totally safe.

However, reduction of cyanide residues to relatively low levels may be required in some situations to protect downstream aquatic areas. The use of existing technology to recover cyanide for re-use may also be feasible in some situations. However, because of the uncertainties regarding what level of WAD CN is safe and the impracticability of a general 1 mg WAD CN/L limit, it is concluded that a benchmark concentration approach for mitigating the risk to birds is not satisfactory unless supported by other measures. A risk management framework combining concentration controls with measures to minimise exposure and ongoing monitoring and response programs is therefore proposed for the protection of wildlife at TSFs. This is outlined in detail in the recommendations section.

A similar conclusion can be reached for protecting mammals, where again the TRV was based on acute toxicity data. However, it is also noted that a high percentage of reported mammal deaths at TSFs were thought due to the animal becoming bogged in the dam, rather than cyanosis. Hence while a combination of measures is again thought appropriate, measures such as fencing to prevent access are likely to be preferable. Prevention of access to birds or mammals is essential at heap leach operations, where reduction of concentrations is not a feasible approach.

While TSFs, decant ponds, and associated infrastructure containing cyanide solutions do not constitute natural waters, risk assessment confirms the potential for cyanide-contaminated water to harm aquatic life if waste were released into downstream waters. Existing standards and controls on TSF design and operation are considered adequate to minimise the risk of such aquatic exposure occurring, with managed release into receiving waters based on the ANZECC/ARMCANZ (2000a) guidelines.

Impacts on surrounding vegetation due to cyanide contamination are possible, but other factors are also likely to contribute to impacts arising through seepage and rising groundwater (e.g. salinity). Site rehabilitation plans ultimately address the restoration of vegetation on and near TSFs or other structures.