Exposure from flotation use in base metal ore processing

30.3Summary of environmental exposure assessment

Sodium cyanide manufacturing facilities in Australia are located in industrialised areas, where there is limited habitat for wildlife. These facilities must meet strict operational requirements managed by environmental agencies and/or local councils. There are licence limits on emissions of gases and particulates to the air or cyanide or nitrogen-containing substances formed from cyanide to water. To meet these, where possible cyanide is recovered and returned to the NaCN process, otherwise waste streams are treated to destroy cyanide residues. These measures are expected to prevent exposure of wildlife to harmful levels of cyanide.

Wildlife should not normally be exposed to cyanide during the transport, delivery and storage of NaCN. However, wildlife could potentially be exposed through an accident, leakage or other incident during transport. The extent of exposure would depend on the amount of cyanide released and the clean-up action taken. Delays in reacting to an incident and completing recovery/destruction of cyanide spills increase the risk of environmental contamination spreading or wildlife being exposed (e.g. by drinking pooled water, or contamination of a pond or stream).

Wildlife exposure through use in gold mining

Tailings Storage Facilities (TSFs) and associated infrastructure may provide habitat for wildlife and may also be accessed by livestock. Habitat in and around a TSF may be attractive to various species for swimming, diving, roosting, foraging, breeding and protection from predators. TSFs and their surroundings may provide various food sources, including terrestrial and airborne insects, live prey and carcases for raptors, and in poorly managed tailings facilities, potentially also phyto- and zooplankton and aquatic macroinvertebrates. TSFs and associated ponds may attract wildlife as a source of drinking water, particularly when there are few alternative water sources available, such as in arid areas. Poorly managed tailings facilities are therefore a major source of environmental exposure.

Studies have identified a large number of bird species at TSFs at gold mines or other mining operations, including species such as waterbirds and waders which spend significant time in or near water, ranging to species which fly in once or twice per day to drink, and including protected wildlife species. Various species visit at night (e.g. waterfowl and bats), when they are unlikely to be seen. Observations indicate that visitation by reptiles, amphibians and terrestrial mammals such as kangaroos are far outweighed by that by birds and by bats. Exposure to cyanide residues (concentrations from as low as 1 mg WAD CN/L to well above 100 mg WAD CN/L) at mining sites may occur via multiple pathways, including direct consumption of cyanide in drinking water, incidental ingestion or contact with cyanides in water and sediment during feeding, inhalation of HCN gas, and skin contact with cyanide-containing waters. Adult birds may potentially expose their young during feeding (regurgitation of swallowed water/sediment). Secondary exposure of predatory or scavenging wildlife could also occur.

Exposure to aquatic organisms is not a concern in the TSF or associated dams and ponds, but is a possible concern if loss of cyanide residues occurs in runoff, overflow or seepage from a TSF, or in water draining from other areas of the site. Fish, aquatic invertebrates, algae and plants in downstream areas could then be exposed to cyanide residues, as could birds and animals consuming affected organisms or drinking contaminated water. Aquatic and terrestrial areas could also be potentially exposed through failure of a TSF. Exposure of vegetation (via root contact) and soil-dwelling organisms could potentially occur to groundwater in the vicinity of TSFs and heap leach operations. Groundwater also needs to be considered as it may be extracted for beneficial uses such as livestock watering and irrigation.

Licences require the installation of bores at mining operations to monitor groundwater in the vicinity of structures such as TSFs, and these specify permissible limits for WAD CN (typically 0.5 mg /L). Available data indicate measured levels in groundwater in the immediate vicinity of TSFs in Australia range from < 0.01 to > 100 mg/L WAD CN, but the measured concentrations fell to low levels 100-200 m from the measured TSFs. In some situations dam design and local hydrology/hydrogeology lead to significant seepage. Excessive rising of the watertable and/or movement of seepage water must then be prevented by the use of recovery bores, ranging from a few to many bores. Ongoing monitoring is essential to ensure seepage is managed satisfactorily.

During gold ore beneficiation and processing some emission of HCN to air occurs, but overall the loss of HCN to air during these processes is only very minor (~1-2%). However, after material containing cyanide is deposited in tailings storage facilities, HCN emission to the atmosphere is expected as a significant means of dissipation. Some loss of HCN to the atmosphere may also occur from other mining and industrial uses.

While worldwide use of NaCN for gold ore processing may be a significant anthropogenic source of HCN in the atmosphere, on a global scale modelling using measured data suggests this is a minor source of release of HCN to the atmosphere compared to biomass burning. The estimated atmospheric lifetime for HCN is ~5 months, limiting the potential for long term increases in atmospheric concentration. Releases of HCN from gold industry or other uses of sodium cyanide are therefore considered unlikely to lead to significant increases in concentration of HCN in the atmosphere, and HCN is not considered to be of concern as a greenhouse gas or ozone depletor.

Modelling exposure of wildlife from gold mining use for risk assessment

While wildlife may be exposed to cyanide in a TSF by various means, for risk assessment purposes estimated exposure of wildlife to cyanide has been calculated based on the drinking water exposure route, as it is considered the most likely and most important pathway. As actual data on drinking water intake are limited, equations have been used which estimate drinking water intake rates for avian or mammalian wildlife species based on their bodyweight. These equations were developed from actual data for northern hemisphere species ranging widely in bodyweight. The calculated water intake rates are considered estimates only and daily water intake by birds and mammals may be more or less than estimated, particularly depending on environmental conditions (e.g. temperature).

Drinking by wildlife may occur once or at a number of times during the day, and this is critical to estimate exposure to wildlife species from cyanide due to the potential for metabolism of small repeat doses without adverse effects. As a worst case scenario, it is assumed that even if multiple drinks are taken, all drinking water is obtained only from contaminated sources. Similarly, it is assumed that no aversion occurs due to sublethal cyanide exposure or other components in the water (e.g. salinity).

Calculations were made for various concentrations of WAD CN across the range reported in Australian TSFs (1-600 mg/L) for birds and small mammals (bodyweight ranging from 0.01-1.5 kg body weight).

Waste produced in base metal flotation processes flows to tailings dams, hence there are similar potential routes of environmental exposure to those arising from cyanide use for beneficiation in gold mines. However, available information indicates that in general a high percentage of the cyanide used is converted to insoluble metal complexes during use, with some loss as HCN. Significant exposure of wildlife to cyanide in tailings facilities therefore appears less likely than with gold beneficiation. Therefore consideration of environmental exposure from goldmine use in tank leaching systems should adequately cover potential exposure resulting from base metal flotation use.

Electroplating, metal cleaning and metal surface treatment operations occur at enclosed industrial sites, generally with any unconsumed cyanide being treated to destroy it prior to disposal. Hence, potential exposure of wildlife is limited to exposure to waste delivered to landfill or released to the sewer if the waste were not adequately treated prior to discharge.