4.1. RISKS FROM MINES
There are two basic types of mine, open cast and underground. Each type has specific risks associated with it, but some are common to both. Any excavation, whether underground or open cast, is subject to degradation and may eventually collapse or cave in. Open cast workings are subject to normal erosion forces leading to, for example, slope instability and failure of rock walls, with the ensuing risks of injury and damage to adjacent property. The collapse of underground workings may cause damage at the surface to property and possible injury to the local population. Unsealed mine shafts pose a risk of accidents to unsuspecting forest workers, hikers, etc. Abandoned adits might invite people to enter and explore them, being unaware of the unsafe conditions.
Mine workings frequently extend below the groundwater table, thus providing direct pathways for interaction with the atmosphere. The introduction of oxygen and resulting oxidation of sulphide minerals in the ore or host rock will lead to the generation of acid mine drainage. This is a common phenomenon and is not specific to uranium mining. The effects of acid drainage are discussed in the following section. In turn, direct access to groundwaters also opens up pathways for the introduction of contaminants into the groundwater.
The mining operations themselves may have introduced contaminants into the (aqueous) environment. Examples include degradation products of explosives, machinery oil or transformer fluids.
The causes of acute failure in uranium mining and milling residue impoundments are no different from those at other mines. The causes include:
earthquake induced instability and failure owing to cracking or liquefaction;
physical weakness of embankments and slopes leading to breaching; erosion from heavy rain or adjacent waterways leading to dispersal of residues and thinning of retaining dams or overtopping by decant water; cracking induced by settlement; piping; slumping of material into the containment causing overtopping and/or erosion of the wall or its physical destruction; and spillway collapse caused by heavy overflow of decant or slurry after severe rainfall.
There are also many forms of chronic failure. These include:
(a) Dispersal of radioactive dust — uncovered residues are allowed to dry out, followed by wind blown distribution of dust;
(b) Erosion of residues from the outer surfaces of the containment;
(c) Acid drainage through the floor and/or walls of the containment;
(d) Contaminated surface runoff draining into natural waterways.
Both acute and chronic events are commonly associated with non-radio-logical impacts related to heavy metals and other toxic compounds in the tailings, pore water and decant water. The generation of acid drainage from residues containing sulphide minerals is of particular concern. Acid drainage tends to exacerbate the probability of environmental contamination because of the solubility (and hence greater mobility) of many metals under acidic conditions. It may also affect the bioavailability of toxic metals and compounds.
Hence the neutralization of tailings prior to deposition, when required, has become common practice in more recent times. Surface capping of solid residues to prevent the ingress of seepage waters helps to reduce the generation of acid drainage.
4.3. HUMAN HEALTH RISKS
Sudden failure of any engineered structure may cause death through drowning, crushing or suffocation. Uranium mining and milling residue containments are no different from water containment dams in that regard.
Landslides involving uranium mining and milling residues have killed people and destroyed property. However, the failure of uranium tailings containments does not feature highly on the list of dam failures to have caused death or major
proportion of the total number of mine dams.
Although other types of mining residues may also contain naturally occurring radioactive materials the particular human health risk that is associated with uranium mill tailings is the risk from radioactivity. In turn uranium mill tailings may also pose risks associated with their inventory of heavy metals and other chemical elements, such as arsenic. People in uranium mining districts may be exposed to radiation doses from mining, milling, transport of radioactive materials, radioactive dust and contaminated water and foodstuffs.
Tailings may be a significant proportion of the health hazard because of the manner in which they were commonly disposed of. Their disposal at the surface over relatively large areas allows significant flux of radioactive gases and interaction with surface water. In many historical uranium mining areas both have constituted a pathway from the immediate surroundings of the mine site and affected the general public.
The long half-lives of radionuclides from uranium tailings and the demon-strated risks associated with them have given rise to high levels of concern among the general public and in government — in some places exacerbated by official secrecy and lack of data on health impacts.
Perhaps the most direct implication of mining and milling residues as a radiation source sufficient to cause human health impacts relates to their re-use for building materials. This has happened in many places around the world.
4.4. ENVIRONMENTAL IMPACTS
Many articles in the literature purport to describe impacts on the natural environment from uranium mining in general, and the placement/disposal and management/mismanagement of mill tailings in particular. However, the description of environmental impact that most of these articles present is limited to a qualitative description of impact in terms of contaminated soil, surface waters, groundwaters, lake sediments, etc. Any quantitative description is usually given in terms of the area/volumes affected and concentrations of radioactive elements, or activity levels and doses. A body of literature exists on the contamination of environmental media directly linked to consumption and therefore potential uptake of radionuclides by humans, principally soils and water. In contrast there are relatively few articles that set out to quantify the level of harm to the biosphere caused by uranium mining and milling residues.
Research into the biological impacts of the nuclear fuel cycle is a relatively new science and is strongly biased towards research on the risks,
into the biological impacts from the uranium production part of the nuclear fuel cycle has grown from:
(a) A realization that fisheries in the vicinity of some uranium mines were deteriorating;
(b) A realization that radionuclides could be taken up and concentrated by plants, thereby posing a potential risk to vegetation as well as to animals higher in the food chain, including humans;
(c) A realization that radionuclides could be taken up and concentrated by animals, thereby posing a potential risk to the health of individuals, communities and ecosystems as well as animals higher in the food chain, including humans;
(d) Documentation on the concentration of radionuclides in bottom sediments and their long term availability to bottom feeders and potential for multiple cycling through the aquatic food chain over the long term;
(e) Development of transfer pathway models for the transfer of radiological dose through the food chain;
(f) Concern that tangible impacts on human health from radioactive contam-ination associated with uranium mines and tailings piles may also be having significant impacts on the environment;
(g) Questions over the validity of the ICRP’s assumption that if human beings were adequately protected other species would also be protected.
Since the late 1970s there has been a general increase in concern for the environment, presumably due to evidence that the actions of humans are causing visible and significant environmental changes. Furthermore, close attention is now being paid to the larger implications of harm being caused to elements of particular ecosystems with consequent impacts on biodiversity. It is perhaps for these reasons that the issue of environmental protection in the context of ionizing radiation is being addressed in many countries.
The impacts on the environment from uranium mining and milling residues are not all related to the radionuclide content alone. The presence of other contaminants can exacerbate the availability of the radionuclides to the environment, and in some cases the other contaminants have direct harmful effects in themselves. In this regard, the potential for uranium mining and milling residues to cause environmental harm is little different from that of other forms of mining, and the resultant impacts may be quite similar. Indeed it is not adequate to consider the radiological risk only. The other effects may include:
(2) The chemical toxicity of heavy metals and metallic compounds;
(3) The chemical toxicity of non-metallic minerals and compounds in the ore or introduced during processing (e.g. sulphuric acid, kerosene);
(4) Acidity, resulting from sulphidic (ore) minerals or acid introduced during milling;
(5) Increased turbidity in surface waters;
(6) Increased salinity.
The types of non-radiological contaminants that may cause harm are dependent on the mineralization in the ore body, the gangue mineralogy, the overburden mineralogy and the processing technique used in the mill. Elevated acidity plays a major role in increasing the mobility of metals in aqueous solution, including uranium, as well as copper, arsenic, cadmium and other metals. The effects of heavy metal pollution are complex and strongly dependent on local geographic and climatic factors, the mix of chemical constit-uents present, and on the nature of the affected organisms. Effects can be lethal or non-lethal. Chronic sub-lethal poisoning can affect growth, reproduction, behavioural patterns, and result in lower resistance to disease, as well as causing the organism to have body concentrations of elements above standards permitted for consumption. In some instances the non-radiological hazards have a much larger effect than the radiological contaminants.