Since no means exist to make the wastes from uranium mining harmless, reclamation of these wastes can only intend to confine the contaminants and prevent their spreading into the environment by the best means possible. This confinement must prevent the release of radon gas and gamma radiation, the wind erosion of contaminated material, and the release of seepage to surface waters and groundwater.
In the early years of uranium mining after World War II, the mining companies often left sites without any clean up after the ore deposits were exhausted: often, in the United States, the mining and milling facilities were not even demolished, not to mention reclamation of the wastes produced; in Canada, uranium mill tailings were often simply dumped in one of the numerous lakes.
The untenability of this situation was for the first time recognized by U.S. legislation, which defined legal requirements for the reclamation of uranium mill tailings in 1978 [UMTRCA1978]. On the basis of this law, regulations were promulgated by the Environmental Protection Agency (EPA) and the Nuclear Regulatory Commission (NRC) [EPA1983a], [EPA1983b], etc. These regulations not only define maximum contaminant concentrations for soils and admissible contaminant releases (in particular for radon), but also the period of time, in which the reclamation measures taken must be effective: 200 - 1000 years. The reclamation action thus not only has to assure that the standards are met after completion of the reclamation work; but for the first time, a long-term perspective is included in such regulations. A further demand is that the measures taken must assure a safe disposal for the prescribed period of time without active maintenance. If these conditions cannot be met at the present site, the tailings must be relocated to a more suitable place.
Considering the actual period of time the hazards from uranium mining and milling wastes persist, these regulations are of course only a compromise, but they are a first step, at least. Regulations for the protection of groundwater were not included in the initial legislation, they were only promulgated in January 1995 [EPA1995].
Based on these regulations, various technologies for the safe and maintenance-free confinement of the contaminants were developed in the United States during subsequent years. The reclamation efforts also include the decontamination of homes in the vicinity built from contaminated material or on contaminated landfills.
In Canada, on the contrary, authorities decide on a site-by-site basis on the measures to be taken for reclamation; there are no legal requirements. The Atomic Energy Control Board (AECB) has only promulgated rough guidelines; and it decides, together with the mine and mill operators, on the necessity of measures to be taken. Therefore, it is no surprise that the Canadian approach results in a much lower level of protection.
Alternatives for Uranium Mill Tailings Management
When speaking about uranium mill tailings management, one solution seems to be near at hand: bringing the wastes back to where they came from, into the mined-out mining cavities and pits. Unfortunately, this is generally not a satisfying solution. Although the uranium was removed from the ore, the tailings are no less hazardous than the ore. Quite the contrary: the tailings still contain 85 % of the radioactivity and all toxic contaminants are present in the ore; moreover, the contaminants are now, due to the mechanical and chemical properties of the material, much more mobile and susceptible to release into the environment.
Bringing the wastes back to an underground mine is therefore in most cases not an acceptable option; after the halt of the pumps, the material would be in direct contact with groundwater. Further, there is often only a small part of the old galleries accessible.
The situation is similar for the option of bringing the tailings back to a former open pit mines. Here, also, the tailings will be in direct contact with groundwater, or can contaminate it through seepage. This option can only be considered, if groundwater contamination can be permanently excluded due to the presence of proven natural or artificial tight layers. Its advantage is the relatively good erosion protection.
In France, on the other hand, the concept of dumping the tailings in former open pits in groundwater is pursued at several sites in recent years. In this case, a highly permeable layer is installed around the tailings, to allow free groundwater circulation around the tailings. Since the permeability of the tailings themselves is lower, it is anticipated that nearly no exchange of contaminants between tailings and groundwater takes place. A similar method is being tested in Canada for the disposal of uranium mill tailings in lakes (called "pervious surround disposal").
In most cases, there will be no choice other than dumping the tailings above ground. In this instance, the protection requirements can be realized more easily in a controlled way, but additional measures for erosion protection must be included.
In any case, the site must be suitable for the disposal by consideration of its geology and hydrology: it must not be located on geological faults, must not be endangered by earthquakes; impermeable geological layers should be present; it should not be located in the flood plain of rivers; groundwater level should be as deep as possible; possible seepage excursions should not endanger groundwater; the site should not be located too far from suitable deposits of clays needed for covers and liners; and it should be located remote from settlements, etc.
During site investigations, monitoring of groundwater flow has to be carried out. The age of groundwaters can be determined by isotopic methods; this knowledge allows the identification of links between groundwater and surface water, for instance. Only after sufficient site data has been gathered, can groundwater flow be modelled by three-dimensional computer modelling. Based on the monitoring data gathered and on modelling results, the impact of anticipated or existing contaminant releases can be predicted and respectively followed up.
If a suitable site has been found, appropriate liners and covers have to be installed to confine the wastes in the optimum and most durable way.
For an intermediate disposal of tailings, lined ponds are suitable; radon release can in this case be minimized by a water cover of 1 m minimum.
Management of Existing Uranium Mill Tailings
If an existing tailings deposit is to be reclaimed according to the requirements of a safe long-term disposal, detailed site investigations have first to be performed to allow a thorough hazard assessment. If the deposit presents an immediate hazard, preliminary management options (such as the installation of a cover against dust releases, collecting of seepages) can be performed, as long as they don't impede the measures to be taken later for a safe long-term disposal.
At first, it has to be determined if the deposit can be reclaimed in its original place. This requires detailed hydrogeological site investigations. Under certain circumstances, it may become necessary to remove the whole material temporarily, to allow installation of a liner, onto which the material is then brought back. This was the case for example at the Canonsburg (Pennsylvania, USA) site. Under very unfavourable circumstances, it may become necessary to permanently relocate the tailings to a more suitable site. In the United States, this option was selected for those sites where the tailings were located in densely populated areas on sites at high flood risk.
For a safe long-term disposal, the following measures are required:
- If no natural impermeable layers are found on the site, a liner must be installed beneath the contaminated material, to prevent groundwater inflow and seepage releases. Appropriate materials have to be selected for the liner, that permanently keep their properties even under impact from the deposited tailings. The liner might consist of multiple layers to meet all requirements.
- The following measures serve to enhance the mechanical stability of the deposit: dehydration of the slurries, flattening of the slopes, installation of an erosion prevention technique.
- On top of the deposit, a cover has to be installed, for protection against release of gamma radiation and radon, infiltration of precipitation, intrusion of plants or animals, and erosion. Generally, this cover consists of several different layers to meet all requirements. Under certain circumstances, the liner under the deposit can be omitted, if a suitable cover is installed.
- Collection and treatment of seepage is necessary for as long as the measures taken for lining and covering are not yet fully effective, to assure the release of treated water only. According to the U.S. regulations, liners and covers must in the long term assure the release of minor amounts of seepage only, and the safety of the deposit must be based on passive measures.
- Finally, it has to be determined, to what extent contaminated material was used for construction purposes or in landfills on properties in the vicinity . The contaminated properties have to be included in the reclamation programme.
Reclamation After In-Situ Leaching
After termination of an in-situ leaching operation, the waste slurries produced must be safely disposed, and the aquifer, contaminated from the leaching activities, must be restored. Groundwater restoration is a very tedious process that is not yet fully understood. So far, it is not possible to restore groundwater quality to previous conditions.
The best results have been obtained with the following treatment scheme, consisting of a series of different steps [Schmidt1989], [Catchpole1993b]:
- Phase 1: Pumping of contaminated water: the injection of the leaching solution is stopped and the contaminated liquid is pumped from the leaching zone. Subsequently, clean groundwater flows in from outside of the leaching zone.
- Phase 2: as 1, but with treatment of the pumped liquid (by reverse osmosis) and re-injection into the former leaching zone. This scheme results in circulation of the liquid.
- Phase 3: as 2, with the addition of a reducing chemical (for example hydrogen sulfide or sodium sulfide). This causes the chemical precipitation and thus immobilization of major contaminants.
- Phase 4: Circulation of the liquid by pumping and re-injection, to obtain uniform conditions in the whole former leaching zone.
But, even with this treatment scheme, various problems remain unresolved:
- Contaminants, that are mobile under chemically reducing conditions, such as radium, cannot be controlled,
- if the chemically reducing conditions are later disturbed for any reasons, the precipitated contaminates are re-mobilized,
- the restoration process takes very long periods of time,
- not all parameters can be lowered appropriately.
Since the alkaline leaching scheme is the only one used in the Western world in-situ operations, most restoration experiments reported refer to this scheme. Therefore, nearly no experience exists with groundwater restoration after acid in-situ leaching, the scheme that was applied in most instances in Eastern Europe. The only in-situ leaching site restored after sulfuric acid leaching so far, is the small pilot scale facility Nine Mile Lake near Casper, Wyoming (USA). The results can therefore not simply be transferred to production scale facilities. The restoration scheme applied included the first two steps mentioned above. It turned out that a water volume of more than 20 times the porevolume of the leaching zone had to be pumped, and still several parameters did not reach background levels. Moreover, the restoration required about the same time as used for the leaching period [Nigbor1982].
An elimination of the hazards presented by the legacy of uranium mining and milling is not possible by any management options.
For the reclamation of uranium mill tailings, concepts for a comparatively safe long-term disposal have been developed.
For the restoration of groundwater after in-situ leaching, there exists no satisfying management option.