The use of radioactive material, specifically uranium, for the source of nuclear weapons and power spurred the movement toward exploration and mining of uranium ore in the United States. The uranium mill tailings, the waste by-product created during uranium extraction from the ore, posed concern because of the radioactive nature of the tailings and the additional chemicals used for extraction. At first, like many earlier waste practices, tailings were handled in a way that put the environment and humans at risk. This resulted in requiring remediation of the disposal sites, particularly through the Uranium Mill Tailings Remedial Action Program (UMTRAP) under the Uranium Mill Tailings Radiation Control Act (UMTRCA). Uranium mill tailings are, in many ways, similar to any other waste because they must be isolated from the surrounding environment or made inert to where they no longer pose a risk. However, unlike other wastes, the timescale and volume of the mill tailings are orders of magnitude greater, making traditional remediation techniques alone inadequate. A comparison of case studies will encompass the previous points and include a more in-depth look into some key remediation techniques. The case studies include the historic Grand Junction incident, the inactive mill site of Durango, CO used as a model study, and the ongoing remediation of the MOAB site in Utah.
To gain a thorough understanding of the background and overall procedure for the remediation of uranium mill tailings, a few key components should be emphasized. This emphasis will include a summary of the uranium mining processes, the chemical properties of uranium, and the adverse health effects uranium mill tailings pose to the environment and humans. Legislation passed regarding the handling of tailings are a result of the aforementioned points, and therefore, are pertinent to understanding the evolution of the uranium mining industry as well.
Table 1: Concentrations Dry Waste Basis and Distribution of Uranium in Soils (Gavrilescu 2008)
Photo 1. Durango, CO, Uranium Mill Tailings Site Prior to Remediation 1986 (Robinson 2004)
Mill tailings are the largest source of radioactive material in the United States, albeit low radioactivity versus the highly radioactive wastes created from nuclear power plants (Robinson 2004). Because of the leaching process and nature of the original ore, the tailings can also contain high amounts of heavy metals, inorganic compounds, and organic compounds that can prove to be as environmentally harmful as the radionuclides (ASCE 1986). These compounds are referred to as co-contaminants, and in some cases, such as MOAB, UT are actually the primary contaminants of concern. However, the nature of these is considered more traditional and therefore the majority of focus will be on radionuclides until the specific case studies are discussed.
Figure 2: Decay Chain of Uranium-238 (Landa 1978)
“The first is the diffusion of radon gas directly into indoor air if tailings are misused as a construction material or for backfill around buildings. When people breathe air containing radon, it increases their risk of developing lung cancer.
“Second, radon gas can diffuse from the piles into the atmosphere where it can be inhaled and small particles can be blown from the piles where they can be inhaled or ingested.
“Third, many of the radioactive decay products in tailings produce gamma radiation, which poses a health hazard to people in the immediate vicinity of tailings.
“Finally, the dispersal of tailings by wind or water, or by leaching, can carry radioactive and other toxic materials to surface or ground water that may be used for drinking water.”
Table 2: EPA Exposure Pathways (Robinson 2004)
Figure 3: Exposure Pathways (ASCE 1986)
Inhalation exposure is typically attributed to Th-230, Ra-226 and the decay daughters of Rn-222 (ASCE 1986). The radionuclides enter the atmosphere through the emanation of Rn-222 directly or, as mentioned earlier, transported on soil particles by wind (Landa 1978). Once the radionuclides are in the atmosphere they can enter the lungs of humans. The radionuclides, now deposited in the lungs, proceed to decay and emit radiation, causing lung cancer (ASCE 1986). One should note that because of Rn-222’s short lived half-life (4-days), the majority of radiation exposure to lungs is caused by the decay daughters, even though the mobility of the Rn-222 gas is what initially puts the threat into the air (Landa 1978). Inhalation exposure of radon daughters is the cause of nearly 90% of the total dose of radiation humans receive (ASCE 1986).
Ingestion exposure to humans can occur in a variety of ways. A more direct way would be for humans to drink surface or groundwater contaminated directly from the leaching of the uranium mill tailings (ASCE 1986). More indirect ways would be for plants, specifically crops planted near a tailings site, to intake radionuclides from contaminated soil or water and humans to eat the plants. Another similar process would be for animals, livestock or hunting game, to graze on said above vegetation and then, in turn, be consumed by humans (ASCE 1986). Ingestion is the least contributor of radiation exposure.
External exposure is predominately a result of gamma radiation due to the limited range of alpha particles and low penetration depths of alpha and beta particles; alpha particles penetrate skin tissue to a depth of 0.1 cm while beta particles penetrate to a depth of 1.0 cm, therefore making them predominately internal exposure health risks (ASCE 1986). Undispersed tailings exposed at the surface, soil particles with radionuclides adsorbed on them transported by wind or water, and radionuclides in the air are the primary sources of external gamma radiation (ASCE 1986). In the case of the Grand Junction incident, large doses of external radiation were received by the populace because of the way tailings were used, for example the backfill of homes. External gamma radiation accounts for approximately 8% of the total dosage (ASCE 1986).
Despite radiation levels from tailings being low compared to fuel rods and weapons development wastes, the prevailing school of thought is that because the levels are above background radiation, there is a greater probability of cancer development and genetic damage (ASCE 1986). However, there are some studies that indicate no elevated negative impacts on health, such as the analysis from the Church Rock tailings in New Mexico (IAEA-TECDOC-1403). The results from the Church Rock tailings are disputed, however, because it is believed the pre-site conditions used to compare were, in themselves, already affected by mining activity, so the results could be misleading (IAEA-TECDOC-1403). Regardless, cases of higher lung cancer rates for uranium mine workers (ASCE 1986) and almost double the rate of leukemia for the county of Grand Junction, CO in comparison to rest of Colorado State (IAEA-TECDOC-1403) indicate that uranium mill tailings are a health risk, especially to those in close proximity. As a result, legislation was enacted to limit the number of deaths, and an evaluation by the UMTRA program estimated a total of 1289 lives over 100 years can be saved through remediation (Robinson 2004).
The development of laws regarding uranium mill tailings follows the overall progression of environmental remediation and regulation with a few differences. Uranium mining became increasingly larger in-scale in the early 1940’s under the jurisdiction of the Manhattan Engineer District (MED) to find and develop uranium for nuclear weapons. Successive agencies replaced the MED until in 1977, the Department of Energy (DOE) obtained jurisdiction and maintains it to this day.
The inactive tailings site in Grand Junction, CO led to a case in 1966 that is analogue to the Love Canal incident. The uranium mill tailings at Grand Junction were reused for a variety of construction purposes instead of simply being stockpiled, as was typical at the time. The tailings were “not only used for backfill around residential and other structures, but also for aggregate in concrete block and paving material, and even for children’s sand boxes” leading to the increased leukemia rates as mentioned previously (ASCE 1986). What the Love Canal case did as far as initiating research, remediation, and regulation for environmental reform, Grand Junction did for uranium mill tailings and uranium mining. Grand Junction lead to additional research in 1974 beyond the reports of the health effects mentioned in Section 2.3, and an engineering assessment of inactive tailings was completed in 1977 (ASCE 1986).
This led to the passing of the Uranium Mill Tailings Radiation Control Act, passing responsibility of remediation for past sites to the DOE and giving authority to the EPA to set standards for active mines regarding disposal and handling of tailings. The Uranium Mill Tailings Remediation Action Program (UMTRAP) was the program resulting from UMTRCA in 1978. The sites that are considered under UMTRCA can be split into two categories, Title I and Title II. Title I sites, 22 originally in 1978 but currently at 31 (DOE website), were the pre-1970 inactive sites that resulted from mining of uranium intended for nuclear weapons (Robinson 2004). 24 of the Title I sites are listed below in Table 2, with pertinent data such as cost of cleanup and amount of material removed (DOE). Title II sites are typically post-1970 and are mines that mined uranium bound for nuclear reactors (Robinson 2004). Title I sites are the sole responsibility of the DOE while Title II sites are the responsibility of the mining companies until reclamation of the mine and tailings is complete, upon which responsibility is transferred to the DOE for long-term monitoring.
As a result of the UMTRCA, the Department of Energy developed maximum acceptable concentrations for potentially toxic metals such as Uranium and Radon. These concentrations were developed to ensure the safety and stability of freshwater access. In remediating tailing sites, it is important to adhere to the guidelines and threshold set forth by the UMTCRA. In sites with a complex soil stratification and non-uniform distribution of contaminates, creative multifaceted solutions are necessary to account for variability.
Figure 4: UMTRCA Site Locations as of 2015 (DOE)
Table 3: UMTRCA Site Details (DOE)
The sites transferred to the DOE are currently in varied stages of completeness, with some smaller, less complex clean ups only needing routine maintenance and monitoring. While each project is unique and has individual difficulties, there are a few main methods that can be slightly altered to adapt to individual project needs. In the simplest terms, the tailings need to be moved off site and the underlying contamination controlled. Traditional treatment methods for remediation of groundwater often consist of pumping out the affected water and treating it externally. While this method is reliable, it is often very time and cost inefficient for bigger projects with wider spread contamination. Remediating all of the UMTRA sites is a complex economic and environmental issue and, thus, much time and energy has been exhausted to develop more efficient practices. For all of the remediation projects, many different techniques and methods are developed and tested before one is ultimately chosen.
Remediation of uranium mill tailings has techniques and procedures similar to all geoenvironmental clean-up programs, the only difference being the radioactive nature; although, heavy metals and leaching chemicals are of primary concern at some sites. In brief, the first option of any remediation program is to assess the site and demonstrate no threat exists now or in the future with continuous monitoring (non-intervention), or the second, and more costly option, is to reduce the contaminants to levels that are in compliance with regulations (intervention). Intervention can furthermore be categorized as containment, treatment, or removal (Gavrilescu 2008). Non-intervention and the different intervention strategies are discussed in greater detail below before being paralleled with the corresponding case studies.
Because non-intervention is the cheapest option, it is typically vetted first before other options are considered. As mentioned above, an assessment of the site is performed to determine if the radionuclides pose a threat to the surrounding environment. Monitoring of the site is the main justification for non-intervention, as well as the likelihood of natural processes to occur as discussed below. Surface and deep wells are installed at key depths depending on the hydrogeology of the site and are monitored accordingly. A typical layout for monitoring of the sites is provided below in Figure 5. Additionally, montioring of the atmosphere for radon concentrations should also take place. Maximum contaminant levels (MCLs) set by the EPA for water quality are as follows: Beta/photon emitters=4mrem/yr, gross alpha particle=15pCi/L, combined radium226-228=5pCi/L, and uranium=30 micrograms/L (EPA).
Figure 5. Typical Natural Attenuation Monitorig Layout (IAE-TECDOC-424)
Factors considered when determining if non-intervention is suitable include the ability of the natural environment to retard movement of the radionuclides through natural attenuation or the reduction of contaminant levels to a satisfactory concentration by physical, chemical and biological processes, i.e. dilution and dispersion. Dilution is dependent on the solubility, adsorption to surfaces, bioavailability and toxicity of the radionuclides (Gavrilescu 2008). Dispersion is dependent on the particle size and density that the radionuclides are adsorbed too, limiting physical processes such as wind erosion (IAEA Tech Report 424). Dilution and dispersion are not preferred because the net contamination level is not reduced but only local levels are lowered to acceptable thresholds. Thus, natural attenuation is the preferred non-intervention strategy.
Natural attenuation depends on the surrounding environment to accomplish the same end result as an engineered system would do. This can be a chemical process such as precipitation, where the radionuclides are fixated to other chemicals. An example of this is U(VI), a relatively mobile form of uranium in water because of its stability. When U(VI) is reduced to U(IV), however, the reduced uranium is more likely to precipitate and become less mobile (IAEA Tech Report 424). Sorption and complexation by organics like peat are other natural chemical processes. Biological processes such as biomineralization, biosorption and microbial phase transfers are also natural processes that can attenuate contaminants, but are only mentioned in brief (IAEA Tech Report 424). The physical process of decay can also be considered for natural attenuation, but because of the long half-lives of most radionuclides in tailings, it is not very feasible. However, the physical process of filtration is possible if the radionuclides are bound to particles that, because of their size, will not flow in groundwater through natural media (IAEA Tech Report 424).
Remediation becomes more involved if unacceptable levels of contaminants are discovered in monitoring wells or if the site is deemed unsuitable for natural attenuation. Therefore, specific engineered systems are designed and implemented to control the radionuclides and co-contaminants. This involves using the remediation strategies of containment and removal.
Containment is a widely used technique and is implemented at most sites with the purpose of isolating or immobilizing the contaminants (Gavrilescu 2008). Tailings cells can be compared to municipal solid waste landfill cells in that the main objective is to limit water from coming into contact with the waste material. However, because the systems are to be designed for 1000-year disposal life where reasonable, and at minimum 200 years, the cover systems are sloped to 5H:1V versus the typical 3H:1V or 2.5H:1V of landfills (Robinson 2004). Temporary capping of sites is also an interim strategy used while more permanent solutions are looked into (Gavrilescu 2008).
For permanent cover systems, long-term stability and reduction of radon emanation are, among a few other items, the main design characteristics. To begin, particulates from unstabilized tailings piles can be blown by wind distances up to a mile away at dosage rates greater than those recommended, 25 mRem/yr (Landa 1978). Additionally, because external exposure is a health concern, georadiological barriers are used to reduce the amount of radiation energy emitted, thus serving a dual purpose besides isolating the tailings from wind and water (Inyang 2005). Cover thickness in regards to limiting gamma radiation depends on the attenuation coefficients of soil and water, the density of the barrier, and the intensity of the initial gamma radiation (Inyang 2005). Design of the cover system in regards to long-term stability depends more on filter criteria, long-term climatological models, and ease of maintenance or complete lack thereof (Abt 1991). Past design criteria used Sherard filter criteria as a means of determining compatibility between the various layers, but work by Abt (1991) allowed for a design that took into consideration precipitation based on local hydrographs, interstitial water velocity, and drainage time through the materials (Abt 1991). For more details refer to the references. Two different types of cover systems are provided below in Figures 6 and 7 (IAEA-TECHDOC-1403). One should notice the commonality of a radon (georadiological) barrier, a drainage layer, and various protection layers. A completed cover system can be seen in Photo 2.
Figure 6: Cover Design for the UMTRA Estes Gulch Containment Structure (IAEA-TECDOC-1403)
Figure 7: Cover Design for the UMTRA Monticello Containment Structure (IAEA-TECDOC-1403)
Photo 2. Durango, CO UMTRAP site remote disposal cell at Bodo Canyon, 1992 (Robinson 2004)
Subsurface barriers with low-permeability, such as slurry walls, sealable-joint sheet piles, and grout curtains, are another form of isolation used in many other remediation cases but are useful to tailings that can contaminate groundwater (IAEA-TECDOC-424). In the MOAB, UT case study (Section 4.1), a clean freshwater barrier is used to isolate the co-contaminate ammonia leaking from the tailings pile and prevent it from entering a nearby stream (Karp 2005).
Immobilization of the radionuclides is more expensive than the isolation techniques above but are necessary for many sites, especially if the tailings are highly toxic and highly mobile (Gavrilescu 2008). Unlike physical isolation, immobilization has the intent of affecting the contaminant itself and changing it into a form that is less mobile (IAEA-TECDOC-424). Immobilization can be done in-situ using chemical, biochemical, or thermal methods, or it can be done ex-situ at an on-site or off-site plant (IAEA-TECDOC-424). In-situ treatment is the enhanced and expedited version of natural attenuation. Chemical immobilization injects grouts and other chemicals into the ground to change the pH and redox conditions, and in so doing, stabilize the contaminants making them less likely to leach out (IAEA-TECDOC-424). Chemical injection must be done to reduce further contamination by the chemicals intended for treatment and is dependent on the hydraulic properties of the final state of the contaminant. Thermal treatment vitrifies the soil by increasing the temperature until a melted, solid product is formed, but this is economically unfeasible for many tailings due to the large volume (IAEA-TECDOC-424).
Another form of immobilization is the use of permeable reactive barriers (PRBs) installed below ground at key locations. Unlike the impermeable barriers used for isolation, these barriers are constructed to allow groundwater to flow through them. The barriers are made with materials intended to react with the contaminants in the groundwater and immobilize them within the barrier (Dwyer 1997). The procedure typical entails removing a portion of the aquifer material, replacing it with the reactive barrier, and then using impermeable barriers to funnel the groundwater in to the reactive material (IAEA Tech Doc 424). The advantages to this are that no by-product sludges are produced, there is little maintenance cost if designed properly, and there is no active cost of pumping used in removal techniques (IAEA Tech Doc 424). The downside to PRBs is that they are only cost effective at relatively shallow depths (Gavrilescu 2008). A PRB was part of the Durango site remediation.
Removal of the contaminants, like the previous techniques, relies on chemical, physical and biological processes. The process known as pump and treat, a prevalent technique in the 1990s and currently being used in the MOAB project, assumes that contaminant levels can be reduced through ion exchange or sorption and the precipitation process. Pump and treat is a widely used remediation technique for a variety of remediation projects, with a general schematic of the process seen below in Figure 8.
Figure 8: General Pump and Treat Schematic (IAEA Tech Report 424)
Pump and treat methods specific to uranium removal from contaminated groundwater rely on chemical mechanisms that make uranium more soluble, as juxtaposed with the immobilization technique that attempts to make uranium insoluble. By making the uranium soluble and suspended in aqueous solution, the uranium laden water can be pumped out of the ground, treated at the surface, and then injected back into the ground. Uranium solubility can be enhanced by meeting the following conditions: uranium solubilization by exposing it to chelating ions like carbonate, increasing availability of complexing anions, and uranium oxidation to the hexavalent state, U(VI) (Gavrilescu 2009). Chelating ions and complexing anions work by binding to the uranium cation, and in so doing greatly increase the ability of the uranium to be leached and extracted (Gavrilescu 2008). However, a report by the National Academy of Sciences provided an extensive assessment of the pump and treat method and found it be very inefficient (IAEA Tech Report 424). Similarly, Gavrilescu (2008) states the methods are often expensive and the procedure can prove difficult in isolating the treatment of the target metals when there are a variety of other competing ions. However, this method was used to treat the co-contaminant ammonia at the MOAB site as discussed in Section 4.0.
Other methods of removal include extracting the tailings and washing them to remove the radionuclides and other contaminants and then replacing the clean soil back. This can be done with chemicals, similar to the leaching process originally performed on the ore, or it can be done using the physical process of screening. Screening capitalizes on the affinity radionuclides have for certain particle sizes, as mentioned in Section 2.1 and Table 1 (Gavrilescu 2008) and separates them to reduce the overall concentrations.
Located in the heart of America’s scenic parklands and a mere 750 feet from the Colorado River, contaminated soils from years of operation of the Atlas Uranium Mill posed a threat to the civilizations and ecosystems that utilize the area. When the mine ceased operation, 10.5 million tons of tailings and contaminated soils rested in a 130 acre unlined pile (Karp 2005). In 2001 possession of the site was transferred to the DOE, which was tasked with removing the piles without damaging the surrounding area.
Figure 9: Moab Remediation Site Location
Initial site evaluations indicated that contaminated groundwater from the pile was entering the Colorado River, a major issue due to the societal and environmental importance of the Colorado River. The first course of remedial action was to limit the contamination of groundwater in the piles and limit the amount of groundwater seeping into the River. For more permanent remediation, the DOE determined that relocating the piles was the best way to prevent contaminated water from entering the watershed. The groundwater improvements at the site continued during pile removal since the underlying ground area still contained hazardous materials.
The DOE implemented many tactics to control the groundwater entering the river while a more permanent solution was developed. A system of vertical band drains was installed to allow a passageway for pore fluids to escape the pile and extra overburden mass was added to increase the pressure and the rate of fluid extraction. (Karp 2005) Through this, pore fluid volumes were reduced and large amounts of uranium and other harmful substances such as ammonia were extracted from the site. Another technique used to remove contaminates was the construction of evaporation ponds. Groundwater was pumped into a shallow 4 acre pond and allowed to evaporate exposing contaminates that can be collected and removed. In addition to reducing contaminates, freshwater injection wells were installed to add pure water and dilute the concentration of contaminants in the groundwater. (Karp 2005) This is vital because it will maintain volumetric flows while reducing contaminant concentrations entering the river. An aerial view of the configuration of pipes and the pond can be seen in Figure 10.
Figure 10: Aerial View of Moab Evaporation Pond and Surrounding Area
The permanent disposal site for the uranium pile is located approximately 30 north of the mining site. The disposal process consists of loading the mill tailings onto railcars that are covered and shipped to the disposal site. Tailing removal began in 2009 and it is expected to take 15-20 years to complete the removal of 130 acres of tailings.
The previous case study detailed the work of removing the tailings from the mines to a distant site. With the source of contamination removed, it is important to ensure that contaminates that have leached below the ground surface be prevented from spreading and to limit the leaching of the displaced contaminated soil. The Durango UMTRCA Project site was previously operated as a lead smelter plant and to furnish Vanadium before it was used for uranium ore processing. In use since 1880, the underlying soils at the site developed excessive concentrations of toxic metals such as uranium, selenium, and manganese. (DOE 2002) Groundwater contamination has the potential to spread to local drinking water and thus poses a threat surrounding communities. The location of the site in relation to the town can be seen in Figure 11.
Figure 11: Durango Remedial Site Location
To assess the severity and extent of contamination, the DOE monitored groundwater at the site. Table 4 details the observed initial concentrations along with UMTRCA standards. (DOE 2002) With several contaminants having average concentrations exceeding acceptable standards, remedial efforts were taken to improve the quality of the groundwater at the site. To address the issue, the DOE developed several alternative action plans and determined that natural flushing and continued water monitoring would reduce groundwater concentrations to below UMTRCA standards in a few decades.
Table 4: Observed vs Acceptable Groundwater Concentrations of Heavy Metals
One possible solution developed for the disposal site was a reactive barrier treatment system. A typical PRB system is provided below in Figure 10. In this design, a series of impermeable walls would be placed to direct and control any resulting groundwater. With an underground flow path established, the groundwater can be forced through low permeability treatment zones (Dwyer and Marozas, 1997). For the contents of the treatment zones, many different mechanisms such as chemical, physical, and biological treatment systems were evaluated and considered. Several inorganic reactive materials previously used for the removal of uranium were tested and ultimately a combination of metallic iron and iron foam was chosen for treatment. Through testing, it was also determined that the rate of contaminant reduction was significantly increased when the iron was in contact with a copper catalyst. (Dwyer and Marozas, 1997) These specific metals were chosen since they are widely available, environmentally harmless, and capable of operating for long periods without the need for human upkeep. These treatment methods were implemented and continually monitored and evaluated over time and adapted to produce optimal effects.
Figure 12: Trench and Gate PRB Design (IAEA-TECDOC-424)
In addition to the plans developed, the DOE was responsible for preparing environmental assessments on non-intervention alternatives to serve as a baseline for evaluating the remedial efforts of other strategies. The environmental assessment of both plans detailed the affected area and environmental consequences of issues such as ground and surface water, land and water use, human health, and ecological risk were evaluated. (DOE 2002) It is these environmental assessments as well as economic and social impacts that are then analyzed to determine the best course of action.
Since the discovery of the potential harmful effects of exposure to radioactive elements, much research has been done to determine sustainable, efficient, and safe practices for removing toxic chemicals from the environment. In 1978, the US Department of Energy implemented the Uranium Mill Tailings Radiation Control Act to set guidlines and standards for the handling of radiological materials. Many earlier references detailed pump and treat options which obtain desirable results but are neither cost nor time efficient for larger scale projects. Thus, many in situ remediation techniques, such as permeable reactive barriers and filtration, are used. Like most large scale projects, there is not one be all solution for the remediation of uranium contaminated soils. However, each project must be thoroughly examined and tested in order to develop an acceptable solution. Studies of the projects in Moab, Utah and Durango, Colorado as well as many other succesfull cleanups can serve as lessons for the future. By learning from previous successes and failures, several decades of malpractice can be reversed.