Abstract
The purpose of this paper is to provide an overview of the experience of the US Environmental Protection Agency (EPA) in cleaning up radioactively contaminated sites. In the USA, EPA regulates the radiological clean-up of uranium mill tailings sites, some Department of Energy legacy sites within the US nuclear weapons complex, and Superfund National Priorities List sites. The approach to site remediation decisions, including the determination of clean-up levels, varies according to the enabling legislation granting EPA these authorities. Past practices that gave rise to many of the existing exposure situations at legacy sites were permissible before the advent of environmental clean-up legislation. The Uranium Mill Tailings Radiation Control Act of 1978 authorised EPA to set applicable radioactivity concentration standards for soil clean-up at inactive uranium mill sites and vicinity properties. For the other categories of sites mentioned above, remediation goals are typically based on not exceeding a target excess cancer risk range established under the Comprehensive Environmental Response, Compensation, and Liability Act (also known as ‘Superfund’). EPA’s regulations for cleaning up various contaminated sites in existing exposure situations often result in residual doses that are typical of optimised doses in planned exposure situations. Although the clean-up levels selected may differ from those adopted in other countries, recommendations from the International Commission on Radiological Protection are reflected in the exposure assessment methodologies used in their establishment.
Keywords
1. RADIOACTIVELY CONTAMINATED SITES IN THE USA
The types of radioactively contaminated sites in the USA can be categorised chronologically to some extent. The oldest sites are those where naturally occurring radioactive material (NORM) was present before the existence of radioactivity was discovered. Even today, these areas of elevated natural background radiation are seldom subject to remediation, except where programmes exist to test for indoor radon, and fix houses found to have high concentrations of radon decay products. Other than these undisturbed NORM sites, some of the oldest contaminated sites in the USA arose from processing radium following its discovery. Radium contamination resulted not only from careless practices that were designed to extract radium, but also from the mining of vanadium and other rare earth elements which left high concentrations of radium behind as a mining byproduct.
The discovery of nuclear fission and the development of nuclear weapons as well as nuclear power gave rise to contaminated sites dating from the mid-20th Century. This era produced sites associated with the exploration, mining, and milling of uranium and thorium ore; the building and testing of nuclear weapons; and the improper disposal or release of radioactive waste containing fission products, activation products, and transuranic isotopes. Additional sources of contaminated sites arose from medical, industrial, research, and other uses of radioactive material during the time when disposal often simply meant burial on site without regard for groundwater contamination or off-site migration. These various types of contaminated sites, and the experience of the Environmental Protection Agency (EPA) in the assessment and remediation of these sites are discussed in the following Sections.
1.1. Radium sites
Following the discovery of radium in 1898, scientists and amateur enthusiasts became fascinated with its properties, including radioluminescence which caused watch dials to glow in the dark. In the USA, many small companies began producing a variety of products containing radium salts. In addition to the well-known radium dial industry, some companies produced health and beauty products that claimed almost magical benefits from ingestion or topical application of radium compounds. For every legitimate use of radium, there were many others that would prove its uselessness, and often found it to be life threatening. As the evidence for the harmful effects of radiation became better understood, many of these companies either went out of business or ceased the manufacturing of radioactive products.
Other legacy radium sites resulted from the mining of rare earth elements such as vanadium. The extraction processes for these commercially valuable non-radioactive minerals sometimes left behind high concentrations of radium as a mining byproduct. The fate of these contaminated properties depended on whether or not actions were taken at the time to document the presence of radium and clean it up. Even when attempts were made to clean up the radium, residual concentrations were often left behind which were later found to be unacceptably high by today’s standards. In some cases, sites were remediated several times over the course of decades. Today, radium sites in the USA are often cleaned up under EPA’s Superfund Program, as described in Section 2.2.
1.2. Sites contaminated from atomic energy and weapons development programmes
Many of the contaminated legacy sites in the USA are associated with the development of nuclear weapons and commercial atomic energy dating from the 1950s. The Manhattan Engineer District (MED) and the Atomic Energy Commission (AEC) promoted research and development activities in these areas, many of which were highly classified at the time. Uranium exploration and mining became a priority as the need for sources of fissile uranium-235 increased. In addition to uranium, there was also some interest in mining for thorium-232 to supply the thorium fuel cycle. Although the thorium fuel cycle has its proponents, uranium-235 is still the principal fissile radionuclide used in commercial nuclear fuel in the USA. As a result, uranium-contaminated sites represent the largest share of contaminated legacy sites in the USA.
When EPA was formed in 1970, the authority for setting standards for radioactivity in the general environment was transferred from AEC to EPA. In 1974, AEC was abolished, and its authority to regulate the uranium fuel cycle and the possession of radioactive material was transferred to the US Nuclear Regulatory Commission. Many of AEC’s remaining functions were transferred to the Energy Research and Development Administration, which later became part of the US Department of Energy (DOE). DOE has responsibility for managing most of the legacy sites from the AEC and MED era. Following clean-up, many sites will require long-term institutional controls and ongoing site management activities. Some sites have been remediated completely, but in 2001, DOE listed 129 contaminated sites that require long-term stewardship (Wells and Spitz, 2003).
In 1974, the Formerly Utilized Sites Remedial Action Program (FUSRAP) started to address sites with contamination resulting from US atomic energy and nuclear weapons programmes from the 1940s to 1960s. These sites were contaminated with low levels of uranium, thorium and radium, and their respective decay products. The clean-up responsibility for the FUSRAP sites was transferred from DOE to the US Army Corps of Engineering in 1998. These sites are being cleaned up using the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) process described in Section 2.2. Following clean-up, FUSRAP sites were transferred to DOE’s Legacy Management Program.
1.3. Legacy uranium milling sites
After uranium ore is mined, the next step in the fuel cycle involves extracting the uranium from the ore. According to EPA (1982), between 1948 and 1978, approximately 145 million metric tons of ore were processed to recover about 150 metric tons of yellowcake (U3O8). The quantity of mill tailings left behind after processing was essentially unchanged from the original quantity of ore. A typical inactive mill tailings site from 1978 included a tailings pile covering approximately 20 hectares. The mill tailings piles consisted of sandy material that was deposited in slurries, and contained the residual radioactivity from the uranium ore and other hazardous constituents (EPA, 1982).
Since 1978, DOE has managed the remediation of 22 designated inactive uranium mill tailings sites. To date, this has resulted in the creation of 19 disposal cells that contain encapsulated uranium mill tailings and related contaminated material. Over 30 million cubic metres of radioactively contaminated mill tailings are now disposed into these engineered disposal cells (DOE, 2015).
2. US LAWS FOR MANAGING CONTAMINATED SITES
2.1. Uranium Mill Tailings Radiation Control Act
By 1978, the US Congress recognised that the mill tailings from both inactive and operating uranium mill tailings sites posed a hazard to the public from residual radioactivity and other toxic constituents present in the tailings. The Uranium Mill Tailings Radiation Control Act (UMTRCA) of 1978 was passed to address clean-up of these sites (UMTRCA, 1978). UMTRCA gave EPA the authority to set standards for the management of the tailings piles, and for the clean-up of contaminated land and buildings.
For contaminated land, EPA set a general clean-up standard for radium-226 of 185 Bq kg−1 in the top 15 cm of soil and 555 Bq kg−1 below 15 cm. EPA provided additional standards that require radon-222 emanation from tailings piles to remain below 0.74 Bq m−2 s−1 (EPA, 1983).
2.2. Comprehensive Environmental Response, Compensation, and Liability Act
CERCLA was passed in 1980 to address the problem of abandoned or poorly controlled sites containing hazardous waste, including radioactivity (CERCLA, 1980). It also provides for the management of accidental spills and other emergency releases of pollutants into the environment. CERCLA is commonly referred to as the ‘Superfund Program’, in part because it provides a mechanism for covering the cost for cleaning up a site. The basic principle of the Superfund Program is that those responsible for contaminating a site should pay for its remediation. The Superfund funding mechanism allows the achievement of stringent risk reduction goals.
Except for uranium mill tailings sites, most EPA radiation site clean-ups are performed using the process described in the National Contingency Plan, which is the set of EPA regulations that implement the Superfund Program (EPA, 1990). For clean-up under the Superfund Program, a site must first be listed on the National Priorities List (NPL). NPL listings are determined by a hazard ranking system score that considers all onsite contaminants except for natural background radiation.
The CERCLA clean-up process begins with a preliminary site assessment and site investigation (EPA, 2016). Data collected during this phase support the determination of the site’s hazard ranking score. If the score reaches a higher limit, the site is put on the NPL for eventual remediation under the Superfund Program (EPA, 2011). The next steps are remedial investigation and a feasibility study, which include more complete site characterisation and baseline risk assessment (EPA, 2016). This phase produces a set of options for cleaning up the site, taking into consideration the risk reduction associated with each option. Finally, a remedy is selected and made official through a record of decision. The final steps include clean-up activities, post-clean-up reviews, and eventual removal of the site from the NPL (EPA, 1990).
For all carcinogens present at a site, both radiological and chemical, the target residual risk goal is 10−4–10−6 excess cancers for an average member of the US population (EPA, 1990). The upper end of this risk range equates to no more than approximately one lifetime excess radiogenic cancer being expected among 10,000 individuals exposed at the final clean-up level. Assuming an exposure duration for any particular site of 25–30 y, this results in a clean-up goal of approximately 100–150 µSv y−1 to a reasonably maximally exposed individual at a site containing radionuclides alone.
Publication 103 recommends public dose reference levels for existing site clean-ups in the range of 1–20 mSv y−1, with optimisation below the reference level (ICRP, 2007). CERCLA-based regulations in the USA result in radiation site clean-up goals that yield public doses which are typically 10–100 times lower than this range [i.e. on the lower end of the International Commission on Radiological Protection’s (ICRP) recommended dose constraints range for planned exposures]. Clean-up goals are generally determined for unrestricted future use of the site. However, when unrestricted future use is not achievable, risk objectives may be met by limiting future use of the site to commercial or industrial activities, or other restricted access uses (e.g. a park or game reserve).
3. EXAMPLES OF CONTAMINATED SITE CLEAN-UPS IN THE USA
3.1. Lansdowne, Pennsylvania site (legacy radium site)
From 1924 to 1944, a physics professor at the University of Pennsylvania enriched radium ore in his home. In 1964, the Pennsylvania Department of Health performed a partial clean-up of the property. Pennsylvania informed EPA of the site in 1983, and it was listed on the NPL in 1985. As there were no surviving responsible parties, clean-up of the site was funded federally under the Superfund Program. EPA assessed the threat from radium and radon in the original house and in vicinity properties. The remedial action resulted in the removal of 1300 metric tons of contaminated building rubble and 3700 metric tons of contaminated soil. The site was removed from the NPL in 1991.
3.2. Montclair/West Orange, New Jersey site (legacy radium site)
This site includes 469 residential properties and 10 municipal properties, covering a total area of approximately 50 hectares that has been remediated under EPA’s Superfund Program. The site was contaminated from nearby radium-processing facilities that operated in the early 1900s. The State of New Jersey discovered high indoor radon and γ levels in 1983, and the site was listed on the NPL in 1985. It has been estimated that 170,000 m3 of radium-contaminated soil was spread across the site over time. The site remedy involved excavation and off-site disposal of all radium-contaminated soil, followed by restoration of the properties, which required some residents to be temporarily relocated. Clean-up began in 1990, was completed in 2004, and the site was removed from the NPL in 2015.
3.3. Niagara Falls Storage Site (FUSRAP)
The Niagara Falls Storage Site was used by MED and AEC from 1947 to 1952 to store residues from processing uranium ore. The site is 77 hectares on federal land. Radioactive waste is impounded in a 4-hectare interim waste-containment structure containing 184,000 m3 of soil contaminated with uranium-238, thorium-230, caesium-137 and radium-226. The site also includes approximately 3000 m3 of legacy Belgian Congo ore containing radium-226 with an average concentration of 19 MBq kg−1. The US Army Corps of Engineers is in charge of ongoing site management and clean-up activities at this site. The site clean-up is following a CERCLA process.
4. SUMMARY
There is general consistency in contaminated site clean-ups in the USA because EPA regulations issued under CERCLA (1980) and UMTRCA (1978) have guided the remediation of most legacy radiation sites since the 1980s. Clean-up of various contaminated sites in existing exposure situations in the USA often results in residual doses to the public well below Publication 103 (ICRP, 2007) reference levels for existing exposure situations. However, EPA’s management of contaminated sites makes use of the ICRP process of optimisation to determine clean-up goals. Also, the Superfund risk assessment methodology for determining risk-based clean-up goals relies on ICRP biokinetic and dosimetric models for the assessment of organ doses following radionuclide intakes and exposures.