The Use of Retrospective Health Risk Assessments
L
i ! US. Department of Energy (DOE) nuclear weapons complex is composed of more than 14 facilities in 13 states covering 3350 square miles of land and employing more than 100,000 people ( I ) . The weapons complex is a collection of factories and laboratories dedicated to metal fabrication, chemical separation processes, and electronic assembly. Forty-five years of operation of these large i n d u s t r i a l facilities has resulted in the release of enormous quantities of radionuclides and chemicals. Thousands of solid waste management units (SWMUs) (units or areas from which hazardous constituents might migrate) have been identified as a result of Resource Conservation and Recovery Act (RCRA) compliance activities throughout the weapons complex (Table 1). The majority of the sites have contaminated groundwater and billions of cubic meters of contaminated soil and sediments. DOE’S five-year environmental restoration and waste management plan projects the expenditure of tens of billions of dollars for the investigation, man-
1270 Environ. Sci. Technol., Vol. 26. No. 7.1992
i
.1L1.
problems in-and refi the results of-the first studies (9-14). This process leaves a confused and angry McLaren/Hart Environmental public that must be addressed by Engineering Corporation health officials. Alameda, CA 94501 The public has become more knowledgeable about environmenagement, and clean-up of contami- tal issues over the past decade and nation at the weapons facilities (2). has demanded greater accountabilHigh-profile weapons facilities ity from officials with regard to enthat handle substantial quantities of vironmental hazards. In the past, in radioactive material, particularly the interest of national security, fedplutonium, have been the subject of eral facility managers felt little regrowing public concern for a num- sponsibility for informing the pubber of decades (3-6). Epidemiologic lic of environmental problems or studies or case reports suggesting a hazards caused by operations. Now correlation between the presence of these facilities find themselves ina nuclear facility and the occur- creasingly regulated by other fedrence of disease periodically reach eral agencies and fighting a wave of the popular press, generating de- public anger and distrust that mands for action. However, the threatens their continued operation studies seldom point the way to ef- (3, 6). DOE and state health departments fective actions that public health officials can take to address public increasingly have sponsored retroconcerns. Those epidemiologic spective risk assessments to address studies generating the most interest public concerns. These retrospecsuggest that clusters of cancer cases tive assessments are also instruamong nuclear weapons facility em- mental in properly designing epideployees or the neighboring public miologic studies. The objective of may exist or could appear (7,8). The such assessments is to predict the publication of studies indicating dose of contaminants and their asthe existence of a cancer cluster fre- sociated risks, for workers or the quently stimulates subsequent stud- public, that may have resulted from ies that point out methodological past facility operations. These stud~
STEPHEN R. RIPPLE
0013-936X/92/0926-1270$03.00/0 @ 1992 American Chemical Society
ies require the characterization of past emissions of hazardous substances and the assessment of potential exposures for the various populations of concern. The retrospective risk assessments being performed at weapons complex sites are unlike much of the risk assessment work being performed for the purposes of RCRA or Comprehensive Environmental Response,. Compensation, and Liability Act (CERCLA) compliance. RCRA and CERCLA risk assessments are developed to predict future risk and are constructed so as not to underestimate the potential risks. Such an approach can lead to an exaggeration of the actual future risks that individuals will experience (15). Risk assessments under RCRA and CERCLA are often constrained by regulatory assumptions designed to standardize, to some extent, the approach used in the assessment in order to make risk management decisions more consistent and to encourage responsible parties to take actions that will effectively reduce future risk. These actions may include efforts to break new ground in science or technology, such as the development and installation of more effective con-
trol technologies. In contrast, the objective of the retrospective risk assessments being performed at the weapons facilities is to accurately estimate the actual past risks and their attendant uncertainties. The other objective is to inform the public and support the development of appropriate health studies of the exposed populations. As such, retrospective assessments attempt to avoid the repetitive use of conservative assumptions that can significantly overestimate past risk. My purpose here is to discuss some of the retrospective health risk assessments under way at federal facilities and, additionally, progress on a first-of-its-kind retrospective risk assessment currently being conducted for the Rocky Flats Plant near Denver, Colorado. The approaches used in these studies to understand past health hazards are those that would be used to understand the hazards posed by the long-term operation of any largescale industrial facility.
Nevada Test Site, the Hanford Nuclear Reservation (WA), the Fernald Feed Materials Production Facility (OH),and the Rocky Flats Weapons Plant. Studies at the Nevada Test Site have focused on the characterization of radiation doses to downwind residents as a result of atmospheric weapons testing conducted primarily during the 1950s (16). The studies have characterized radionuclide production and fallout from the many atmospheric tests (17)and have estimated exposure rates (18), analyzed vegetation interception of fallout (19), estimated radionuclide ingestion through the development of a complex food
Studies at DOE facilities Retrospective assessments of human exposure have recently begun or are well under way at a number of federal facilities including the Eniviron. Sci. Technol., Vol. 26, No. 7, 1992 1271
chain model ( 2 0 ) , and developed both internal and external dose estimates for individuals as well as for many other related investigations (22,22).
Initial work at the Hanford Nuclear Reservation has focused on the release of radioactive iodine between 1944 and 1947 and its potential impact on area residents, as well as the release of radioactive cooling water to the Columbia River between 1964 and 1967 (23-26). The results of the Hanford work will support the design of an epidemiologic study sponsored by the Centers for Disease Control and performed by the Fred Hutchinson Cancer Research Center in Seattle that will examine thyroid problems in persons exposed to these emissions as children. Dose reconstruction studies have also been initiated at the Fernald Feed Materials Production Center in Ohio. The Fernald Dosimetry Reconstruction Project is examining the historic releases of radionuclides from the facility with the objective of quantifying off-site doses to humans to support the design of an epidemiologic study. Each of these studies deals with past events that resulted in the release of contaminants and in the potential for exposure of the public in the vicinity of the facilities. The studies all aim to describe these releases and to characterize the risk that the public has borne as a result of these events.
The Rocky Flats Plant study The Rocky Flats Plant is about 16 miles northwest of downtown Denver. The plant occupies 384 acres in the middle of a 6550-ane preserve that serves as a buffer zone between the plant and surrounding communities. The plant consists of more than 134 buildings with 2.5 million square feet and currently employs more than 6200 workers. Although the public knew little about the site or its mission during the 1950s and 1960s, two major fires involving plutonium in 1957 and 1969, a waste disposal practice initiated in the late 1950s that led to the spread of plutonium in soil (in the “903 Pad” area), and the inadvertent release of tritium to area drinking water reservoirs attracted considerable public attention (4,271. The health studies &e., the retrospective risk assessment and other yet-to-be-defined studies) at the Rocky Flats Plant are the result of growing public concern about the health hazard posed by past opera1272 Environ. Sci. Technol.. Vol. 26, No. 7. 1992
An epidemiologic ~
study suggesting a correlation between cancer incidence and proximity to Rocky Flats was a milestone in the development of public concern. ~
tions and accidents at the plant. An epidemiologic study suggesting a correlation between cancer incidence and proximity to Rocky Flats was a milestone in the development of public concern (8). The local public has become increasingly aroused about the potential impact on health as the population in the two counties closest to the plant (Jeffersonand Boulder counties) has grown more than five-fold from just over 100,000 to more than half a million between 1950 and 1980. Concern reached new heights following a June 1989 raid by approximately 100 FBI and EPA agents who sought documentation of alleged criminal acts and mismanagement. As a result, the state of Colorado, as part of an agreement-in-principle with DOE, obtained funding to sponsor a retrospective assessment of off-site health effects. Dose reconstruction efforts currently under way for the Rocky Flats Plant differ from those at the other nuclear facilities in that the scope was not limited to radionuclides, but included the thousands of chemicals used by the plant. The plant manufactures components for nuclear weapons from radioactive and nonradioactive materials, utilizing some materials in very large quantities (Table 2). Most of the concern about the human and ecological risks from nuclear facilities has focused on the hazards associated with radionuclides. However, in the case of Rocky Flats, the public was well aware that other chemical contaminants had been released. Chemicals have been released as the result of fugitive air emissions from plant operations, disposal practices that
have contaminated groundwater, and liquid discharges and site runoff to holding ponds. Heretofore, retrospective assessments of chemicals and radionuclides released from a facility have been evaluated separately at most nuclear sites. Such independent evaluations have resulted from the differences in the historic evolution of regulations governing these materials, as well as the training of professionals who deal with chemical and radioactive materials. The techniques developed to understand the uptake of radionuclides are not unlike those used in the 1980s to understand the uptake of chemicals in the environment (28-30). Although characterizations of the health hazards of chemicals and radionuclides are separated by different technical jargon and reporting conventions, in many ways they require the same basic technical approaches (32).For example, in the exposure assessment step, process emissions must be quantified and their transport through the environment must be modeled. The release and transport of both radionuclides and chemicals are related to the specific chemical and physical properties of the materials; their environmental fate can be influenced by transformation, degradation, or decay processes. An objective of the Rocky Flats study was to treat chemicals and radionuclides equally when investigating their potential for cresting off-site health hazards. The study will need to address the differences in the way risks from chemicals and radionuclides are characterized and typically regulated. A significant difference has been the development of risk factors from dose-response studies. Risk factors for radiation exposure historically have been based on best estimates @.e.,mean values) of the dose-response for fatal cancers or genetic effects. However, risk fac-
Trichloroethane
134,337gallons 4675 gallons
tors for chemical exposure typically have been based on “upper bound estimates” &e., 95% upper confidence limit on the mean) for cancer incidence rather than fatality. Other differences include the regulation of exposures to radiation in excess of background on a maximum annual exposure basis, whereas regulation of chemical exposure does not exclude background and is typically on the basis of cumulative lifetime exposure. Finally, the level of “acceptable” cancer risk implicit in NCRP and EPA radiation dose limits are in the range of 10-~to IO-‘, with NCRP recommending a de minimis level of exposure consistent with a risk level of IO4 [32,33). The risk level frequently referred to as de minimis for chemical exposure is lo4 (34). The Colorado Department of Health (CDH) secured a contractor to perform a two-year project identified as the Toxicologic Review and Dose Reconstruction Study and set up a 12-member Health Advisory Panel to oversee the conduct of the
study (Figure 1).The panel is composed of state health department officials, scientists of national renown, and representatives of local government, national environmental groups, and the local public. It is charged with ensuring that the study is of the highest scientific quality and that it is performed completely independently. The study was designed to address all historic releases of hazardous materials from Rocky Flats that could have affected public health. CDH recognized that, although operations and incidents have received considerable scrutiny in recent years, there has been little opportunity to thoroughly examine events from the early decades of plant operation when secrecy was high and public involvement was low. In addition, it is hoped that a more comprehensive assessment of the potential cumulative impacts on the offsite populations would determine the need for epidemiologic studies and support their design. Overview of study approach. The Rocky Flats Dose Reconstruction Study design reflects the complexities of performing a comprehensive historical assessment. It is composed of 12 tasks [Figure 2) that represent both qualitative and quantitative investigations. The initial tasks deal with the review and compilation of historical information for the purpose of selecting the specific radionuclides and chemicals that warrant detailed study as well as any accidents or incidents that may have affected the off-site public. This objective is being met through the performance of Tasks 1 , 2 , and 3.
Subsequent tasks address the development of the quantitative information needed for off-site exposure assessment. Tasks 4 and 5 address the identification of release points and the development of release estimates for each material and for each accident of interest. The facility operators were strongly motivated to control radioactive emissions-specifically plutonium, because it was costly to produce and its loss or release was a potential threat to national security or public health. The plant routinely monitored building effluents since operations began in 1953. It is this effluent monitoring record that likely will provide the basis for routine radioactive emissions reconstruction, provided all significant emissions were monitored and appropriate sampling and analytic methods were employed. Chemical emissions from the facility have been largely unmonitored historically. The development of historical estimates of chemical emissions is likely to rely on the limited information available for processspecific chemical use. The reconshuction of accidental releases will rely on a combination of engineering calculations and effluent monitoring data to quantify releases. In some cases, depending on the availability of data, environmental monitoring data will be used to back-calculate accident emissions. After developing estimates of release of the materials of concern under normal operating conditions and resulting from accidents, contaminant transport and exposure pathway modeling will be performed under Task 6. The potential
Environ. Sci. Technol., VoI. 26. No. 7, 1992 1273
for each of the specific compounds of interest to move through various environmental media or through the food chain is evaluated as part of the pathway analysis. Environmental transport models are then employed for the purpose of predicting the amount of each contaminant that was likely present in each relevant environmental media over the past 40 years. Information on land use and the location and size of populations surrounding the plant is used to evaluate the plausibility of the various exposure pathways and to develop appropriate exposure scenarios used to complete the final dose assessment activity under Task 8. The information needed to develop appropriate exposure scenarios includes the nature of recreational and occupational activities and local habits-such as swimming or fishing in affected waters, or consuming substantial quantities of vegetables grown in affected areasthat can influence the degree to which individuals may have contacted contaminants. The dose estimates and their associated uncertainties developed under Task 8 will be used to evaluate the likelihood that individuals may have experienced adverse health effects as a result of plant emissions. The following material discusses the processes of Tasks 1and 2: identification of materials of concern from a list of more than 8000 substances used at Rocky Flats. Other papers are planned for presenting details on the subsequent assessment steps. The results of efforts to quantify both plant emissions and off-site doses to the public are likely to be made available in the literature in 1993-94. The first step: identifying contaminants of interest. The Rocky Flats Plant has two main missions: the production of "triggers" for nuclear weapons, and the processing of retired weapons for plutonium recovery. The first mission primarily involves the highly skilled machining of plutonium, uranium, beryllium, stainless steel, and other materials into specialized weapons parts. The plant also receives retired weapons components that must be chemically processed to recover valuable plutonium. The main objective of the recovery process is the separation of the plutonium from its principal decay product, americium. Initial work on the project has focused on developing a qualitative 1274 Envimn. Sci. Technol., VoI. 26, No. 7. 1992
understanding of the potential health impacts of the Rocky Flats Plant. The work included a comprehensive look at all the materials used at the plant since 1952.An effort to collect and review plant records was initiated to identify materials used on-site. Classified and unclassified records were evaluated on the plant site, in local document repositories, and from other locations, primarily DOE libraries. Accordingly, a number of project staff required DOE security clearance to perform this activity. The need to control and account for radionuclides used by the plant was well established even when the plant first began operations. This is primarily because regulations governing the use of radionuclides have been promulgated for nearly 40 years. In contrast, most chemicals could be purchased and disposed of during the same period without any sort of permit. As a result, plant registries and inventories of radionuclides have been maintained in various forms virtually since the plant began operations. However, the tracking and documentation of chemical use is a relatively recent concern, and written records of chemical use at Rocky Flats prior to the 1970s are scarce. Therefore, the project has had to rely on extensive interviews with plant employees and retirees for the early history of chemical use at the plant. In addition to reviewing inventories and registries, investigators examined effluent monitoring and environmental monitoring data to determine whether the contaminants detected in the environment were those used at the plant. These investigations led to the identification of dozens of radionuclides and more than 8000 chemicals and commercial products that have been present at the plant site. The next task was the development of an approach to identify those chemicals and radionuclides that may have had the potential to cause off-site health impacts. This task was fairly straightforward for the radionuclides because many of the compounds identified as being present at the plant were in the form of sealed sources or were present in very small quantities for analytic purposes and were not likely to have been released off-site. The isotopes of plutonium, uranium, and americium that are associated with production materials, as well as tritium and thorium, became the subject of further investigations be-
cause they have been either present at the plant in significant quantities or detected in plant effluents or the environment. Screening process for list of 8000 nonradioactive materials. The development of an objective process for screening the chemicals of possible concern from a list of 8000 materials presented a formidable challenge. The screening process we developed consisted of the steps depicted in Figure 3. A team of three professionals from different disciplines (chemistry, industrial hygiene, and toxicology) tackled the identification of key hazardous chemicals. They independently placed the materials into one of three categories: (1) those warranting formal evaluation as potential
materials of concern, (2) those that were identified only by their tradenames and whose chemical contents would need to be determined prior to reclassification to either the first or third categories, and (3) common household or commercial products that were generally recognized as nontoxic or of low toxicity. The lists generated by the three individuals were then reviewed and differences in the classifications were discussed and resolved. Those chemicals falling in the first category were formally screened with the intent of identifying approximately 15 chemicals of concern for further study. The purpose of the three-stage screening process of Task 2 was to select the chemicals warranting detailed study because it was clear that the vast majority could not have posed a n environmental health hazard. Stage 1 of the screening identified known environmental toxicants including: known or suspected human or animal carcinogens or chronic toxicants identified by EPA and given carcinogenic slope factors or reference doses in the Integrated Risk Information System maintained by EPA, known reproductive and developmental toxicants as identified by the California Department of Health Services under Proposition 65 or in the Catalog of Teratogenic Agents (35,36), chemicals known to have been released to the environment, and chemicals present on plant inventories in quantities greater than 5 kg [these were retained for further detailed selection screening because we wanted to further consider all chemicals present in MYsignificant quantity). Chemicals not falling into any of the groups and present in inventory
quantities of less than 5 kg were excluded from further consideration. The 5-kg cutoff is based on a conservative screening exposure analysis approach used as part of the Stage 2 screening, which demonstrated that even extremely toxic chemicals present in quantities of 5 kg were unlikely to pose an off-site hazard. Stage 1 screening resulted in the identification of 629 compounds for further screening. Stage 2 of the chemical screening process was driven by an analysis that led to a gross estimate of the quantity of a chemical that would need to be present at the facility to pose a n off-site hazard. T h i s amount could then be compared to the reported inventory quantities. The process is illustrated in Figure 4. The first step required the determination of the maximum allowable average lifetime daily dose to an individual that corresponded with a level of cancer risk (1 x IOd) selected for screening purposes or an acceptable level of exposure (e+, the EPA’s reference dose). This average daily dose was then converted to a concentration in air or drinking water based on the average adult body weight and the breathing or water ingestion rate. The plant emission quantity that would be required to produce the calculated air or water concentration for the most exposed individual is then estimated using a screening-level air dispersion model from EPA known as SCREEN, and simple transport and mixing calculations for water-borne effluents. For the air pathway, the most affected individual was assumed to be located at the facility fenceline 24 h per day. The water pathway calculations were based on the assumption that the chemicals released annually by the plant to the nearest drinking water reservoir were mixed within a sin-
gle volume of the reservoir. The maximally exposed person was assumed to derive all of his or her drinking water from this reservoir, and 10% of the release from the plant was assumed to be present in finished drinking water. It was also assumed that the plant used 10 times the highest reported inventory quantity of each chemical on an annual basis and that 25% of this annual usage was released to both the air and the water for each year of operation. The 46 chemicals that were identified as being present in sufficient quantity to pose a hazard based on these Stage 2 screening assumptions were subjected to the third and final stage of screening. The Stage 3 screening of the remaining chemicals involved a preliminary review of the actual storage and use practices for the 46 chemicals identified by the Stage 2 screening process. This evaluation permitted a more critical review of assumptions regarding usage rates and the potential for release, as well as a more realistic evaluation of the potential hazard the remaining chemicals posed. The use of these qualitative and quantitative screening criteria and some knowledge of actual use and storage practices have led to the identification of the materials listed in Table 3 as being of potential concern for off-site health impacts. These materials and the selected radionuclides are now the subject of detailed historical use investigations to more rigorously evaluate the doses that off-site populations may have received from the plant. Current Rocky Flats dose reconstruction project efforts. Current activities involve evaluation of efflue n t m o n i t o r i n g data a n d t h e development of source term estimates in order to estimate the historical off-site concentrations of the
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materials of concern. Environmental transport models are being selected and used to predict the concentrations of the emissions of concern in various environmental media. The plant’s location adjacent to the front range of the Rocky Mountains poses a unique air modeling challenge because of the complex meteorologic environment. A modeling code developed specifically for Rocky Flats (i.e., the Terr a i n Responsive Atmospheric Code), which utilizes complex wind field information, is being evaluated for use on the study. Also, dust resuspension and particle deposition are important issues because of documented plutonium contamination of surface soils from leaking waste drums at the 903 Pad location. The Fugitive Dust Model will be employed for the evaluation of the 903 Pad contamination. Predictions of contaminant concentrations in the off-site environment made using the emissions estimates and transport models will be compared to the historical environmental monitoring data for air and soil at select locations to evaluate the consistency of the predictions. In addition, the study may lead to the identification of soil and sediment sampling that could be performed for the environmentally persistent contaminants, in order to confirm the study results. Relevant environmental exposure pathways, including food chain pathways, are being identified, and appropriate models will be employed to calculate potential human uptake of the contaminants of concern. The locations of residents and businesses over the past 40 years, as well as the agricultural uses of nearby lands, are being characterized in order to quantify the frequency and extent of potential expo-
sures to plant-related contaminants in the past. Quantitative uncertainty
estimates are being included in each step of the assessment process so that Monte Carlo simulation techniques can be employed in the calculation of the dose estimates and their uncertainties. All this information will be used to describe the historical doses of contaminants from Rocky Flats that off-site individuals may have received and the attendant health risk. The Rocky Flats Dose Reconstruction Study and others like it offer unique opportunities to address public concerns regarding the impact nuclear facilities have had on the public’s health. These investigations call for the wide-ranging study of various historic events and the incorporation of state-of-the-art environmental science. The studies are on the frontier of many risk assessment issues as a result of the following: theneed for estimates ofthe most realistic historical risks these facilities have posed, recognition that the impetus for these studies is public demand for information and that the process and results should be widely accessible, and the desire to use the results to make informed decisions regarding the need for more specific community health studies and epidemiologic investigations. As a result, the approach developed by these studies serves as a template for addressing similar public concerns associated with large facilities in a variety of settings. In addition, the Rocky Flats study should help to further integrate the evaluation and commnnication of health risks arising from both chemical and radionuclide emissions.
Stephen R. Ripple is a monogiog principal with the ChemRisk Division of M c L a r e d H a r t Environmental Engineering Corporotion. He h a s on M.P.H. degree in environmental health sciences a n d an M.B.A. degree from the University of California-Berkeley, He is currently project manager of the Toxicologic Review a n d Dose Reconstruction project for the Rocky Flats Plant being performed under the auspices of the Colorado Deportment of Health a n d a similar feasibility study a t the Oak Ridge Reservation for the Tennessee Department of Health. His technical interests include the development of unified approaches to the assessment of multipathway health risks from chemicals a n d r a d i o n u clides ond the communication of these risks to the public.
References (1) Complex Cleanup: The Environmen-
(2)
(3)
(4) (51
(6)
(7)
tal Legacy of Nucleor Weapons Production; U.S. Congress. Office of Technology Assessment; US. Government Printing Office: Washington, DC, February 1991;OTA-0-484. Environmental Restoration a n d Waste Manogement, Five-Year Plan, Fiscal Yeon 1992-1997; U.S.Department of Energy: Washington, DC. August 1991. The New York Times, 1990, Feb. 15. A22. The Denver Post. 1988, Nov. 25. 1A. The Son Fmncisco Examiner, 1990, July 12, AZO. The New York Times, 1988, Dec. 18. Wing, S. et al. JAMA 1991, 265(11), 1397-1402.
( 8 ) Johnson, C. J. AMBIO 1981, X(4), 178-82.
(91 National Cancer Institute. Cancer in
TABLE 3
Materials of concern for the Rocky Flats Stu Benzene
Americium -241
Plutonium - 238,239,240,
Populations Living Near Nucleor Facilities: Division of Cancer Etiology, Epidemiology and Biostatistics Program: Washington, DC, July 1990. (IO) Crump. K. S.;Ng, T.; Cuddiby, R. G. Am. J. Epidemiol. 1987, 126(1), 12735. 111) Selvin, S. G. et al. JNCI 1987, 7331, 417-23. (12) Gilbert, E. S. et al. Radiation Research. 1989.120.1%35. (13)Jablon; S.; kubkc, Z.; Boice, J. D. JAMA 1991, 265(11), 1403-08.
(14)Paustenbach. D. J. et al. 1.Air Waste
Manage. . Assoc. 1990, 4Ol121, 162030. (15) Church, E. W. et al. Health Phys. 1990, 59151, 503-10. (161 b y , P. W.;Heit, P.; Miller, K. M. Health Phys. 1990,59(5). 541-54.
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(17) Thompson, C. B. Health Phys. 1990, 59(5), 555-63. (18) Simon, S. L. Health Phys. 1990,59(5), 619-26. (19) Wicker, F. W. et al. Health Phys. 1990, 59(5), 645-57. (20) Ng, Y. C.; Anspaugh, L. R.; Cederwall, R. T. Health Phys. 1990, 59(5), 693713. (21) Henderson, R. W.; Smale, R. F. Health P h p . 1990, 9(5), 715-21. (22) Pacific Northwest Laboratory. Summary Report: Phase I of the Hanford Environmental Dose Reconstruction Project; Richland, WA, August 1991; PNL-7410 HEDR Rev. 1,UC-707. (23) Pacific Northwest Laboratory. Columbia River Pathway Report: Phase I of the Hanford Environmental Dose Reconstruction Project; Richland, WA, July 1991; PNL-7411 HEDR Rev. 1, UC-707. (24) Pacific Northwest Laboratory. Air Pathway Report: Phase 1 of the Hanford Environmental Dose Reconstruction Project; Richland, WA, July 1991; PNL-7412 HEDR Rev. 1, UC-707. (25) Ballinger, M. Y.; Hall, R. B. A History of Major Hanford Facilities and Processes Involving Radioactive Material; Pacific Northwest Laboratory: Richland, WA; March 1991; PNL6964 HEDR, UC-707. (26) T h e Denver Post, 1951, 50(233); March 23, p. 1. (27) Fries, G. F.; Paustenbach, D. J. J. Toxicol. Environ. Health 1990, 29, 1-43. (28) Radiological Assessment: A Textbook on Environmental Dose Analysis; Till, J. E.; Meyer, H. R., Eds.; U.S. Government Printing Office: Washington, DC, September 1983; NUREG/ CR-3332,0RNL-5968. (29) Exposure Factors Handbook U.S. Environmental Protection Agency. Exposure Assessment Group, Office of Health and Environmental Assessment. U.S. Government Printing Office: Washington, DC, May 1989; EPA/600/8-89/43. (30) Risk Assessment Guidance for Superfund, Volume I, Human Health Evaluation Manual (Part A); US. Environmental Protection Agency. Office of Emergency and Remedial Response. U.S. Government Printing Office: Washington, DC, 1989; EPA/540/189/002, Chapter 10. (33 National Council on Radiation Protection and Measurements. Recommendations on Limits for Exposure to Ionizing Radiation, Report No. 91; NCRP: Bethesda, MD, June 1,1987. (32) US. Code of Federal Regulations, 1991, Title 40, Part 190. (33) Young, F. E. Regul. Toxicol. PharrnaC O l . 1987, 7, 179-84. (34) California H e a l t h a n d Welfare Agency. California Safe Drinking Water and Toxic Enforcement Act (Proposition 65); Title 26, Section 2 2 12102,1989, p. 1388. (35) Shepard, T. H. Catalog of Teratogenic Agents; The Johns Hopkins University Press: Baltimore, MD, 1989. (36) Department of Energy. Final Environmental Impact Statement: R o c k y Flats Plant Site, Golden, Jefferson County, Colorado; DOE: Washington, DC, April 1980.
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