A EPA
EPA/635/R-17/787
IRIS Assessment Plan
www.epa.gov/iris
IRIS Assessment Plan for Uranium (Oral Reference Dose)
(Scoping and Problem Formulation Materials)
[CASRN 7440-61-1]
January 2018
NOTICE
This document is a Public Comment Draft. This information is distributed solely for the purpose of
predissemination peer review under applicable information quality guidelines. It has not been
formally disseminated by EPA. It does not represent and should not be construed to represent any
Agency determination or policy. It is being circulated for review of its technical accuracy and
science policy implications.
Integrated Risk Information System
National Center for Environmental Assessment
Office of Research and Development
U.S. Environmental Protection Agency
Washington, DC

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IRIS Assessment Plan for Uranium
DISCLAIMER
This document is a preliminary draft for review purposes only. This information is
distributed solely for the purpose of predissemination review under applicable information quality
guidelines. It has not been formally disseminated by EPA. It does not represent and should not be
construed to represent any Agency determination or policy. Mention of trade names or commercial
products does not constitute endorsement or recommendation for use.
This document is a draft for review purposes only and does not constitute Agency policy.
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IRIS Assessment Plan for Uranium
CONTENTS
AUTHORS | CONTRIBUTORS	vi
1.	INTRODUCTION	1
2.	SCOPING AND INITIAL PROBLEM FORMULATION SUMMARY	2
2.1. BACKGROUND	2
2.2.SCOPING SUMMARY	3
2.3.	PROBLEM FORMULATION	4
2.4.	KEY SCIENCE ISSUES	5
3.	OVERALL OBJECTIVE, SPECIFIC AIMS, AND DRAFT PECO (POPULATIONS, EXPOSURES,
COMPARATORS, AND OUTCOMES) CRITERIA	6
3.1.	SPECIFIC AIMS	6
3.2.	DRAFT PECO (POPULATIONS, COMPARATORS, EXPOSURES, AND OUTCOMES) CRITERIA 	8
REFERENCES	9
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IRIS Assessment Plan for Uranium
TABLES
Table 1. EPA program and regional office interest in an assessment of uranium	3
Table 2. Draft PECO (populations, comparators, exposures, and outcomes) criteria for the
uranium assessment	8
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IRIS Assessment Plan for Uranium
ABBREVIATIONS
ATSDR	Agency for Toxic Substances and Disease Registry
CERCLA	Comprehensive Environmental Response, Compensation, and Liability Act
EPA	Environmental Protection Agency
IRIS	Integrated Risk Information System
LOAEL	lowest-observed-adverse-effect level
MCL	maximum contaminant limit
MRL	minimal risk level
OW	Office of Water
PECO	populations, exposures, comparators, and outcomes
RfD	oral reference dose
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AUTHORS CONTRIBUTORS
Assessment Team
Paul White (Assessment Manager)
Xabier Arzuaga
Michele Taylor
Marc Stifelman
U.S. EPA ORD/NCEA
U.S. EPA ORD/NCEA
U.S. EPA ORD/NCEA
U.S. EPA REGION 10
Executive Direction
Tina Bahadori
Mary Ross
Emma Lavoie
Samantha Jones
Kris Thayer
James Avery
NCEA Center Director
NCEA Deputy Center Director
NCEA Assistant Center Director for Scientific Support
NCEA Associate Director for Health (acting)
NCEA/IRIS Division Director
NCEA/IRIS Deputy Director (acting)
Contributors and Production Team
Satoru Ito
Ryan Jones
Vicki Soto
Dahnish Shams
Ingrid Druwe
Amina Wilkins
Marian Rutigliano
Roman Mezencev
Maureen Johnson
HERO Librarian
HERO Director
Project Management Team
Project Management Team
Systematic Review Support
Systematic Review Support
Systematic Review Support
Systematic Review Support
NCEA Webmaster
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1. INTRODUCTION
The Integrated Risk Information System (IRIS) Program is undertaking a reassessment of
the noncancer, nonradiological health effects of uranium via oral exposure. Uranium was included
on the December 2015 IRIS Program multiyear agenda fhttps://www.epa.gov/iris/iris-agenda! as
a chemical having high priority for assessment development
IRIS assessments provide high quality, publicly available information on the toxicity of
chemicals to which the public might be exposed. These assessments are not regulations, but
provide a critical part of the scientific foundation for decisions made in Environmental Protection
Agency (EPA) program and regional offices to protect public health.
Before beginning an assessment, the IRIS Program consults with EPA program and regional
offices to define the scope of the assessment, including the nature of the hazard characterization
needed, identification of the most important exposure pathways, and level of detail needed to
inform Agency decisions. Based on the scope defined by EPA, the IRIS Program develops problem
formulations to frame the scientific questions that will be the focus of the assessment, which is
conducted using systematic review methodology.
This document presents the draft assessment plan for uranium, including a summary of the
IRIS Program's scoping and initial problem formulation conclusions, objectives, and specific aims of
the assessment; draft populations, exposures, comparators, and outcomes (PECO) criteria outlining
the evidence considered most pertinent to the assessment; and identification of key areas of
scientific complexity. Brief background information on uses and potential for human exposure is
provided for context.
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2.SCOPING AND INITIAL PROBLEM FORMULATION
SUMMARY
2.1. BACKGROUND
Uranium is a naturally occurring radioactive element, which in nature is a mixture of three
isotopes: 234U, 235U, and 238U. The most common isotope, 238U, makes up about 99% of natural
uranium, and due to that predominance, is thought to be primarily responsible for the chemical
toxicity of uranium. Uranium is "enriched" by processes that remove and concentrate 235U, with the
remaining uranium being termed "depleted." Depleted uranium has an even greater concentration
of 238U than natural uranium and the chemical toxicity of the two are believed to be essentially
identical fATSDR. 20131. Enriched uranium is used in nuclear reactor fuel and in nuclear weapons;
it is not a subject of this assessment. Uranium metal is almost as hard as steel and much denser
than lead. Due to its physical properties, depleted uranium is used as counterweights in aircraft
applications, for shielding against ionizing radiation, as military armor, and in armor-penetrating
munitions.
Uranium is naturally present in many soils with an average concentration in the United
States of about 3 ppm; some areas, particularly in the western United States, have higher
concentrations. Uranium mining milling, and processing operations have released uranium into
the environment leading to elevated levels of uranium in affected soils and dusts fATSDR. 20131. In
response to the presence of hundreds of abandoned uranium mines in the Navajo Nation in the
southwest United States, EPA has commitments for major risk assessment and remediation projects
in that area (US EPA. 20181. Commercially viable phosphate ore deposits in the United States and
elsewhere contain uranium fUlrich etal.. 2014: Sattoufetal.. 20071 and cleanup sites at former
phosphate mines in, for example, the northwest United States have elevated soil concentrations of
uranium. Evaluation of cleanup needs at sites with uranium contamination generally entails
assessment of both the risks from the chemical toxicity of uranium and the radiological risks
multiple elements, where both may contribute importantly to total risk.
The general population is primarily exposed to uranium through food and drinking water.
In most areas of the United States, low levels of uranium are found in drinking water, with a
population mean concentration of about 1 |ig U/L. Higher levels of uranium are seen in water from
wells in uranium-rich rock. Approximately 4% of reporting US drinking water systems (serving 8
million people in total) reported some exceedance of the EPA maximum contaminant limit (MCL)
for uranium of 30 |ig/L fUS EPA. 20161. Large aquifers in the United States great plains and in
California's central valley have locally elevated uranium concentrations (Nolan and Weber. 20151.
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Human daily intake of uranium from typical diets has been estimated to range from 0.9 to
1.5 ng/day. Uranium from soil is adsorbed onto the roots of plants; root crops including potatoes,
radishes, and other root vegetables are a source of uranium in the diet (ATSDR. 20131.
Environmental exposures to uranium from contaminated sites can involve multiple
pathways including ingestion of soil, foods, surface water, or ground water as well as consumption
of locally grown or foraged food. Multiple routes of exposure may be particularly important at sites
that are located on or near Indian Nations fArnold. 2014: ATSDR. 2013: Middlecamp etal.. 2006:
Brugge and Goble. 20021.
Depending on the chemical form of uranium and circumstances of intake, about 0.1-6% of
ingested uranium is absorbed by the gastrointestinal tract and enters the systemic circulation in
humans, with soluble uranium compounds being more readily absorbed. Urinary excretion is the
principal elimination pathway for absorbed uranium. Absorbed uranium is retained in many organ
systems, with the highest levels found in the bones, liver, and kidneys. It is estimated that 66% of
the typical human body burden of uranium is found in the skeleton. Uranium in the skeleton is
retained for a longer period, with a half-life on the order of 70-200 days; most of the uranium in
other tissues leaves the body in 1-2 weeks following exposure fATSDR. 20131.
2.2. SCOPING SUMMARY
During scoping, the IRIS Program met with EPA program and regional offices that are
interested in an IRIS assessment for uranium to discuss specific assessment needs. Table 1
provides a summary of input from this outreach.
Table 1. EPA program and regional office interest in an assessment of
uranium
Program or
regional
office
Oral
Inhalation
Statues/regulations
Anticipated uses/interest
Office of
Land and
Emergency
Management
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CERCLA
Uranium toxicological information may be
used to make risk determinations for
response or remedial actions (e.g.,
short-term removals, long-term remedial
response actions). CERCLA authorizes EPA
to conduct short- or long-term cleanups at
Superfund sites and later recover cleanup
costs from potentially responsible parties.
Uranium is listed as a hazardous substance
under CERCLA and is commonly found at
National Priorities List facilities.
Region 10a
V

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Program or
regional
office
Oral
Inhalation
Statues/regulations
Anticipated uses/interest
OW
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Safe Drinking Water Act
Uranium toxicological information may be
used to inform risk determinations
associated with contaminants commonly
found in water. The maximum contaminant
level goals of 0 ng/L and maximum
contaminant level of 30 ng/L for uranium
were published in 2000 (65 FR 76707).
CERCLA = Comprehensive Environmental Response, Compensation, and Liability Act; OW = Office of Water
a Pacific Northwest States.
Oral exposure to uranium is of concern to the Superfund Program as this element has been
found at approximately 60 Superfund sites, with oral intake driving site exposure assessments.
EPA regulated uranium as a drinking water contaminant in 2000 based primarily on radiological
exposures, but also considered kidney toxicity. The EPA's Office of Water (OW) periodically
updates drinking water regulations and needs an IRIS assessment of uranium that examines the
more recent literature (U.S. EPA. 20171.
This reassessment focuses on nonradiological, noncancer effects associated with uranium
exposure because (1) IRIS assessments historically focus on the nonradiological effects of chemicals
and (2) cancer risks from uranium have generally been attributed to and assessed as the result of
radiation exposures. In addition, this reassessment focuses only on oral exposure because the oral
pathway has been the primary route of exposure for nonradiological environmental exposures to
uranium (e.g., drinking water, soils at contaminated sites). Studies on both natural uranium and
depleted uranium will be considered in this reassessment; studies of enriched uranium or the
radiological effects of uranium are not within the assessment scope. This update will include
examination of potentially susceptible populations, including women of child-bearing age, pregnant
women, infants, and children.
2.3. PROBLEM FORMULATION
EPA's IRIS assessment of uranium dates from 1989 (U.S. EPA. 1989). Much research on the
health effects of uranium has been subsequently published. In 2013, the Agency for Toxic
Substances and Disease Registry (ATSDR) completed its Toxicological Profile for Uranium (ATSDR.
2013). which includes a detailed review of the available human epidemiology and experimental
toxicology data. The ATSDR assessment examines the substantial data available on the kidney,
reproductive, developmental, and other effects of uranium and recommends an
intermediate-duration oral minimal risk level (MRL) of 2 x 10~4 mg U/kg-day for soluble uranium
compounds based on 90-day studies in rats (Gilman etal.. 1998). This MRL calculation uses a
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lowest-observed-adverse-effect level (LOAEL) value of 0.06 mg U/kg-day for renal effects in rats,
divided by an uncertainty factor of 300. This includes a factor of 3 due to the use of a LOAEL, a
factor of 10 for animal-to-human extrapolation, and a factor of 10 for human variability. For
comparison, in EPA's 1989 IRIS assessment, an oral reference dose (RfD) of 3x 10-3 mg/kg-day was
based on kidney toxicity and body weight loss with a LOAEL of 2.8 mg U/kg-day in a 30-day oral
study in rabbits (Mavnard and Hodge. 19491 and used a composite uncertainty factor of 1,000 (U.S.
EPA. 19891.
In this reassessment, EPA will heavily rely on the literature review and scientific analysis
contained in ATSDR's toxicological profile fATSDR. 20131. In addition, EPA will perform a review of
literature published since the development of ATSDR's assessment (literature since 2012) and will
seek to develop an updated RfD based on the noncancer, nonradiological effects from oral exposure
to uranium.
The ATSDR toxicological profile identified kidney, reproductive, and developmental effects
of uranium as being of principal concern, and data on these effects provided the bases for that
assessment's MRL values for different durations of exposure. The IRIS assessment will examine
whether newly available data indicate a need to revise the conclusions for these hazards. Newly
available data will also be examined to see whether additional health hazards of uranium have been
identified that may provide a basis for developing new toxicity values. As described below, the
review of the new literature will be integrated with the evidence compiled in the ATSDR
toxicological profile to develop a revised characterization of health hazards and provide the basis
for the derivation of an RfD for uranium.
2.4. KEY SCIENCE ISSUES
Based on the preliminary literature survey, the following key scientific issues have been
identified that warrant evaluation in this assessment.
• Uranium occurs in the environment in a variety of forms to which humans may be exposed,
including metallic uranium, soluble uranium salts, and poorly soluble uranium compounds.
In developing the IRIS assessment, consideration will be given to the approach used by
ATSDR of providing toxicity values suitable for all soluble forms of uranium versus possible
alternatives, addressing specific forms of uranium (e.g., more soluble versus poorly soluble
versus insoluble species). Taking into account any new research, the assessment will
develop and use a rationale for the specific categories of uranium compounds assessed.
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1	3.0VERALL OBJECTIVE, SPECIFIC AIMS, AND DRAFT
2	PECO (POPULATIONS, EXPOSURES,
3	COMPARATORS, AND OUTCOMES) CRITERIA
4	The overall objective of this assessment is to identify adverse health effects and
5	characterize exposure-response relationships for noncancer, nonradiological effects from ingestion
6	of uranium to support development of toxicity values (e.g., an RfD). This assessment will use
7	systematic review methods to evaluate the epidemiological and toxicological literature for uranium.
8	Given the extent of human and animal toxicology studies, in vitro and other mechanistic studies will
9	not be a focus of the systematic review because toxicity values for uranium are likely to be based
10	directly on human and mammalian studies of uranium's apical effects. The evaluation conducted in
11	this assessment will be consistent with relevant EPA guidance.1 The systematic review protocol
12	will be disseminated after review of the draft assessment plan and will reflect changes made to the
13	specific aims and the PECO criteria in response to public input
14 3.1. SPECIFIC AIMS
15	• Building on findings from the Toxicological Profile for Uranium fATSDR. 20131. identify new
16	epidemiological and experimental animal studies of the health hazards of uranium as
17	outlined in the PECO criteria. The literature search will be focused on publications since the
18	ATSDR literature search was conducted (i.e., publications from 2012-2017).
19	• Conduct study evaluations (risk of bias and sensitivity) for individual epidemiological and
20	toxicological studies identified in the literature search. The results of this review will allow
21	subsequent analyses to be focused on those new studies that are most informative for the
22	assessment's needs.
23	• Examine whether newly available data indicate a need to update evidence conclusions and
24	toxicity values for principal health outcomes from the ATSDR toxicological profile (i.e.,
25	kidney toxicity, and reproductive and developmental effects of uranium). Also, this review
26	will examine whether newly available data on other health outcomes support identification
'EPA guidance documents: http://www.epa.gov/iris/basic-information-about-integrated-risk-information-
svstem# guidance /
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of additional uranium health hazards and may plausibly support deriving an RfD for
uranium.
•	If newer PECO-relevant studies on health outcomes are identified, these findings will be
considered along with key studies2 cited in the ATSDR toxicological profile for evidence
synthesis/integration and RfD derivation purposes. In this case, both new studies and key
studies used from the ATSDR toxicological profile will be summarized and evaluated jointly
using the methods described below.
•	Extract data on relevant health outcomes from epidemiological and toxicological studies
considered informative.
•	For the identified outcomes with important new data, synthesize evidence across studies
(including both new and key older studies) within the human and animal evidence streams,
using a narrative approach or meta-analysis (if appropriate). For health outcomes
examined by ATSDR where important new studies are not identified, EPA will seek to base
its hazard conclusions on ATSDR's findings unless compelling reasons for further review
are identified.
•	For each of the selected health outcomes, express confidence in conclusions from across
studies within human and animal evidence streams, evaluating each evidence stream
(human and animal) separately.
•	For each health outcome, integrate results across evidence streams (human and animal) to
conclude whether a substance is hazardous to humans. Identify and discuss issues
concerning potentially susceptible populations and life stages. Biological support from
mechanistic studies will be summarized primarily by relying on other published sources
and targeted literature searches, if warranted, to address specific topics that may arise
when conducting the assessment.
•	Derive an RfD as supported by the available data. System- and organ-specific RfD values
will be derived where supported by the database.
•	Characterize uncertainties and identify key data gaps and research needs, such as
limitations of the evidence base, limitations of the systematic review, and dose relevance
and pharmacokinetic differences when extrapolating findings from higher dose animal
studies to lower levels of human exposure.
2Key earlier studies on relevant toxicological endpoints will be identified through the study summaries and
analysis developed by ATSDR. Considerations include: studies providing data in dose ranges proximate to
toxicological findings considered in ATSDR MRL derivation and/or used in important newly identified
literature; studies of relevant durations for toxicity value development (generally studies of subchronic or
chronic duration as well as developmental or reproductive studies using relevant shorter exposure
durations); and studies, which as summarized, were not identified to have major methodological
shortcomings. Accordingly, key studies are generally those that appear to provide informative data on the
health outcomes and may plausibly support deriving toxicity values for uranium.
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3.2. DRAFT PECO (POPULATIONS, COMPARATORS, EXPOSURES, AND
OUTCOMES) CRITERIA
A PECO statement is used as an aid to focus the research questions, search terms, and
inclusion/exclusion criteria in a systematic review. The draft PECO criteria for the uranium
assessment (see Table 2) were based on (1) nomination of the chemical for assessment,
(2) discussions with scientists in EPA program and regional offices to determine the scope of the
assessment that will best meet Agency needs, and (3) preliminary review of the health effects
literature for uranium (primarily reviews and authoritative health assessment documents) to
identify the major health hazards associated with exposure to uranium and key areas of scientific
complexity.
Table 2. Draft PECO (populations, comparators, exposures, and outcomes)
criteria for the uranium assessment
PECO element
Evidence
Population3
Human: Any population and all life stages (e.g., children, general population, occupational, or high
exposure from an environmental source). The following study designs will be considered most
informative: controlled exposure, cohort, case-control, cross-sectional, and ecological. Note: Case
reports and case series will be tracked during study screening but are not the primary focus of this
assessment. They may be retrieved for full-text review and subsequent evidence synthesis if no or
few more informative study designs are available. Case reports also can be used as supportive
information to establish biologic plausibility for some target organs and health outcomes.
Animal: Nonhuman mammalian animal species (whole organism) of any life stage (including
preconception, in utero, lactation, peripubertal, and adult stages).
Exposure
Exposure based on administered dose or concentration, biomonitoring data (e.g., urine, blood, or
other specimens), environmental, or occupational-setting measures (e.g., air, water levels), or job
title or residence. Studies on natural uranium and depleted uranium will be included, studies on
enriched uranium or those specific to radiation exposure from uranium will not be included.
Mixture studies for animals will be included if they have an arm with a uranium compound only.
Human and animal: Oral exposure will be examined. Other exposure routes, including dermal,
inhalation, or injection, will be tracked during title and abstract as "supplemental information."
Comparator
Human: A comparison or reference population exposed to lower levels (or no exposure/exposure
below detection levels) of uranium or to uranium for shorter periods.
Animal: Quantitative exposure versus lower or no exposure with concurrent vehicle control group.
Outcomes
All noncancer health outcomes. In general, endpoints related to clinical diagnostic criteria, disease
outcomes, histopathological examination, or other apical/phenotypic outcomes will be prioritized
for evidence synthesis over outcomes such as biochemical measures.
a Evaluating individual mechanistic studies for uranium is not anticipated to be critical given the extent of the
experimental animal evidence for noncancer outcomes and findings of earlier reviews. For mechanistic information,
this assessment will primarily rely on other published authoritative sources, such as public health agency reports and
expert review articles.
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REFERENCES
Arnold. C. (2014). Once upon a mine: the legacy of uranium on the Navajo Nation. Environ Health
Perspect 122: A44-A49. http://dx.doi.org/10.1289/ehp.122-A44.
ATSDR (Agency for Toxic Substances and Disease Registry). (2013). Toxicological profile for
uranium. Atlanta (GA): Agency for Toxic Substances and Disease Registry (US).
Brugge. D: Goble. R. (2002). The history of uranium mining and the Navajo people. Am J Public
Health 92: 1410-1419. http://dx.doi.Org/10.2105/ATPH.92.9.1410.
Gilman. AP: Villeneuve. DC: Secours. YE: Yagminas. AP: Tracy. BL: Ouinn. TM: Valli. YE: Willes. RT:
Moss. MA. (1998). Uranyl nitrate: 28-day and 91-day toxicity studies in the Sprague-Dawley
rat. Toxicol Sci 41: 117-128. http://dx.doi.org/10.1006/toxs.1997.2367.
Mavnard. E. .A.: Hodge. H. ,C. (1949). Studies of the toxicity of various uranium compounds when
fed to experimental animals. In IC Voegtlin; HC Hodge (Eds.), Pharmacology and toxicology
of uranium compounds (pp. 309-376). New York, NY: McGraw-Hill.
Middlecamp. CH: Phillips. MF: Bentlev. AK: Baldwin. 0. (2006). Chemistry, society, and civic
engagement (part 2): uranium and American Indians. J Chem Educ 83: 1308.
Nolan. I: Weber. KA. (2015). Natural Uranium Contamination in Major US Aquifers Linked to
Nitrate. Environ Sci Technol Lett 2: 215-220.
http://dx.doi.org/10.1021/acs.estlett.5b00174.
Sattouf. M: Kratz. S: Diemer. K: Rienitz. 0: Fleckenstein. 1: Schiel. D: Schnug. E. (2007). Identifying
the origin of rock phosphates and phosphorus fertilizers through high-precision
measurement of the strontium isotopes 87 Sr and 86 Sr. Landbauforschung Voelkenrode
57: 1-11.
U.S. EPA (U.S. Environmental Protection Agency). (1989). Uranium, soluble salts; no CASRN.
Chemical assessment summary. Washington, DC: National Center for Environmental
Assessment, Integrated Risk Information System.
https://cfpub.epa.gov/ncea/iris/iris documents/documents/subst/0421 summarv.pdf.
U.S. EPA (U.S. Environmental Protection Agency). (2017). Six-year review 3 of drinking water
standards. Washington, DC: Office of Water, https: //www.epa.gov/dwsixyearreview/six-
vear- r e vie w- 3 - drinking-wate r- standards.
Ulrich. AE: Schnug. E: Prasser. HM: Frossard. E. (2014). Uranium endowments in phosphate rock.
Sci Total Environ 478: 226-234. http://dx.doi.org/10.1016/i.scitotenv.2014.01.069.
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1	US EPA. (2016). The analysis of regulated contaminant occurrence data from public water systems
2	in support of the third six-year review of national primary drinking water regulations:
3	Chemical phase rules and radionuclides rules. (EPA-810-R-16-014).
4	https: //www.epa.gov/sites/production/files/2016-12/documents/810rl6014.pdf.
5
6	US EPA. (2018). Navajo Nation: Cleaning Up Abandoned Uranium Mines. Available online at
7	https://www.epa.gov/navaio-nation-uranium-cleanup.
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