Technical BRIEF
INNOVATIVE RESEARCH FOR A SUSTAINABLE FUTURE
www.epa.gov/research
Aquifer Restoration after Uranium Recovery
Evaluation of aquifer restoration at sample uranium in-situ recovery sites
Background
In-Situ Recovery.
In-situ recovery (ISR)
of uranium is a
mining practice that
uses groundwater.
First, wells are drilled
into a uranium-
bearing aquifer.
Leaching fluids are
injected into the wells to dissolve the uranium ore into the
groundwater, and then the uranium is recovered by
extracting the groundwater. Uranium extraction by ISR
accounts for more than 90% of production since the mid-
1990s. The uranium deposits where ISR is conducted are
typically found in arid areas of the United States, such as
Texas, Colorado, and Wyoming.
Groundwater Quality. Water stress, especially as a
result of increasing populations and drought, is motivating
communities and businesses to consider future drinking
water sources that previously would not have been
considered. To allow for the possible future use of
groundwater found in a mining area, operators are
required—per 40 CFR 192—to restore water quality to
pre-mining concentrations for metals, metalloids, anions,
and total dissolved solids after ISR mining operations
cease. Several approaches have been used to restore
groundwater quality, including groundwater sweeping,
reverse osmosis treatment, subsurface injection of
chemical reductants, bioremediation, and monitored
natural attenuation. Even though mining companies are
required to restore groundwater quality to pre-mining
conditions, several water quality studies indicate that
trace metals, anions, and leaching fluids remain in the
aquifer after restoration is completed (Anastasi and
Williams, 1984; Davis and Curtis, 2007; Borch et al., 2012;
Stover, 2004).
Contaminant Migration. Aquifers near ISR operations
may also be vulnerable to contaminants migrating through
undetected or mischaracterized faults from the injection
zone into any hydraulically connected aquifers. Additional
research is needed to identify if contaminants are present
at or near ISR sites and understand the potential for
migration of contaminants, which would be useful to EPA's
Underground Injection Control (UIC) program.
Permitting and Exemptions. ISR operations are
required to have a Class III UIC permit and an approved
aquifer exemption if injecting into an underground source
of drinking water. The UIC regulations allow EPA to
exempt aquifers from the protections afforded by the Safe
Drinking Water Act. To qualify for exemption, the aquifer
must not currently serve as a source of drinking water and
will not serve as a source of drinking water in the future,
among other criteria that must be met.
A mining operator who received an aquifer exemption
could use the entire aquifer, or a portion of the aquifer, for
mineral extraction or disposal purposes in compliance with
the UIC requirements under the Safe Drinking Water Act.
The EPA evaluates the boundaries of the proposed
exempted area using available data so that nearby
underground sources of drinking water continue to be
protected by the Safe Drinking Water Act. A better
understanding of contaminant transport within the mining
area may improve EPA's ability to evaluate proposed
exemption boundaries and protect surrounding drinking
water sources.
State of the Science
In 2016, EPA's Office of Research and Development (ORD)
published a state-of-the-science review paper, "Potential
Aquifer Vulnerability in Regions Down-Gradient from
Uranium In-Situ Recovery Sites"
http://dx.doi.Org/10.1016/i.ienvman.2016.08.049, which
covers background information on the geology and
geochemistry of uranium roll-front deposits; leach
solutions that mobilize the uranium during ISR; potential
interactions with other contaminants of concern, such as
arsenic, selenium, molebdynum, vanadium, and radium;
potential aquifer vulnerabilities related to in-situ leaching;
data gaps, appropriate monitoring strategies, and key
considerations for modeling applications. It also provides
guidance for further focused laboratory, field, and/or
transport and fate modeling research, in the review,
knowledge gaps in understanding how contaminants may
migrate during and after ISR operations were identified.
U.S. Environmental Protection Agency
EPA/600/F-17/342 | September 2017

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EPA Research
Research Recommendations. Based on the state of
the science review, EPA's ORD recommended additional
research activities focused on (1) methods of establishing
geochemical stability of potential groundwater
contaminants after the restoration period concludes, and
(2) understanding the potential for vertical and horizontal
migration of contaminants around ISR mining sites.
Research will focus on the potential for contaminants to
be present in the aquifer after restoration, which
contaminants might be present, and, if present, the
potential for those contaminants to migrate to nearby
underground drinking water sources.
Research Approach. ORD's research approach involves
reviewing existing data; developing field methods that can
be used to trace contaminant movement (such as isotopic
tracers); conducting laboratory studies to explore the
geochemical behavior of uranium and associated trace
metals; modeling approaches to simulate groundwater
flow and transport of injected fluids and contaminants
that could potentially impact underground sources of
drinking water and influence aquifer exemption boundary
determinations; and performing field studies that provide
data for and verify the modeled approaches (proposed).
Research Objectives. The objectives of the field
studies are to (1) verify that reducing conditions are re-
established at ISR sites; (2) evaluate water quality
characteristics down-gradient from post-mining ISR
operations compared to ore-zone water quality, and
where possible, to pre-mining water quality conditions;
and, (3) document natural attenuation pathways of
contaminants of concern.
Proposed Methodology. Field studies to investigate
whether aquifer sediments will rebound or recover to pre-
restoration concentrations will be conducted in several
mining districts so that a range of characteristic geologic,
geochemical, and hydrologic environments can be
compared. Laboratory studies utilizing materials from the
identified field research sites or from other sources to
investigate geochemical interactions between redox-
sensitive metals and aquifer solids are being conducted.
These studies are aimed at better understanding the
processes that control uranium mobilization and
attenuation in groundwater, relationships with other
contaminants of concern (e.g., As, Mo, and Se), and
development of risk management approaches for
restoring groundwater resources after ISR operations have
ceased. In-house capabilities are being developed to
determine uranium isotopic ratios in groundwater which
can be used as a tracer to evaluate vertical and horizontal
migration of uranium in groundwater.
Expected Outcomes. ORD's research will provide state-
of the-science approaches and new data to inform
standards for protecting the environment from hazards
associated with uranium solution mining and processing.
Results may also help EPA consider contaminant
movement in aquifers when determining aquifer
exemption boundaries and permit conditions. ORD's
studies are expected to add to the following:
•	Improved knowledge of the biogeochemical processes
that control the long-term stability of contaminants in
groundwater and surrounding rock.
•	Identification of geochemical factors that impact
contaminant stability and their role in determining
effective restoration criteria.
•	Development of tools for assessing the potential for
contaminant migration and attenuation.
•	Improved understanding of how groups of
contaminants that are found in the same aquifer
behave in site-specific instances.
•	Identification of strategies for minimizing impacts from
groups of contaminants based on appropriate scientific
data and site restoration approaches.
Additional Information
epa.gov/radiation/radionuclide-basics-uranium
epa.gov/radiation/health-and-
environmental-protection-standards-uranium-and-
thorium-mill-tailings-40-cfr
• 40 CFR Part 192
Websites:
• EPA's uranium information:
References Cited:
•	Anastasi, F.S., Williams, R.E. (1984). "Aquifer
restoration at uranium in situ leach sites."
International Journal of Mine Water 3, 39-37.
•	Davis, J.A., Curtis, G.P. (2007). "Consideration of
geochemical issues in groundwater restoration at
uranium in-situ leach mining facilities." U.S. Nuclear
Regulatory Commission, NUREG/CR-6870, 86 pp.
•	Borch, T., Roche, N., Johnson, T.E. (2012).
"Determination of contaminant levels and
remediation efficacy in groundwater at a former in
situ recovery uranium mine." Journal of.
Environmental Monitoring, 14,1814-1823.
•	Stover, D.E. (2004). "Smith Ranch ISL uranium
facility: The well field management program: Recent
developments in uranium resources and production
with emphasis on in situ leach mining." IAEA
TECDOC-1396, pp. 189-203.
Technical Contacts: Rick Wilkin, (580) 436-8874, wilkin.rick@epa.gov; and Susan Mravik, (580) 436-8553, mravik.susan@epa.gov
Communications Contact: Michelle Latham, (513) 569-7601, latham.michelle(a)epa.gov
U.S. Environmental Protection Agency
EPA/600/F -17/342 September 2017

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