Water Recycling for Climate Resilience
through Enhanced Aquifer Recharge and
Aquifer Storage and Recovery
d EPA
February 2023
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Disclaimer
The information in this document was funded by the U.S. Environmental Protection Agency under
Contract 68HERH19D0033. This document is not legally binding on any party and does not constitute a
statute or regulation, nor does it modify any statute or regulation. If there is any conflict with any
statute or regulation or other law, the statute or regulation or other law governs.
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Table of Contents
I. ^
Preface and Intended Audience iv
1. Background 1
2. Companion Reports and Terminology 2
2.1 Report on EAR with Stormwater 2
2.2 Report on UIC Applications 2
3. Drivers and Business Case for EAR and ASR 3
3.1 Hydrologic Demands and Impacts from Climate Change 3
3.2 Environmental, Ecological, and Social Considerations 4
3.3 Integrated Management of Water Supplies 5
3.4 Economic Drivers 5
4. Current Practice of EAR and ASR in the United States 7
4.1 Geographic Scope and Distribution of Technologies 8
5. Sources of Recycled Water for EAR and ASR 10
5.1 Treated Municipal Wastewater 10
5.1.1 Pathogen Risks 10
5.1.2 Chemicals and Compounds of Emerging Concern 11
5.1.3 Other Water Quality Parameters 11
5.2 Agricultural Return Flows and Drainage 12
5.2.1 Water Quality and Regional Considerations 13
5.3 Stormwater 14
6. Subsurface Water Quality Changes and Technical Considerations 15
6.1 Mixing Effects in the Aquifer 15
6.2 Soil Aquifer Treatment and Changes in the Unsaturated Zone 16
6.3 Organic Carbon and Nutrients 17
6.4 Redox Conditions 18
6.5 Arsenic and Metals Mobilization 18
7. Treatment Needs Associated with Sources of Water and End-Uses 19
7.1 Treatment before Recharge 19
7.1.1 Preventing Clogging during Recharge 19
7.1.2 Considerations of Existing Aquifer Characteristics 20
7.2 Treatment after Recovery 20
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8. Methods of Implementation: Opportunities and Constraints 21
8.1 Site Selection 22
8.2 Clogging 22
9. Regulatory Environment 24
9.1 California 24
9.2 Massachusetts 25
9.3 Nevada 25
9.4 Pennsylvania 25
10. Costs and Benefits 27
10.1 Cost of Treatment 27
10.2 Cost of Conveyance 27
10.3 Quantifying Benefits 28
11. Summary and Conclusions 30
References 31
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Preface and Intended Audience
\
On February 27, 2020, the U.S. Environmental Protection Agency (EPA) released the National Water
Reuse Action Plan: Collaborative Implementation (WRAP, Version 1). Through this initiative, EPA seeks
to advance water reuse across the water sector through a series of actions taken by EPA, other federal
agencies, industry associations, and relevant stakeholders. WRAP Action 7.4 (Increase Understanding of
Current Aquifer Storage and Recovery Practices) was developed to promote the understanding of
aquifer storage and recovery (ASR) practices by summarizing the current state of practice and detailing
the different factors that should be considered when developing such projects.
As part of Action 7.4, this report focuses on enhanced aquifer recharge (EAR) and ASR in the context of
water reuse, providing state- and local-level decision-makers information on using recycled water in EAR
and ASR applications. It explores the key considerations to evaluate when planning EAR and ASR as part
of integrated water resources management and climate resilience planning. More specifically, it
describes:
¦ The drivers for EAR and ASR.
¦ The extent of practice of EAR and ASR in the United States.
¦ Sources of recycled water for these projects.
¦ Subsurface water quality changes and technical considerations.
¦ Treatment needs.
¦ Methods of recharge.
¦ The regulatory environment.
¦ Potential costs and benefits.
While decision-makers must account for site-specific conditions when evaluating EAR and ASR projects,
this report provides a general overview of current projects using recycled water and identifies the types
of issues to address before implementation. Two companion reports from other EPA programs that
address other aspects of ASR and EAR are described in Section 2.
The information herein draws from existing EPA resources such as the 2012 Water Reuse Guidelines and
the 2017 Potable Reuse Compendium, as well as a search of available literature. EPA also held virtual
meetings with staff from the United States Department of Agriculture (USDA), the Ground Water
Protection Council (GWPC), and the American Water Works Association (AWWA) to solicit perspectives
on major issues related to the use of recycled water in EAR and ASR applications. While not intended to
be a comprehensive scientific literature review, this report provides a robust introduction to topics with
references to additional resources to empower state and local communities considering an EAR or ASR
project using recycled water to build climate resilience and support other local water management
priorities.
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1. Background
Enhanced aquifer recharge (EAR) is a practice in which groundwater aquifers are recharged by artificial
means, including injection wells and infiltration practices such as constructed basins or trenches. Aquifer
storage and recovery (ASR) is a sub-category of EAR, in which an aquifer is replenished, and the recharge
water is explicitly intended for later extraction and use. EAR differs from natural aquifer recharge by
including practices that intentionally recharge aquifers beyond naturally occurring processes. Both EAR
and ASR methods can store water for many beneficial uses and to address various hydrologic and water
quality issues affecting water supplies or environmental
flows. Understanding these benefits more fully is becoming
increasingly important as communities seek to build
resilience to more frequent and severe drought events now
exacerbated by climate change.
Currently, ASR and other forms of EAR are most often
implemented using treated drinking water and surface
water. However, treated municipal wastewater,
stormwater,1 agricultural drainage and return flows, and
other sources of recycled water have great potential for
increased use in EAR and ASR applications. Recharging
aquifers with recycled water can create an alternative
wastewater management strategy, and while aquifer
recharge with recycled water is practiced in some regions of
the country, in many others it represents a new—and largely
underutilized—contribution to local water supplies.
Although it requires specific considerations for water quality
and treatment, recycled water can be a valuable source of water, especially in areas where water
quantity is a major concern.
Applications of EAR and ASR with recycled water can address various needs, such as storage and
treatment for potable reuse, storage for use in irrigation, land subsidence mitigation, reducing or
eliminating surface discharges of wastewater, and protection against saltwater intrusion. Differing
regional drivers and potential benefits have resulted in a variety of operational and proposed EAR and
ASR projects around the United States, all with unique challenges and solutions that can help inform
future projects and integrated water management strategies to enhance resilience to the impacts of
climate change.
This document focuses on the
intentional recharge of aquifers
with recycled water. It mainly
addresses ASR, but it also
considers broader applications
including EAR, as both share
many of the same operational
practices and issues. Although
EAR and ASR can be practiced
with many different sources of
water, including surface water
diversions, this report focuses
exclusively on the use of recycled
water.
1 Recharge with stormwater is explored more fully in the EPA companion report Enhanced Aquifer Recharge of
Stormwater in the United States: State of the Science Review (EPA, 2021; see Section 2.1 for more details).
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2. Companion Reports and
Terminology
•
2.1 Report on EAR with Stormwater
In July 2021, EPA published a review and summary of the scientific and technical literature on the use of
urban stormwater for EAR: Enhanced Aquifer Recharge of Stormwater in the United States: State of the
Science Review, referred to in this document as the "Stormwater EAR Report." The report discusses
technical aspects of capturing and using urban stormwater, such as site selection, fate and transport of
pathogens and organic compounds, mobilization of subsurface contaminants, and pretreatment of
water prior to recharge operations. Therefore, this current report does not include detail on urban
stormwater capture in EAR or ASR, aside from scenarios where stormwater may be incorporated into
wastewater reuse applications.
2.2 Report on UIC Aquifer Recharge and ASR Applications
EPA is preparing a report on aquifer recharge (AR) and ASR that focuses on the current state of injection
practices within the Underground Injection Control (UIC) Program: Review of Aquifer Recharge and
Aquifer Storage and Recovery via Underground Injection, referred to in this document as the "UIC
AR/ASR Report." It includes an overview of the regulations of AR and ASR via injection wells, a summary
of relevant state and federal regulations, an inventory of AR and ASR injection wells within the United
States, and a discussion of technical and operational challenges associated with AR and ASR via injection
wells. Therefore, this current report does not present details on the UIC program.
Description of Terms for Aquifer Recharge
Several terms are used to describe the replenishment of aquifers, some of them largely synonymous. The
descriptions below are included for the purposes of this report only and are not intended to represent
official definitions.
~ Aquifer recharge (AR): The movement of water from the surface or unsaturated zone into the
saturated zone (e.g., an aquifer). Recharge is a natural process, but in the context of this report, it refers
to the intentional resupply of water to an aquifer.
~ Enhanced aquifer recharge (EAR): A suite of recharge practices with various goals, site requirements,
recharge water types, and infrastructure. A project is considered an EAR project if it results in more
recharge than would otherwise be expected through just natural processes. This document uses "EAR"
interchangeably with "managed aquifer recharge" (MAR), a term used extensively in the literature.
~ Aquifer storage and recovery (ASR): The recharge of water for later recovery and use. It is a type of
EAR practice. ASR systems that recharge via injection can use the same well for both injection and
withdrawal or use separate wells (in close proximity) to inject and withdraw water. ASR systems can
also use other EAR practices such as infiltration basins to recharge the aquifer for later recovery and
use. ASR is a subset of EAR. For instance, a project to replenish an aquifer to protect the aquifer against
seawater intrusion and ground surface subsidence would be considered an EAR project, but not an ASR
project, because there are no associated plans to recover the water used to replenish the aquifer.
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3. Drivers and Business Case for EAR
and ASR
EAR and ASR are relevant to a wide range of water management settings and scenarios. However, the
implementation of EAR and ASR has generally been prompted by several factors associated with the
overuse of groundwater due to increased demand over the last half of the 20th century (Dillon et al.,
2019). These factors relate to the reliability of water supplies, the environmental and ecological impacts
of overexploitation, economic considerations, and the need for a suite of options for integrated water
resource management (Asano, 2006).
Increased groundwater demand has been driven by societal shifts that include population growth and
increased energy consumption in urban areas, along with increased irrigation for food production and
other agricultural products (Jakeman et al., 2016). Technologically, increased withdrawal has been
facilitated by the availability of electric power and submersible pumps (Dillon et al., 2019).
Approximately 4,500 km3 (over 3.5 billion acre-feet) of groundwater depletion occurred globally
between 1900 and 2008. Within that period, depletion rates significantly increased starting in 1950, with
the highest rates occurring from 2000 to 2008—the most recent year with available data (Konikow,
2011). Water quantity concerns are especially acute in arid regions where water supplies are already
scarce and natural aquifer recharge is limited. Demand for water is also particularly high in coastal areas,
where high population densities result in heavy water usage.
3.1 Hydrologic Demands and Impacts from Climate Change
From a hydrogeologic standpoint, overexploitation of an aquifer occurs because natural recharge is
slower than the rate of groundwater withdrawals. This is often exacerbated by the fact that
groundwater residence times in an aquifer can be extremely long (Asano, 2006). For example, the High
Plains Aquifer, which underlies eight states and provides a third of the groundwater extracted in the
United States, has water older than 13,000 years in some areas (Fienen and Arshad, 2016). Therefore,
even if withdrawals were reduced or stopped in an overexploited aquifer, recovery would be slow, and
water levels would remain lower than before widespread extraction (Casanova et al., 2016; Angelakis
and Paranychianakis, 2003).
Increased groundwater demands are occurring in the context of climate change, with the associated
uncertainties and changing weather patterns (Miller et al., 2021). Generally, climate impacts include
increased drought, which further stresses water supplies, while higher rainfall intensity can exceed the
rate at which water infiltrates into soils, causing flooding and reducing the amount of water that
recharges groundwater (Bekele et al., 2018; Dahlke et al., 2018). The Fourth National Climate
Assessment (Lall et al., 2018) specifically cites climate change as a major driver in the frequency,
duration, and distribution of drought, especially in the Southwest. The Assessment also cites
groundwater depletion as exacerbating drought risk. This is further reinforced by the most recent report
from the Intergovernmental Panel on Climate Change (IPCC, 2022), which concluded with high
confidence that climate change will result in increases in frequency, intensity, and severity of droughts.
Concerns about uncertain water availability and increased demand fall under the broad umbrella of
water security, a common driver for water reuse. Using recycled water for various applications helps
mitigate this insecurity by turning a waste product into a valuable resource (Dillon et al., 2019). Recycled
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water in an aquifer recharge setting can provide a steady supply with relatively predictable volume
during drought or other conditions where the natural supply of water is uncertain or subject to
variability (Angelakis and Paranychianakis, 2003). It is an especially attractive option where water from
traditional sources, such as surface water or other aquifers, is not readily available in sufficient
quantities to support all drinking water supply needs. In this context, the aquifer used for storage is
effectively a bridge between wastewater treatment facilities and drinking water infrastructure (Dillon et
al., 2019).
Case Study; Orange County Water District
The Orange County Water District's Groundwater Replenishment System in Southern California is an
advanced wastewater treatment facility that recharges the Orange County Groundwater Basin. Its current
capacity is 100 MGD; of this amount, approximately 35 MGD is used for a seawater intrusion barrier, with the
rest recharged through infiltration basins (Kiparsky et al., 2021). This allows for the aquifer to be recharged
while ensuring it is also protected from future seawater intrusion. A project like this one could be
implemented with several different sources of water, but using treated municipal wastewater is an
increasingly common practice due to consistent availability. By using recycled water, the region has become
less reliant on imported sources ofwaterwhile protecting their existing groundwater supplies.
3.2 Environmental, Ecological, and Social Considerations
Environmental and ecological concerns prompting interest in EAR and ASR relate to the interactions
between groundwater and surface water and the need to protect surface water quality and quantity for
overall ecosystem health. Reductions in groundwater discharge due to lowered groundwater levels can
damage groundwater-dependent ecosystems such as wetlands, riparian forests, and springs fed by
baseflow from underlying aquifers (Dillon et al., 2009). In California, excessive groundwater pumping has
reduced baseflow to the point that flow in some rivers and streams has slowed or dried up (Fleckenstein
et al., 2004). EAR and ASR can be implemented to address these impacts of overdraft, thereby indirectly
protecting surface water resources.
EAR and ASR practices using recycled water can also protect surface water quality by reducing effluent
discharges and associated nutrient loadings (Page et al., 2018). For example, the Hampton Roads
Sanitation District (HRSD) in southeastern Virginia is in the early stages of implementing its Sustainable
Water Initiative for Tomorrow (SWIFT), which will use a series of groundwater injection facilities using
treated municipal wastewater. While water supply augmentation will be a benefit of this initiative, the
primary driver is to manage nutrient discharges in accordance with the Chesapeake Bay Total Maximum
Daily Load. Because HRSD can achieve nutrient reductions at a lower cost than neighboring jurisdictions
investing in improvements to comply with permit requirements, it is estimated that the implementation
of SWIFT may result in up to $2 billion in cost savings through nutrient credit trading (Green Nylen,
2021). A case study in this report provides more information on HRSD's SWIFT project (see page 28).
Excessive groundwater withdrawal causes other environmental and social impacts related to declining
water quantity. The loss of hydraulic pressure in an aquifer due to heavy withdrawal can cause land
subsidence and endanger buildings and other surface infrastructure in addition to decreasing the
existing and future storage capacity of the aquifer. Land subsides when groundwater pumping opens
pore space, which then collapses. Porosity within the aquifer is then sometimes permanently lost, along
with storage capacity for water. Seawater intrusion can also occur in near-ocean groundwater basins
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with active groundwater production wells. These wells can often draw down the groundwater within the
basin such that seawater begins to flow through the aquifer toward the well. As this happens, the
seawater mixes with the previously fresh water within the aquifer, often significantly degrading the
groundwater quality. In many cases, groundwater from the aquifer can no longer be used for its
intended purpose without expensive treatment and may take decades to recover to its previous
freshwater quality.
Recharge of coastal aquifers with the injection of water can provide additional hydraulic pressure to
prevent or mitigate the inward migration of saltwater and protect the water quality of inland freshwater
aquifers (Bekele et al., 2018). Saltwater intrusion has been a serious enough concern to prompt the
installation of injection wells for saltwater intrusion barriers in California and Florida. The use of recycled
water helps address these issues, while providing greater certainty for future water supplies. It also
gives communities more confidence in their water supplies, since recycled water is a steadier, more
climate-resilient water source than many natural supplies.
3.3 Integrated Management of Water Supplies
There is an increasing awareness of the need for the integrated management of water resources that
addresses local and regional needs, and EAR and ASR can play a valuable role. As noted above, EAR and
ASR are useful complements to surface water storage, as these approaches can help protect the water
supply against surface influences such as evaporative loss, algal blooms, atmospheric deposition of
contaminants, etc. (Hartog and Stuyfzand, 2017; Zheng et al., 2021). With greater seasonal variations in
water availability due to climate change (Casanova et al., 2016), EAR and ASR also provides a way to
remedy mismatches between water availability and demand (Hartog and Stuyfzand, 2017).
By developing a local water supply that includes alternative sources through water reuse, some
communities will also become less reliant on imported water. This is especially important for
communities in western states that rely on water from the Colorado River Storage Project or the Central
Arizona Project, which require water to be transported large distances with allocations that can
fluctuate based on availability. EAR and ASR provide water storage options in areas where there is a lack
of suitable sites or land area for surface reservoirs (Asano, 2006). As urbanization, population growth,
and water demand continue to exert stress on water availability and quality, the ability to bolster
existing aquifers and to store and retrieve previously underused water resources, such as recycled
water, holds promise in resilient water management planning (Dillon et al., 2019; Casanova et al., 2016;
Hartog and Stuyfzand, 2017). Climate change further stresses these demands and makes it critical that
communities diversify their water supply portfolio rather than relying on a single source of drinking
water. Through a diverse supply portfolio with recycled water, a community's supply is more resilient to
climate stresses and able to withstand more common and persistent drought events.
3.4 Economic Drivers
In addition to environmental and social drivers, there are also economic drivers for pursuing aquifer
recharge using recycled water. In some cases, using recycled water for aquifer recharge to supplement
groundwater supplies is less expensive and energy intensive than other drinking water supply options,
such as desalination. In addition, storage in an aquifer can preclude the need for infrastructure such as
pipelines or canals and reduce loss by evapotranspiration (Asano, 2006; Jimenez-Cisneros, 2014). For
example, in Southern California, reliability concerns and the high costs of importing water have long
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prompted interest in use of recycled water for EAR/ASR and other water reuse applications (Dahlke et
al., 2018), EAR can also be a lower-cost option for treated wastewater disposal than other methods
(Asano, 2006). More information about costs and benefits of EAR and ASR approaches is provided in
Section 10 of this report.
West Basin Municipal Water District's Edward C. Little Water Recycling Facility in El Segundo, California, produces five different
types of recycled water for different uses including groundwater recharge.
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4. Current Practice of EAR and ASR in
the United States
EAR and ASR practices have emerged as water stress has increased in many communities, driven by
increasing demand for water and a decline in natural sources (including groundwater). Before the
evolution of intentional EAR and ASR practices, recharge occurred passively in many places via drainage
wells for flood management, septic systems, and infiltration and percolation during irrigation. These
incidental routes of recharge were unmanaged with respect to water quality or other health and
environmental considerations (Dillon et al., 2019). Over time, unintentional recharge of aquifers began
to be managed, resulting in improvements in water quantity and quality. Progress in the use of EAR is
largely due to significant developments in technologies and management practices over the past 60
years (Dillon et al., 2019).
Common sources of recharge water have typically been surface water diversions and water from other
aquifers, generally recharged via infiltration basins (Casanova et al., 2016). However, recycled water has
been used to some degree for decades, including in projects in the western and southwestern United
States. Early EAR practices in the United States evolved from irrigation with the implementation of
recharge basins for spreading—particularly in Arizona, which saw several pilot projects in in the 1960s
and 1970s (Dillon et al., 2019). The Flushing Meadows pilot project (Salt River Valley, Arizona) and the
23rd Avenue recharge project (Phoenix, Arizona) both used several infiltration basins and recharged the
aquifers with treated municipal wastewater. The passage of the treated municipal wastewater through
soil that these infiltration-based projects employed, and the associated water quality improvements,
eventually become known as soil aquifer treatment (SAT). Additionally, some entities began using
injection wells due to limited available land area.2
When treated to an appropriate level, wastewater effluent has become an acceptable source of water
for EAR and ASR. Advancements in treatment technologies, such as the development of membranes
suitable for membrane bioreactors in the early 1990s, played an important role in the advancement of
both EAR and ASR (Dillon et al., 2019). In addition, some aquifers previously considered to be too
brackish for beneficial use have been transformed into productive water resources through infiltration
or injection of higher-quality water and the resulting dilution of the brackish water (Dillon et al., 2019).
Case Study; Central Arizona Water Conservation District
Under authority provided in 1993 by the Arizona State Legislature, the Central Arizona Water Conservation
District (CAWCD) seeks to give landowners and water providers a mechanism to demonstrate that they have
an assured water supply. As part of a first-of-its-kind public/private agreement in 2014 between CAWCD and
Liberty Utilities, the Liberty Aquifer Replenishment Facility began construction in 2016 and began operation
in February 2017. Located in Goodyear, Arizona, this 51.77-acre facility was constructed to receive water that
is not sold for irrigation from Liberty's Palm Valley Water Reclamation Facility for recharge into the regional
aquifer in an area where water levels are currently declining at a historic rate (Liberty Utilities, 2017).
2 See EPA's Underground Injection Control program and the UIC AR/ASR Report (in preparation) for additional
information on aquifer recharge and ASR injection wells.
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In the United States, general growth of EAR and ASR over the last few decades has been estimated at 5
percent per year (by volume) but may in fact be underreported due to incomplete data availability
(Dillon et al., 2019; Zhang et al., 2020). The average volume of water used for EAR grew from 302 million
m3 (about 250,000 acre-feet) per year in 1960-1970 to 2,569 million m3 (about 2.1 million acre-feet) per
year in 2011-2015 (Dillon et al., 2019). This follows similar trends in 33 other countries, where EAR
volumes have increased from 1,029 million m3 (about 835,000 acre-feet) per year to 9,945 million m3
(about 8.1 million acre-feet) per year during the same timeframe (Dillon et al., 2019). Though data were
not available to indicate how much of this volume represented use of recycled water, as population
increases and water supplies in some regions continue to be strained, treated wastewater will likely be
considered as a consistent resource for use in EAR and ASR projects.
4.1 Geographic Scope and Distribution of Technologies
EAR and ASR are also increasingly common outside the United States as a water management strategy,
as seen in the map of EAR projects which includes ASR, throughout the world (Figure 1, Stefan and
Ansems, 2018). The map shows a broad mix of project types in Europe, the United States, China, and
elsewhere. (The database used to produce Figures 1 and 2 can be found at https://ggis.un-
igrac.org/view/marportal.)
G Spreading Methods
O Induced Bank Filtration
• Well, Shaft & Borehole Recharge
• In-Channel Modification
• Rainwater & Run-off Harvesting
= O Specific MARtype V
SPREADING METHODS
• Infiltration Ponds and Basins
O Flooding
O Ditch and Furrow
© Excess Irrigation
O Reverse Drainage
INDUCED BANK FILTRATION
O Induced Bank Filtration
WELL SHAFT AND BOREHOLE RECHARGE
• ASR/ASTR
• Dug Well/ Shaft/ Pit Injection
IN CHANNEL MODIFICATIONS
© Recharge Dam
• Sand Storage Dams
A Subsurface Dam
O Channel Spreading
RUN OFF / RAINWATER HARVESTING
• Barriers and Bunds
©Trenches
• Rooftop Rainwater Harvesting
EQ
Figure 1: EAR projects throughout the world (Stefan and Ansems, 2018)
Figure 2 is a map from a database of EAR projects in the United States (Stefan and Ansems, 2018). In
general, this map suggests these projects are located in more populous areas. In the map
* Dark blue locations are those using wells for injection.3
¦ Orange locations represent infiltration ponds and basins (spreading methods), used in EAR
projects in California, Utah, Arizona, Washington, Oregon, North Dakota, Kansas, New York, New
Jersey, and Florida. (There are also spreading projects in Alaska and Hawaii, not shown in the
map.)
3 See the UIC AR/ASR report (in preparation) for a more comprehensive inventory of AR and ASR wells in the
United States.
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* Light gray locations represent induced bank filtration projects in Washington, Nebraska, Kansas,
Texas, Indiana, Kentucky, New Jersey, and Ohio. (This report does not cover such projects in
detail.)
IA MAR Portal
T
m
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• ih-ChannH Mnrtlflcur.in
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Figure 2: EAR projects in the continental United States (Stefan and Ansems, 2018)
Although Figures 1 and 2 do not distinguish which aquifer recharge projects use recycled water, they
illustrate where recharge is currently implemented as a viable and valued part of water management.
Additionally, they demonstrate that EAR is not a practice limited to water-stressed regions. Rather,
these projects represent examples of methods, siting considerations, and other aspects of recharge
projects that have commonalities with situations where recycled water is used for recharge. As of 2018,
injection wells appeared to be used more often than infiltration in Florida and the northwest states of
Oregon and Washington. On the other hand, infiltration projects are more common in the arid
Southwest (Utah, Arizona, and California), as shown in Figure 2's orange locations. This region receives
relatively sparse rainfall, resulting in fewer perennial streams or rivers, with higher temperatures and
lower humidity leading to higher evapotranspiration rates (Gelt et al., 1999). Accordingly, depth to
water in this region is typically greater, allowing for seepage of recharge water to depths below the
reach of most plants. These factors make recharge of shallow groundwater by infiltration more
attractive, allowing for underground storage of water from spring snowmelt and storm events and
reducing the potential for evapotranspiration.
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Water Recycling for Climate Resilience in Enhanced Aquifer Recharge and Aquifer Storage and Recovery | 9
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5. Sources of Recycled Water for EAR
and ASR
EAR and ASR projects use a variety of sources of water for recharge including surface water (diversions
from lakes, streams, rivers), stormwater, treated municipal and industrial wastewater, treated drinking
water, groundwater from another aquifer, and agricultural return flows and drainage. The source of
water being used for recharge depends on factors such as availability, quality of the source of water, the
required quality of the recharge water to ensure compatibility with the native groundwater, and
eventual end-uses. Some projects use more than one source of water (e.g., stormwater and treated
wastewater), which can represent a more integrated approach to water resource management, further
diversifying water supply. Historically, surface water has been a primary source of recharge water,
especially given its abundance in temperate humid climates (Casanova et al., 2016).
Considerations for undertaking EAR and ASR with recycled water include the availability of the water to
be recycled in terms of volume, timing, and proximity. Additionally, annual cycles in agricultural settings
affect how much recycled water is available for irrigation versus recharge. Water rights in the western
states may also influence whether treated municipal wastewater is required to be discharged instead of
being reused (see the text box on page 26 for more details on this topic) (EPA, 2012).
5.1 Treated Municipal Wastewater
Treated wastewater from municipal wastewater treatment plants is commonly used for EAR and ASR in
regions with limited water resources. Industrial wastewater is less common for recharge outside of its
contribution to municipal wastewater flows, as it can contain reactive and recalcitrant organic
compounds and heavy metals. However, it can be used for recharge if the water is of adequate quality.
According to Rauch-Williams et al. (2018), about 32 billion gallons per day (BGD) of municipal
wastewater is treated in the United States each year, with about 2.5 BGD currently recovered for some
form of reuse after treatment. Of this 2.5 BGD, just over half is used for landscape and agricultural
irrigation, and the rest is used for a variety of other end-use applications. In California, 18.4 percent of
treated municipal wastewater is reused, with about 69 percent of that amount used for non-agricultural
purposes, including groundwater recharge (Borch et al., 2021). While reuse is practiced in many
different settings, treated municipal wastewater has the advantage of being relatively consistent in
volumes throughout the year in locations where there is a robust wastewater supply in relative
proportion to water demand. The consistent availability of treated municipal wastewater is especially
important for communities looking to build resilience to climate change, since they can assure that part
of their water supply portfolio will be available even in times of drought. However, water quality
considerations for municipal wastewater must be resolved as sufficient treatment is needed prior to
reuse and recharge (Page et al., 2018).
5.1.1 Pathogen Risks
For potable reuse applications, including recharge of drinking water aquifers, pathogens are generally
considered to be the primary acute risk to public health. Pathogens of greatest concern are typically
bacteria (e.g., Campylobacter), enteric viruses (adenoviruses and noroviruses), and enteric protozoa
(Giardia and Cryptosporidium) (Nappier et al., 2018). Pathogen concentrations in raw wastewater can
vary by location, but estimates can be derived for common reference pathogens to develop treatment
Water Recycling for Climate Resilience in Enhanced Aquifer Recharge and Aquifer Storage and Recovery 110
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benchmarks for potable reuse (EPA, 2017; Soller et al., 2018). Treatment processes such as ozone, high-
dose UV, membrane processes, and other disinfection processes are implemented to reduce pathogen
concentrations in wastewater effluent to an acceptable level as determined by the relevant regulations
(see Sections 6 and 9 for more detail). In general, the risk benchmark for pathogens in potable reuse is
to achieve a less than 1 in 10,000 annual risk of infection which is the generally accepted level of risk for
conventional drinking water (Regli et al., 1991). Some states are now developing regulations and
treatment requirements for specific pathogen groups for both indirect and direct potable reuse.
5.1.2 Chemicals and Compounds of Emerging Concern
If not treated appropriately, chemicals and other compounds of emerging concern can be a chronic risk
for public health in water reuse applications. The Chemical Abstract Service of the American Chemical
Society lists over 394,000 chemicals, many of which are found in different types of wastewater.
Furthermore, identifying such chemicals in wastewater can be difficult. Although many are removed via
different mechanisms during conventional wastewater treatment, some will persist (Prasse et al., 2015)
and advanced treatment may be needed for some reuse applications, especially potable reuse. Aside
from limits on select chemicals from federal and state regulations, a common strategy for monitoring
chemical removal in water reuse is through performance-based surrogates used as proxies for different
classes of chemicals. California has taken such an approach for health- and performance-based
indicators and surrogates as part of a monitoring strategy for compounds of emerging concern in
recycled water (Drewes et al., 2018).
5.1.3 Other Water Quality Parameters
For other parameters related to water quality, treated effluent may contain higher concentrations of
organic carbon, but lower concentrations of suspended solids compared to other sources of water such
as stormwater (Page et al., 2018). The higher organic carbon content has implications for the redox
status in the aquifer and vadose zone, affecting the overall geochemical environment. Microbial
consumption of organic matter consumes oxygen, and if organic matter concentrations are high enough,
this depletion can drive the system towards anoxic (oxygen-free) conditions. In anoxic subsurface
environments, iron oxides can dissolve, releasing associated contaminants into the water (see Sections
6.3 through 6.5 for more detail). Dissolved organic carbon can also contribute to the formation of
disinfection byproducts when water is chlorinated for disinfection. Characterization of the types of
organic carbon present in effluent that will be used for recharge will help in anticipating subsurface
processes and any potential treatment issues to be addressed.4
Recycled municipal wastewater may also pose concerns due to elevated salinity and other elements
such as boron (EPA, 2012). The salinity in treated municipal wastewater depends on the original content
in the raw wastewater, such as human activity including the use of water softeners, types of waste being
discharged, and other factors (EPA, 2012). Higher salinity in recycled water relative to surface water
presents management challenges for EAR and ASR, particularly in agricultural settings.
For nutrients, loadings often vary seasonally because of effects on microbial treatment processes (Page
et al., 2018). Nitrogen species in secondary treated wastewater effluent include ammonia, nitrate,
4 EPA's 2012 Guidelines for Water Reuse includes further discussion on the components of organic carbon in
wastewater.
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nitrite, and organic nitrogen (Goren et al., 2014). In some facilities, the nitrogen is present as nitrate,
while in others, nitrogen is primarily present as ammonium (Goren et al., 2014). Recycled water that has
not been nitrified or denitrified can contain over 20 mg/L of ammonia (EPA, 2012), and some states
require nitrogen removal for some classes of recycled water. For example, California requires total
nitrogen of less than 10 mg/L for groundwater recharge to protect public health (California Code of
Regulations, Title 22, §60320.110). The significance of nutrients in the recharge water depends on the
intended use upon withdrawal (Dillon et al., 2009). For aquifers used as a drinking water supply, nitrate
is a significant issue and care should be taken to ensure that recharge does not endanger groundwater
quality. This is especially true in communities that are dependent on private wells without centralized
drinking water treatment.
If an aquifer is in hydraulic connection to groundwater-dependent ecosystems, the potential loading of
nutrients needs to be acceptable for the species present (Dillon et al., 2009). In addition, if the
groundwater may be used as a potable supply in the future or if recharge may affect private wells,
human health effects need to be considered (EPA, 2012). In a system using SAT, some of the nutrient
concerns can be mitigated, but performance for nutrient removal will be variable and system-specific
(e.g., anaerobic conditions favor the reduction of nitrate) (Bekele et al., 2011; Page et al., 2018).
However, in cases where an aquifer is used for irrigation, and not for drinking water purposes, the
nutrients present in the recharge water may be beneficial if the overall water quality is sufficient.
5.2 Agricultural Return Flows and Drainage
Agricultural return flows and drainage typically refer to excess irrigation water or natural precipitation
that generally flows to surface water. While there are opportunities to reuse return flows and drainage
for recharge, there are significant incentives to limit return flows through more efficient practices or to
directly reuse return flows for irrigation. The primary reason fields are drained is to lower the water
table. This is often accomplished through tile drainage systems to drain excess water that may otherwise
affect root health in crops.
Case Study; Arvin-Edison Water Storage District
Located in California's Central Valley, the Arvin-Edison Water Storage District has been practicing aquifer
recharge to support agricultural irrigation since the 1960s. During wet years, excess surface water is
infiltrated through 1,500 acres of spreading ponds that can be withdrawn later during dry years. Before
aquifer recharge was introduced, overdraft was up to 113,000 acre-feet per year and threatened the long-
term sustainability of agriculture in the region. However, between 1966 and 1999,4.2 million acre-feet of
water was stored in the aquifer (National Research Council, 2008). While this is not a direct example of EAR
or ASR implemented with recycled water, it is a specific example of ASR being practiced to support
agricultural irrigation.
Based on the 2017 Census of Agriculture (USDA, 2019), nearly 100 million acres of farmland use either
tile drainage or artificial surface drainage. Much of this land area is concentrated in the Midwest, with
significant amounts in the West as well (USDA, 2019). Drainage is usually managed by allowing water
levels to remain closer to the surface in the spring when root depth is shallow and rainfall is high, then
slowly lowering the water table in front of the growing roots (Skaggs et al., 1994). Although the drained
water is often transported away from the location on the surface, it can be captured for recharge. An
Water Recycling for Climate Resilience in Enhanced Aquifer Recharge and Aquifer Storage and Recovery 112
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estimate of the potential drainage volume in the United States (based on the fraction of farmland
drained by tile but not counting precipitation) suggests that about 220 billion gallons of water per year
may be available for reuse. Of that amount, 65 percent is attributed to western states. However, to
utilize this available water, infrastructure such as storage, pumping, distribution, and potentially
treatment would be needed (Hejase et al., 2022).
5.2.1 Water Quality and Regional Considerations
Constituents of concern in agricultural drainage used for EAR and ASR include nitrate, phosphate, salts,
and other chemicals such as fertilizers and pesticides (Borch et al., 2021). Trace elements can also be
present—arsenic, molybdenum, and selenium, among others—and mobilized during irrigation (Tanji and
Kielen, 2002). Water quality will vary from location to location, but there are some general trends.
Compared to applied irrigation water, subsurface drainage generally has higher salinity with more
nitrogen and potentially more pesticides and phosphorus. This change in water quality depends on a
variety of factors including irrigation method, soil characteristics, the use of fertilizers and other
chemicals, the drainage system, climate, and site operations (Tanji and Kielen, 2002). While it is more of
an operational consideration, sediment contained in agricultural drainage can result in clogging and
impede infiltration.
There are also regional differences in drainage water quality. In eastern states, nutrients in agricultural
drainage are considered valuable for reuse, and treatment may not be needed. In the West, drainage
can be highly saline; in the San Joaquin Valley in central California, for example, total dissolved solids can
reach 5,000-20,000 mg/L. Such highly concentrated drainage poses greater water quality concerns, and
treatment is more likely to be needed before the water can be used for recharge (Hejase et al., 2022).
While the quality of agricultural drainage has been an issue for its impacts on wildlife and ecosystem
health in the western states for quite some time (USGS, 2003), there are still research needs for the
application of agricultural drainage in EAR and ASR. Many of the same compounds and trace elements of
concern for surface water contamination would also be of concern for EAR or ASR, but more information
is needed on the influence of SAT and other subsurface processes on water quality and treatment
needs. Once contaminated, aquifers are difficult, expensive, and time-consuming to remediate, making
evaluations of water quality and treatment needs crucial.
Recharge on Agricultural Land
There is interest in California and other states with irrigated agriculture in investigating the potential for
aquifer recharge using agricultural land. This concept would apply water on agricultural land in excess of
crop needs when water is in sufficient supply and demand is low (Bachand et al., 2012). The practice would
not convert existing agricultural land into spreading grounds or infiltration basins as land would be kept in
production during growing seasons. Using recycled water in the same way would take this concept further
by using a water source that would likely be in consistent supply even in times of low precipitation.
However, it would introduce additional considerations such as the quality of the recycled water and the
efficacy of SAT, impacts on groundwater quality, as well as the effects on agricultural production. In
addition, there are some existing regulatory constraints: for example, some states require that recycled
water for irrigation be applied at the agronomic rate to prevent over-irrigation and protect groundwater
quality. A complete compilation of state-level regulations for water reuse is being developed through EPA's
REUSExplorer, found at http://www.epa.gov/reusexplorer. More discussion on this topic is found in
Richardson etal. (2018).
Water Recycling for Climate Resilience in Enhanced Aquifer Recharge and Aquifer Storage and Recovery 113
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5.3 Stormwater
Stormwater runoff from rain and snowmelt events that flow over land or impervious surfaces is often
viewed as a nuisance or a drainage problem for communities to manage. As some communities face
water stress, stormwater is increasingly being viewed as a resource that can be captured, treated, and
used for different purposes. Among these end-uses is aquifer recharge through EAR or ASR. While the
availability of stormwater is intermittent and dependent on climatic conditions, it can significantly
contribute to a local water supply. For example, in California it has been estimated that up to 3 million
acre-feet per year of stormwater is available for capture in the urban areas of California, depending on
the amount of rainfall, in a given year. This is roughly on par with the potential for enhanced urban
water efficiency and wastewater reuse (Cooley et al., 2022). With climate change projected to increase
the frequency and severity of rain events as well as drought (Lall et al., 2018), capturing stormwater
when it is available may become more crucial to supporting sustainable groundwater supplies. Much like
municipal wastewater, stormwater has different chemical and microbial constituents that can pose risks
to groundwater quality through EAR or ASR applications. Details on treatment needs and other
considerations are largely dependent on site-specific conditions and more detail on the use of
stormwater in EAR can be found in EPA's Stormwater EAR Report.
The Pure Water Monterey project in Central California uses municipal wastewater, stormwater, agricultural drainage, and
agricultural wash water in its potable reuse facility to replenish the Seaside Groundwater Basin.
Water Recycling for Climate Resilience in Enhanced Aquifer Recharge and Aquifer Storage and Recovery j 14
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6. Subsurface Water Quality Changes
and Technical Considerations
«
EAR and ASR systems can change the chemistry of the recharge water and the water in the aquifer
through chemical and biogeochemical processes that depend on the properties of the recharge water as
influenced by treatment before recharge occurs, the chemistry of the groundwater in the aquifer, the
mineralogy of the aquifer, and the properties of the vadose zone (e.g., if the system is infiltration-based
and provides SAT). For example, the groundwater may have achieved equilibrium with the aquifer
matrix, and the addition of new water can be expected to trigger geochemical reactions (Page et al.,
2018). While these subsurface processes are often quite complex, there is a long history of projects
successfully navigating challenges to implement EAR and ASR projects using recycled water. An initial
understanding of these processes and a site-specific investigation are needed, but there are typically
feasible solutions (including treatment, detailed in Section 8) to the different issues that might arise.
The sections below briefly
describe several types of
Figure 3: Unsaturated and saturated zones in soil (Source: United States
subsurface changes and _ . . Ic .
° Geological Survey).
processes.
6.1 Mixing Effects in the Aquifer
Where recharge waters and native groundwater mix, the water chemistry will be the result of any
dilution effects from the physical blending, along with geochemical or biogeochemical reactions.
Recharge water and ambient groundwater may differ in salinity, and salinity can be higher in treated
wastewater than other recharge water sources, such as surface waters or stormwater. Once the
recharge waters mix with the ambient groundwater, the salinity of the recovered water will likely differ
from that of the recharge water (Page at al., 2018). For this reason, it is helpful if recharge water salinity
is lower than or similar to groundwater salinity. If the aquifer contains high-quality groundwater, the
Percolation of recharge water through the vadose zone during SAT can also prompt geochemical and
biogeochemical processes that provide treatment of the recharge water (Page et al., 2018). Processes
affecting the composition of the recharge water as it flows through the vadose zone (also known as the
unsaturated zone) and saturated zone (Figure 3) include biodegradation of dissolved and particulate
organic matter, nitrification and denitrification, adsorption of ammonium, adsorption of trace elements,
adsorption of organic compounds, cation exchange, and precipitation of phosphates (Goren et al.,
2014). Pathogen removal also
occurs during this time through
attenuation as well as
inactivation depending on the
soil characteristics and travel
time. Additional pathogen
removal is achieved based on
residence in the aquifer with
greater removal correlated
with longer residence times
(EPA, 2017).
Surface water
Gravel
? •.
Saturated zone ?.
_ • <=* ^
• tw» * sji—.
' to « & Ground water .
Creviced rock
Water (not ground water) held by molecular attraction
surrounds surfaces of rock particles
- - Approximate level of the water table
All openings below water table
full of ground water
Water Recycling for Climate Resilience in Enhanced Aquifer Recharge and Aquifer Storage and Recovery j 15
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additional salinity could be detrimental to crops if the recovered water is used for irrigation and other
applications such as potable reuse. To remediate this, treatment processes, such as reverse osmosis may
be needed to ensure that salinity is at an appropriate level.
When the recharge water is less saline than the groundwater (e.g., a brackish or saline aquifer) and the
aquifer matrix contains fine clay particles, certain types of clay minerals (e.g., montmorillonite) will
expand. The swelling clogs pore spaces but is reversible if the salinity increases again (Maliva, 2020). The
influx of fresh water can also mobilize (disperse) clays from the aquifer matrix. They will then be
transported in the water and can reduce aquifer permeability when they become trapped in pore
spaces. This type of clogging is not reversible. Whether and how much clay will swell and disperse is site-
specific, depending on aquifer mineralogy and the water chemistry changes occurring when the
recharge water mixes with the native groundwater (Maliva, 2020).
Mixing of waters may also alter the geochemical conditions in the aquifer such that minerals can
dissolve (e.g., calcite) or precipitate (e.g., iron [hydr]oxides), which can affect the permeability and
porosity of the aquifer (EPA, 2021; Page et al., 2018; Bekele et al., 2011). Other changes, such as
decreases in calcium and magnesium in groundwater from dilution, can cause clays to swell and
disperse; this can cause clogging, lowering hydraulic conductivity (EPA, 2021). Such changes will affect
project operations factors such as injectivity, stability of well infrastructure, and transport of
contaminants.
6.2 Soil Aquifer Treatment and Changes in the Unsaturated Zone
SAT is generally considered part of the overall treatment process for treated municipal wastewater
being used for recharge through infiltration. Depending on conditions, passage through the vadose zone
during SAT can result in the partial attenuation or removal of several types of pollutants often found in
treated municipal wastewater. These include pathogens, nitrate and ammonium, protein-based organic
matter, disinfection byproducts (e.g., N-nitrosodimethylamine [NDMA], haloacetic acids [HAAs], and
total trihalomethanes [TTHMs]), and pharmaceuticals and other emerging contaminants (Trussell et al.,
2018; Hartog and Stuyfzand, 2017).
As an example of treatment during SAT, a field trial of infiltration of secondary treated wastewater
through a 9-meter-thick calcareous vadose zone achieved the following reductions: 30 percent for
phosphorus, 66 percent for fluoride, 62 percent for iron, and 51 percent for total organic carbon (TOC)
with a residence time of four days in the vadose zone and less than two days in aquifer. This treatment
also achieved partial reductions in pharmaceuticals (oxazepam and temazepam), and pathogens (Bekele
et al., 2011). There was no reduction in total nitrogen, but there was a reduction in total organic
nitrogen and ammonia and an increase in nitrate due to aerobic conditions in the vadose zone. These
reductions illustrate that the benefits of SAT vary by constituent and can change over time. For example,
phosphorus removal may decline as adsorption sites eventually become saturated (Bekele et al., 2011).
More specific to pathogens, several studies on the attenuation of microorganisms suggest significant
removal for a variety of different pathogens including up to 8 logio removal of MS-2 virus and 9.5 logio
Cryptosporidium depending on travel time (EPA, 2017; Trussell et al., 2015).
Several factors control the geochemical and biogeochemical processes that treat water during SAT. For
example, travel time is significant, with the desired time depending on the soil characteristics, the depth
to groundwater, and the desired level of pollutant removal and attenuation. In addition, a pH between 5
Water Recycling for Climate Resilience in Enhanced Aquifer Recharge and Aquifer Storage and Recovery 116
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and 8 is best for degrading organic contaminants and minimizing trace metal mobility (Casanova et al.,
2016). The mineralogy of the vadose zone can also exert significant control on the removal of metals
and organics through the sorption sites on iron and manganese (hydr)oxides and clay minerals
(Casanova et al., 2016). These processes are complex and site-specific.
Like pH, reduction/oxidation (redox) conditions exert significant control on subsurface geochemistry and
biogeochemistry in both the vadose and saturated zones. The effects of redox changes can be beneficial
(e.g., managing nutrients) or problematic (e.g., mobilization of metals or other contaminants). An
approach to managing recharge water treatment during SAT is to promote different redox conditions via
flooding and drying cycles to achieve treatment goals such as the removal of trace organic compounds
or nitrogen (Bekele et al., 2011). The ability to optimize an SAT system to promote these redox-
dependent transformations is valuable for managing the water quality concerns associated with EAR and
ASR using recycled water.
Additionally, recharge water may leach salts and other constituents from the soil during infiltration,
adding to the concentrations already in the recycled water used for recharge. This is a particular concern
for groundwater quality in agricultural areas because certain ions (boron, chloride, sodium) can
accumulate in sensitive crops, damaging them and reducing crop yields (Richardson et al., 2018).
Case Study; Monterey One Water
Located in the central coast of California, the Monterey Peninsula Water Management District and Monterey
One Water jointly developed the Pure Water Monterey initiative to replenish the Seaside Groundwater Basin
with recycled water. This project also provides recycled water for landscape and agricultural irrigation in the
northern Salinas Valley, where agriculture—both growing and processing crops—is a major industry (Kenny
et al., 2019; Monterey One Water, 2021). This facility uses a similar treatment program as the Groundwater
Replenishment System in Orange County but is unique in using multiple water sources. In addition to
municipal wastewater, this project incorporates stormwater runoff, agricultural drainage, and agricultural
wash water. This project began operation in 2020 with a design capacity of 5 MGD (Monterey One Water,
2021); since operation began, Monterey has proposed to expand to a capacity of 7.6 MGD and to increase
recharge to the Seaside Groundwater Basin from 3,500 to 5,750 acre-feet peryear (Duffy and Associates,
2020). While this is a unique facility, it demonstrates that a robust treatment and management program can
be put in place to incorporate multiple alterative water sources into a single project.
6.3 Organic Carbon and Nutrients
Organic carbon in recharge water provides a source of energy for microbes, facilitating the degradation
of trace organic pollutants and the reduction of nitrate. Organic carbon in the sediments also promotes
adsorption of metals, removing them from water as it infiltrates through the vadose zone (Casanova et
al., 2016) or moves through an aquifer. In an SAT system, dissolved organic matter is largely removed by
microbial activity in the upper two meters of the vadose zone (Goren et al., 2014), and in relatively short
travel times often measured in days (Trussell et al., 2018).
As noted in Section 7.1, concerns about nutrients depend on the intended end-use of water in an EAR or
ASR system. An awareness of both the incoming concentrations in the recharge water and the
anticipated geochemical and biogeochemical processes in the vadose zone and saturated zone is needed
Water Recycling for Climate Resilience in Enhanced Aquifer Recharge and Aquifer Storage and Recovery 117
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to manage system operations and plan for the recovered water quality. A complex set of systems
control nutrient dynamics in the subsurface. For example:
¦ Ammonium removal is controlled by cation exchange and redox conditions in the soil, as are
transformations among the nitrogen species. Under oxic conditions, ammonium is converted to
nitrate. If nitrate removal is needed, subsurface conditions must be anaerobic; nitrate is
converted to nitrogen gas via denitrification under anaerobic conditions and is released from
the system (Goren et al., 2014).
¦ As noted above, nitrogen removal during SAT can be optimized by cycling between oxic and
anoxic conditions, although efficiency will vary by system and is not always adequate (Bekele et
al. 2011).
6.4 Redox Conditions
Redox conditions play a key role in biogeochemical processes in soils and groundwater, similar to the
role they play in settings such as lakes, wetlands, and rivers. Because they are involved in the retention,
release, and transformations of constituents in both water and soils/sediments, redox conditions affect
the quality of the recovered water in EAR and ASR. This in turn has implications for operational choices
and treatment needs before and after recharge.
In an EAR or ASR system, redox conditions depend on the recharge water quality, soil properties, depth
to water table, temperature, and choices in operational practices (e.g., alternating between oxic and
anoxic conditions) (Goren et al., 2014). As noted above, redox conditions in an infiltration basin (e.g.,
SAT system) can be controlled by flooding. Prolonged saturation of the vadose zone promotes the
development of anoxic conditions, leading to processes such as denitrification and dissolution of iron
and manganese minerals and release of any adsorbed contaminants (Goren et al., 2014). The recharging
effluent may also be subject to diurnal changes due to daily cycles in sunlight and temperature, with
increased dissolved oxygen and pH during the daytime (Goren et al., 2014). In the saturated zone, redox
status may be changed by an influx of more oxygenated recharge water, although this effect is often
more dramatic where recharge is done via injection wells.
6.5 Arsenic and Metals Mobilization
Mobilization of arsenic and other metals from aquifer materials has been a significant concern in some
EAR and ASR systems, especially in Florida (Neil et al., 2014; Fakhreddine et al., 2015; Vanderzalm et al.,
2011; Yang et al., 2016). This water quality issue arises when oxygenated recharge water is introduced
into an anoxic aquifer containing arsenic-bearing minerals. The ensuing oxidation can release arsenic
into the water. The arsenic may either become adsorbed to the surface of secondary iron (hydr)oxide
minerals or remain in the groundwater. Should conditions subsequently become anoxic, the dissolution
of iron minerals will release any arsenic or other contaminants (e.g., metals, phosphate, organics)
adsorbed to the mineral surfaces. This balance between mobilization and retention will be site-specific,
depending on the characteristics of the aquifer, groundwater, and recharge water. The potential for
arsenic mobilization can be mitigated by matching the recharge water more closely to the native
groundwater (e.g., by reducing dissolved oxygen in the water). However, arsenic can also decline during
multiple operational cycles (e.g., repeated injection and recovery) as the pool of arsenic-bearing
minerals is depleted (Maliva et al., 2018). Additional discussion on arsenic and metals mobilization is
found in EPA's 2012 Guidelines for Water Reuse and in EPA's UIC AR/ASR Report (in preparation).
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7. Treatment Needs Associated with
Sources of Water and End-Uses
7.1 Treatment before Recharge
Treatment needs before recharging an aquifer with recycled water vary depending on the source and
quality of the recharge water, the recharge method, the end-use after recovery, and site-specific
characteristics (Asano and Cotruvo, 2004). For example, if the native groundwater is of high quality and
the end-use requires high-quality recovered water (e.g., for potable use), treatment will be quite
important (Page et al. 2018). Treatment can also ensure water used for recharge is compatible with the
subsurface soils, sediments, and groundwater to minimize geochemical reactions such as mineral
dissolution, precipitation, and mobilization of contaminants like arsenic (see Section 6.5 for details).
For EAR and ASR with treated municipal wastewater involving injection wells, treatment requirements
are often more stringent to address specific pathogens and other pollutants (see Section 9 for details on
state regulations). The treatment processes used vary, but generally include a combination of low- and
high-pressure membrane treatment, disinfection through chemical treatment, ultraviolet light, oxidation
through ozonation, or other advanced oxidation processes, such as the combination of ultraviolet light
and hydrogen peroxide (EPA, 2017).
SAT—the process where water passes through the unsaturated zone above a groundwater table,
improving water quality through a variety of physical, chemical, and biological processes that retain,
transform, or degrade contaminants, nutrients, and pathogens—can also be used in infiltration projects
as part of a treatment strategy in place of some engineered treatment systems in locations with the
appropriate hydrology and soil conditions (Goren et al., 2014). The specific treatment technologies and
approaches selected for a project will vary based on the state regulations, source water quality, and
other conditions such as the ability to manage brine concentrate from reverse osmosis.
While treatment can occur after withdrawal, addressing water quality issues in the recharge water is
often more effective. For example, for controlling dissolved organic carbon, trace organics, and
disinfection byproducts, laboratory studies with column experiments suggest that pre-ozonation of
WWTP effluent was better than ozonation after SAT. This is due to ability of ozonation to enhance
organic matter's biodegradability to facilitate greater removal through SAT (Echigo et al., 2015). A soil
column study also found that replacing chlorination before recharge with ozonation of tertiary-treated
effluent prior to SAT resulted in removals of bulk organic matter, and a wider variety of emerging
contaminants and pathogens (Trussell et al., 2015).
In addition to treatment, water quality concerns in recycled water can be addressed through measures
such as blending with surface water, as the concentrations of salts, nutrients, and organic compounds
are reduced through dilution. This strategy depends on the availability of sufficient volumes of water
with an acceptable quality for blending.
7.1.1 Preventing Clogging during Recharge
For systems using infiltration with SAT, the water may need treatment before recharge to reduce
suspended solids and thus prevent clogging. This can be accomplished by coagulation, settling ponds,
and sand filtration upstream of infiltration basins (Casanova et al., 2016; Hartog and Stuyfzand, 2017).
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Also, because microbial growth can cause clogging in the soil, water often needs to be disinfected and
potentially chemically treated to reduce organic matter (Hartog and Stuyfzand, 2017). Treatment with
sand filtration and UV disinfection prior to recharge has also been successful in reducing clogging and
addressing poor infiltration rates. In one study using treated municipal wastewater, adding a filtration
step has been found to improve the average infiltration rate in basins by 40 to 100 percent in one
project (Barry et al., 2017). As an alternative to treatment, the practice of alternating wetting and drying
cycles (e.g., increasing or decreasing the amount of water in a system) in SAT systems can preclude the
need for other forms of treatment by reducing clogging at the basin floor.
7.1.2 Considerations of Existing Aquifer Characteristics
Where the release of arsenic and other metals is a concern, it is desirable to match the quality of the
recycled water to that of the native groundwater as closely as possible based on available economic and
technical capacity. Such adjustments can include dissolved oxygen removal, adjustments to pH and ionic
composition, and control of the concentration of residual oxidants from treatment (e.g., hydrogen
peroxide, free chlorine, and chloramines). All of these factors affect the overall geochemical
environment, and the concentrations of dissolved oxygen and other oxidants affect the oxidation of
arsenic-bearing pyrite and release of arsenic, iron, and manganese. The effect of these oxidants on the
redox conditions depends on the concentration of organic matter in the sediments, which consumes
oxidants (Hokanson et al., 2020). Water quality degradation from the mobilization of arsenic, iron, and
manganese may also wane over repeated cycles as the pyrite becomes oxidized and depleted. This
concept has been explored experimentally (Antoniou et al., 2014).
If salinity needs to be reduced, wastewater can be treated with reverse osmosis or nanofiltration.
Reverse osmosis is highly effective at removing salts and metals, but stabilization with caustic soda or
lime is often needed to ensure the water does not corrode the conveyance infrastructure and is
compatible with the aquifer (EPA, 2017). Nanofiltration requires less energy than reverse osmosis, and
while it will remove less dissolved solids, allowing some monovalent ions such as calcium and potassium
to pass through, it will remove larger molecular weight compounds (EPA, 2017).
7.2 Treatment after Recovery
Water recovered from an aquifer may need treatment if its quality has deteriorated in the subsurface or
if disinfection byproducts form after extraction and treatment. As well, there may be a need to address
the mobilization of iron and manganese, due to the reductive dissolution of iron and manganese
(hydr)oxides. The iron and manganese may oxidize in the near-well environment and cause clogging. If
arsenic has been mobilized in the aquifer, blending or treatment will also be needed.
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8. Methods of Implementation:
Opportunities and Constraints
There are different ways to implement EAR and ASR, and the choice of which one to use depends on
factors such as local hydrology, hydrogeology, and ambient water quality (Dillon et al., 2019). This
section describes two primary methods, infiltration or injection: either may be suitable for recharging
aquifers with recycled wastewater, depending on the site-specific conditions.
Commonly used methods of infiltration include the following:
¦ Infiltration galleries—diversion of water into buried trenches dug in permeable soils for
percolation into an unconfined aquifer. An infiltration gallery can be considered a Class V well
under the UIC program if it is deeper than its widest surface dimension, or if it includes an
assemblage of perforated pipes, drain tiles, or other similar mechanisms intended to distribute
fluids below the ground surface (EPA, 2008).
¦ Infiltration ponds and spreading grounds—diversion of recharge water to off-channel areas not
connected to rivers or other waterbodies to allow for infiltration into an unconfined aquifer.
There are several other infiltration methods used for recharge with surface water diversions and other
sources of water that this report does not discuss in depth. Such methods are better suited for surface
water diversions largely because they are implemented in ephemeral streams or existing surface water
bodies where wastewater may not be conveniently available. However, it should be noted that in some
regions, such as the southwestern United States, there are streams where the flow consists primarily of
treated wastewater through de facto water reuse that could be considered as recharge (Rice et al.,
2013). Infiltration methods are suitable for a range of unconfined aquifers, from porous, high-
permeability aquifers to fine sands (Goren et al., 2014). These methods are generally inexpensive and
fairly simple to construct and maintain (Casanova et al., 2016; Goren et al., 2014), with designs that can
be adjusted depending on the project objectives and characteristics of the basin, source water, and
aquifer(s). Expected residence times are generally in the range of months to years (Miller, 2006), often
depending on aquifer properties and the rate of withdrawal (Goren et al., 2014). All infiltration methods
use SAT, which is further discussed in Section 6.2.
Additional infiltration methods include:
¦ Percolation tanks, check dams, or recharge weirs—dams built along ephemeral streams to
detain the water and give water more time to infiltrate into the riverbed.
¦ Bank filtration—groundwater extraction from a well near or beneath a lake or river, inducing
percolation from surface water. Drawing down the water table by pumping from a nearby well
can induce more water to flow through riverbanks and into a pumping well while helping to
replenish the aquifer.
¦ Recharge releases—dams on ephemeral streams used to discharge water to the downstream
streambed at rates that match the capacity for infiltration into underlying aquifers. This method
is typically focused on recharging aquifers when there is available volume.
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Injection methods include:
¦ Injection well—a bored, drilled, or driven shaft whose depth is greater than the largest surface
dimension, a dug hole whose depth is greater than the largest surface dimension, an improved
sinkhole, or a subsurface fluid distribution system (40 CFR 144.3).
¦ Dry well—an injection well, other than an improved sinkhole or subsurface fluid distribution
system, completed above the water table so that its bottom and sides are typically dry except
when receiving fluids (EPA, 2000).
8.1 Site Selection
The critical site selection factors for an EAR and ASR infiltration project include topographic,
geologic/hydrogeologic, environmental, and social considerations (e.g., Ahmadi et al., 2017)5.
Environmental considerations include soil infiltration capacity, water scarcity, source water availability,
and water quality issues. In addition, it is especially important to understand the "complete geologic
architecture of the subsurface layer" (e.g., fine vs. coarse soil layers and their extent and connectedness)
when siting an EAR or ASR project (Alam et al., 2021).
Examples of topographic and geologic/hydrogeologic factors include:
¦ Slope—for infiltration practices, the slope must be shallow enough to minimize runoff and allow
for infiltration.
¦ Depth to water table—if the system involves SAT, the vadose zone should be thick enough to
allow for pollutant removal.
¦ Soil type—if the system uses infiltration for SAT, soils should be sandy loam, loamy sand, or fine
sand soils that are permeable enough to allow high infiltration rates and also provide removal of
trace organics, nutrients, heavy metals, and pathogens.
¦ Groundwater quality—understanding the ambient and recharge water quality can help in
ensuring compatibility between the recharge water and the existing groundwater in the aquifer
or identifying any anticipated changes in groundwater quality.
Given the various factors involved in site selection, multiple criteria can be taken into consideration to
help with project siting. For example, Ahmadi et al. (2017) conducted spatial analysis with each criterion
represented by a thematic geographic information system (GIS) layer, which can then be integrated for
analysis. The resulting maps were found to be highly valuable for optimizing site selection. As an
example of a statewide approach, the Texas Water Development Board has produced an extensive
evaluation of site suitability for EAR and ASR in Texas, including a multiparameter scoring methodology
(HDR Engineering, 2020).
8.2 Clogging
Clogging with fine particulates, mineral precipitates, and/or biofilms is a frequent concern for both
injection- and infiltration-based EAR and ASR projects. The resulting reductions in porosity and
permeability pose challenges with maintaining injection rates or infiltration rates, especially if the
5 The UIC AR/ASR Report (in preparation) contains discussion on siting for aquifer recharge and ASR via injection
wells.
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recycled water contains a high concentration of suspended solids or is geochemically incompatible with
the subsurface solids and groundwater (Barry et al., 2017).
While existing soil characteristics cannot be changed, operational and treatment practices can be
optimized to prevent clogging. As discussed in Section 7.1.1, treatment of recycled water prior to
recharge can be designed to minimize the potential for clogging—for example, by reducing suspended
solids, minimizing the potential for bacterial growth, and adjusting water quality for compatibility with
the subsurface geochemical conditions (Casanova et al., 2016). Maintenance measures such as the
removal of fine sediments from the bottoms of infiltration basins and control of wet and dry periods are
also important to prevent clogging (Barry et al., 2017). More detailed discussions of clogging and other
challenges are provided in the Stormwater EAR Report (EPA, 2021) and the UIC AR/ASR Report (in
preparation).
Case Study; Montebello Forebay
Since 1962, groundwater replenishment has been conducted at the Montebello Forebay Spreading Grounds
in Los Angeles using diverted surface water, tertiary treated municipal wastewater, and stormwater.
Considered to be the first such project in the United States (National Research Council, 2008), this project
does not require the higher levels of engineered treatment seen in some groundwater injection projects
because of the use of SAT and allows multiple sources of water to be integrated into a single project. To
investigate the health effects of this project and reuse of treated wastewater for groundwater recharge, a
five-year epidemiological study was initiated in 1978. This study concluded that there were no measurable
adverse health effects on the surrounding community (National Research Council, 1994). More information
on this project is available in the 2012 EPA Water Reuse Guidelines (EPA, 2012).
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9. Regulatory Environment
Groundwater withdrawal in the United States is regulated by the states. As groundwater demand has
increased over time, more attention has been given to the potential for certain groundwater basins to
be over-pumped, leading to increased pumping costs, the need for deeper wells, land subsidence, and
seawater intrusion. In the United States, there are no national regulations specifically for water reuse
applications, either potable or non-potable. In lieu of national regulations for water reuse, states have
the authority to develop their own, and several have developed regulations for potable and non-potable
reuse. A complete compilation of state-level regulations for water reuse has been developed through
EPA's REUSExplorer, found at http://www.epa.gov/reusexplorer. Note that for projects using injection
wells, EPA or states with primary enforcement authority regulates the construction, operation,
permitting, and closure of injection wells through the UIC program.6
These regulations vary by state but are generally intended to address different chemical and microbial
contaminants through treatment requirements and standards that often go beyond existing regulations
under the Clean Water Act and the Safe Drinking Water Act. In all states, finished drinking water from a
groundwater source augmented by recycled water must, at a minimum, meet all applicable Safe
Drinking Water Act requirements. There are currently no specific state regulations on using industrial
wastewater, stormwater, agricultural return flows, and other recycled water sources in EAR or ASR
projects, though flows from these sources are often incorporated into wastewater treatment facility
influent.
This section summarizes several states' regulations on EAR and ASR using treated municipal wastewater.
This is not a comprehensive list: various other states (e.g., Oregon, Oklahoma, New Mexico) can also
permit projects on a case-by-case basis with treated municipal wastewater.
Groundwater Management in California
Because the states regulate groundwater management, including extraction and legal rights, it is difficult to
generalize in a national context. In California, the Sustainable Groundwater Management Act (SGMA) was
passed in 2014. SGMA requires local agencies to develop and implement plans to ensure sustainable
groundwater management where there is balance between withdrawal and recharge. This was done to
prevent further overdraft in many of the state's groundwater basins to help ensure that groundwater will
continue to be available, even in times of drought. These plans must be implemented, with sustainability
achieved, by 2042 (CA LAO, 2017). By using recycled water as a source for recharge, some aquifers can be
consistently replenished to help ensure that groundwater is managed sustainably.
9.1 California
For groundwater recharge with recycled water, California regulations include specific treatment
requirements for pathogens and chemicals, and projects must be reviewed and permitted on a site-
specific basis by the Regional Water Board (SWRCB, 2018). For pathogens, there are requirements of a
total of 12 logio enteric virus reduction, 10 logio Giardia lamblia cyst reduction, and 10 logio
6 More information on the regulation of injection wells can be found on EPA's Underground Injection Control
program website.
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Cryptosporidium oocyst reduction, where each log corresponds to removal or inactivation of 90 percent
of initial concentrations. These requirements apply to both groundwater recharge through surface
spreading as well as injection (California Code of Regulations, Title 22, §60320.108).
For the control of chemicals in injection projects, treatment must include reverse osmosis as well as
advanced oxidation to provide a minimum of 0.5 log reduction of 1,4-dioxane (California Code of
Regulations, Title 22, §60320.201). Finished water must meet a limit of 0.5 mg/L for TOC (California
Code of Regulations, Title 22, §60320.218) and is subject to monitoring requirements for various
compounds of emerging concern. However, for projects using surface spreading, only tertiary treatment
and disinfection is typically required, with SAT providing the additional necessary treatment for both
chemicals and pathogens. There is also a minimum retention time of two months in the aquifer.
9.2 Massachusetts
Massachusetts can permit the use of recycled water for aquifer recharge in a Zone II ("That area of an
aquifer which contributes water to a well under the most severe pumping and recharge conditions that
can be realistically anticipated [180 days of pumping at safe yield, with no recharge from precipitation]")
or an Interim Wellhead Protection Area ("a half mile radius from the well or well field for sources whose
approved pumping rate is 100,000 gallons per day or greater"). Zone I is an area closer to a public water
supply well and may not be replenished with recycled water. There are no treatment specifications, but
special conditions are included in permits on a case-by-case basis (Code of Massachusetts Regulations,
Title 314, §5.00 and §20.00).
9.3 Nevada
Nevada has regulations for groundwater recharge with recycled water that are similar to California's.
While Nevada has the same pathogen removal requirements as California, it does not have the same
treatment requirements for chemicals and TOC. The primary difference is that Nevada does not require
the use of reverse osmosis for injection projects (Nevada Administrative Code, §445A.276), largely
because there is no ability to use an ocean outfall to discharge brine concentrate into. Not requiring the
use of reverse osmosis allows alternative treatment options that may better fit the needs of inland
communities.
9.4 Pennsylvania
Pennsylvania has different requirements for groundwater recharge for both surface spreading and direct
injection (Pennsylvania DEP, 2012). In contrast to some other states that use numerical targets for
pathogen and chemical control, Pennsylvania has prescriptive treatment requirements with additional
treatment required for injection compared to surface spreading. More specifically, much as in California,
reverse osmosis is required for direct injection, but not for surface spreading. In addition, direct
injection requires a residence time in the aquifer of 12 months compared to 9 months for surface
spreading.
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A common legal consideration for EAR and ASR projects using recycled water is water rights. Each state has
its own system for determining who has the legal right to surface water and how much they have the right to
use. In general, states in the East (e.g., east of the 100th meridian) apply the riparian doctrine, which
generally means that water belongs to the person whose land borders a body of water; this person may
make reasonable use of that water as long as it does not unreasonably interfere with reasonable use by
others with rights to use of this water (National Agricultural Law Center, 2022). In the West (e.g., west of the
100th meridian), the prior appropriation doctrine is generally used. This doctrine can be summarized by the
phrase "first in time, first in right," meaning that the first person to use or divert water for beneficial use can
obtain rights to that water (National Agricultural Law Center, 2022). Some states, such as California and
Oklahoma, have adopted a hybrid of the riparian and prior appropriation doctrines (National Agricultural
Law Center, 2022). While this simplistically defines the issue, the nuances could affect a community's ability
to use recycled water for EAR or ASR and may require consultation with water law experts in each state.
At the Hawks Prairie Ponds in Lacey, Washington, reclaimed water flows through several vjetland ponds before entering
groundwater recharge basins.
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10. Costs and Benefits
As has been described, EAR and ASR practices vary widely on source of water, end-use, and the site-
specific conditions and geology of an aquifer. Even so, there are some general themes regarding costs
and benefits for different practices. For example, recharge of unconfined aquifers using infiltration
basins and minimally treated water (e.g., relying only on SAT for additional treatment) will cost less than
recharge through injection using water that requires higher levels of engineered treatment (Ross and
Hasnain, 2018). The details of these costs are driven by different factors including the cost of different
unit treatment processes before recharge and the cost of conveyance (including injection). The benefits
largely correspond the drivers mentioned above (see Section 3) and can be evaluated and quantified
using a triple-bottom-line (TBL) framework or similar approach. There are also additional cost
considerations from land acquisition for infrastructure, including the need for land to be used for
infiltration basins.
10.1 Cost of Treatment
The cost of treatment in an EAR or ASR system using recycled water will depend on many factors
including the source of water, its quality, and the treatment standards and requirements based on the
applicable regulations. For example, an EAR or ASR system using captured stormwater as a source will
likely have very different treatment requirements than a system using treated municipal wastewater.
These additional treatment requirements for municipal wastewater are likely to result in greater costs
compared to systems using other sources of water.
For the cost of treatment for municipal wastewater used for EAR and ASR to supplement drinking water
supplies via indirect potable reuse, site-specific information may be available that can be used to
identify some key factors. For example, in 2014 treatment for the Orange County Water District's
(OCWD's) Groundwater Replenishment System (GWRS) was estimated to cost about $700 per acre-foot
(Raucher and Tchobanoglous, 2014). Estimates were based on the actual operating expenses of the
GWRS treatment train, which includes reverse osmosis—an energy-intensive process. This treatment
approach is required in California for projects with municipal wastewater as the source for groundwater
injection.
With projects using different treatment approaches than reverse osmosis, the cost of treatment can be
substantially different, in large part due to differences in energy demand. Evaluations of the costs of
different treatment options for potable reuse have shown that treatment trains using ozonation and
biofiltration are less costly than reverse osmosis-based treatment (Plumlee et al., 2014; Funk et al.,
2019). This is in large part due to the high energy demand of reverse osmosis. The cost may be even
lower for facilities using spreading grounds that take advantage of SAT, but the cost of operating and
maintaining spreading grounds would need to be taken into account.
10.2 Cost of Conveyance
Once recharge water is treated to a suitable quality, it must be conveyed to the point of recharge. The
cost of this conveyance is site-specific and depends on the distance and elevation between the point of
treatment and recharge (Raucher and Tchobanoglous, 2014). Ideally, the two sites would be co-located,
which is feasible for some sources of water that do not require much engineered treatment. For
Water Recycling for Climate Resilience in Enhanced Aquifer Recharge and Aquifer Storage and Recovery | 27
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municipal wastewater, though, the treatment facility may be several miles from the point of recharge.
For example, OCWD's GWRS is about 13 miles from groundwater infiltration basins, resulting in
conveyance costs of $120 per acre-foot as of 2014 (Raucher and Tchobanoglous, 2014).
Case Study; Hampton Roads Sanitation District
As part of its Sustainable Water Initiative for Tomorrow (SWIFT), the Hampton Roads Sanitation District
(HRSD) plans to inject treated effluent from seven of its wastewater treatment plants into the Potomac
Aquifer, which is the primary source of groundwater in eastern Virginia (Lang and Kane, 2017). Unlike in
other communities, the main drivers for this project are improving surface water quality, saltwater intrusion,
and the need to address groundwater overdrafts in Virginia's Coastal Plain (Green Nylen, 2021). As a
discharger to the Chesapeake Bay, HRSD must meet strict nutrient limits for its effluent. While HRSD was
able to meet those effluent discharge limits, there were concerns that regulators might impose more
stringent ones in the future.
Once SWIFT is fully implemented, HRSD is expected to recharge a total of 100 MGD from five wastewater
treatment plants (Green Nylen, 2021). HRSD currently has a research facility operating at its Nansemond
Wastewater Treatment plant, which treats and injects 1 MGD. The projected cost of the full-scale SWIFT
implementation is $1.1 billion in capital costs, with annual operating costs estimated at between $21 million
and $43 million peryear (Green Nylen, 2021). HRSD expects that full implementation will bring a 90 percent
reduction in nutrient load from each of the SWIFT-equipped treatment plants, generating enough nutrient
credits to enable 11 of the municipalities within the HRSD service area to avoid spending $2 billion on
otherwise-necessary improvements. The primary beneficiaries of these credits are HRSD ratepayers that are
funding the SWIFT project. Thus, no charge is to be assessed to them. In addition, HRSD plans to explore
opportunities to sell credits at market price to other interested parties, thereby obtaining another source of
revenue to offset costs (Green Nylen, 2021). This approach is expected to generate millions of pounds of
nutrient pollution credits annually through the Chesapeake Bay Watershed Nutrient Credit Exchange
Program, while also replenishing scarce groundwater resources, staving off saltwater intrusion, and
counteracting land subsidence (Green Nylen, 2021).
10.3 Quantifying Benefits
While the direct costs of EAR and ASR can largely be quantified through economic and financial analyses,
estimating the complete suite of benefits is more difficult. Such benefits are diverse. They include
providing additional water supply and water security for communities, industries, and agriculture;
providing a reserve water supply for emergency use; and improving groundwater quality (Zheng et al.,
2021). For a scenario in which the main benefit is additional water supply, the benefit can be estimated
by looking at the volume of water recovered or supplied and multiplying it by the cost of the supply, or
by comparing it to the alternative cost of production (Zheng et al., 2021).
Costs and benefits are often evaluated in tandem on a life-cycle basis using a variety of economic,
environmental, and social factors through a TBL framework. This type of evaluation is often complex and
includes a variety of potential costs and benefits to consider (see Table 1). The precise metrics used to
evaluate projects vary based on the details of the project, as well as the desired outcome for the
community. How a community views the TBL factors for a proposed project will depend on site-specific
factors including the project goals. Several tools have been created for the evaluation of alternate water
reuse and water supply projects that use various approaches, scoring criteria, and frameworks such as
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multi-criteria decision analysis (Hadjikakou et al., 2019; Piper, 2014). This topic is being explored further
in WRAP Action 7.7. Life Cycle Analysis to Support Cost-Effective Enhanced Aquifer Recharge.
Table 1. Examples of Triple-Bottom-Line Costs and Benefits for a Water Supply Project
Economic
Environmental
Social
Lifecycle cost
Carbon footprint
Impact on public health
Local and regional job
creation
Impact on nutrient discharges
Public acceptance
Amount of water
produced
Impact on other pollutant
discharges
Implementation risk
Avoided costs due to
inaction
Residuals production and
disposal
Impacts on recreation
Drought and climate resilience
An important concept to consider when evaluating the climate resilience benefits of EAR and ASR
projects is the benefits of action, compared to the costs of inaction. While there are upfront costs
associated with any drought mitigation project, including EAR or ASR with recycled water, the costs are
often smaller than costs associated with the impacts of drought. For example, the 2011 drought in Texas
resulted in an estimated $7.6 billion in agricultural losses throughout the state, including livestock and
crop production losses (Texas Comptroller of Public Accounts, 2014). Meanwhile, the Millennium
Drought in Australia that officially ended in 2012 resulted in $AUD 4.5 billion in government emergency
assistance expenditures from 2001 to 2008 (Productivity Commission, 2009). This figure just includes the
amount in emergency aid that the Australian government provided and is not a true reflection of the
broader economic impact that occurred as a result of the drought. While investments in water reuse
through EAR and ASR will not eliminate the economic impact of drought, such projects can be helpful to
mitigate the impacts. It is important that water managers consider not just the direct costs and benefits
of a water supply project, but the avoided costs of climate-change-induced drought.
The Hampton Roads Sanitation District's Sustainable Water Initiative for Tomorrow will replenish the Potomac Aquifer to
improve water quality in the Chesapeake Bay and mitigate land subsidence.
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11. Summary and Conclusions
As the impacts of climate change continue to drive drought, it is more critical than ever for communities
to look toward alternative sources of water. Overreliance on unsustainable groundwater extraction is
only a temporary solution to drought, and recharge with recycled water in appropriate locations can
help restore and protect local groundwater supplies.
Implementing an EAR or ASR project with recycled water can seem daunting with all the different
factors that must be taken into consideration, as each aquifer and potential project location is unique in
its native water quality, geochemical characteristics, and other conditions. Site-specific conditions have
a direct influence on project siting, design, and operations that need to be developed to ensure public
health protection and compliance with applicable state and federal regulations. Regulations vary from
state to state, but water quality goals can be met with a combination of engineered treatment and
natural processes such as SAT. Once site-specific issues and treatment requirements are understood,
many communities can design and implement EAR and ASR projects with recycled water to meet
growing demands for water while also protecting vital groundwater supplies.
There have been successful projects in the United States implementing EAR or ASR with recycled water
to enhance local water supplies and become more resilient to climate change impacts. This report
serves as an introductory guide to lead toward increased understanding of using recycled water for EAR
and ASR, where appropriate, as part of a more resilient and integrated water resource management
strategy. Communities can build off the successes of existing projects and use the information and
resources identified in this report to help make these projects a reality while protecting public health
and the environment.
For more information about water reuse, please contact us:
United States Environmental Protection Agency
Water Reuse Program
Office of Water, Office of Science and Technology
1200 Pennsylvania Avenue, NW (Mailcode 4101M)
Washington, DC 20460
waterreuse@epa.gov
https://www.epa.gov/waterreuse
Office of Water | February 2023 | EPA Publication 832R23001
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Photo Credits
Cover: U.S. Environmental Protection Agency, Washington State Department of Ecology; Pg. 6: West Basin
Municipal Water District; Pg. 14: Monterey One Water; Pg. 26: Washington State Department of Ecology; Pg. 29:
Hampton Roads Sanitation District
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