Policy Brief 2010
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ater
Aquifer Storage and Recovery
What is aquifer storage and recovery?
Aquifer storage and recovery (ASR) is the injection
of treated or untreated ground or surface water
(source water) into an aquifer (an underground layer
of soil, sand, gravel or rock that can yield water to a
well or spring), and recovery of that water through
the same well when it is needed. Injection typically
occurs when excess source water is available, such
as in winter or spring, while recovery usually occurs
during summer peak demand or emergency periods.
ASR may be used for a variety of purposes, including
supplementing municipal, industrial, agricultural or
recreational water supply, long-term water storage, or
restoration of groundwater levels.
What happens to the water that is injected?
It flows outward from the injection well into the
aquifer, and displaces the groundwater that is already
there. This creates a “bubble” of injected water
around the well, and a “mixing zone” around the
bubble, where the injected water mixes with the
existing groundwater. When the injected water is
needed, it is pumped back out, and the bubble
shrinks. Repeated “cycles” of injection and recovery
cause the mixing zone to expand, usually improving
water quality of the existing groundwater with
successive cycles.
Can ASR systems be installed anywhere?
No. ASR systems require suitable source water,
and an aquifer with the right characteristics to store
the injected water. Sand or limestone aquifers with
an impermeable layer above and below to prevent
the injected water from leaking out and existing
groundwater leaking in provide the best possibilities
for ASR. Aquifers with lots of large openings or
caverns where groundwater is moving too quickly
are undesirable, because much of the injected water
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would move too far away from the injection well, and
reduce recovery efficiency. Also, ASR wells are not
good in areas where the receiving aquifer has other
nearby injection wells or wells for water withdrawal,
because that causes confusion over whose water is
being withdrawn. And perhaps most importantly, a
cost-effective ASR system needs a nearby customer
base of sufficient size. The feasibility of ASR wells is
very location specific.
Are there places in Georgia where ASR systems
could work?
Yes. In Georgia, the best places for ASR are in the
south-central and coastal areas where the aquifers
are deep, confined above and below, and not too
cavernous. In northwest and southwest Georgia, the
main limestone aquifers are too cavernous and leaky
to hold the injected water in a bubble, but deeper
aquifers in the southwest should be able to do so.
In the Piedmont areas of north Georgia, the deep
bedrock is not porous enough and would not let the
bubble form properly, making ASR almost impossible.
Can the injected water affect the aquifer or existing
groundwater?
Yes. The injected water may have a significantly
different chemical composition, even after treatment.
During injection, the injected water may react with
chemical components of the existing groundwater
and the naturally-occurring minerals in the aquifer.
This may change the pH of the existing groundwater,
release iron or salts from the aquifer minerals, or
release other naturally-occurring chemicals from the
aquifer such as arsenic or mercury.
Are ASR systems currently operating in the United
States?
Yes. ASR technology has been used for more than 40
years. ASR systems currently store drinking water in
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27 sites in Florida and South Carolina. Many
systems also operate in 13 other states across the
nation. At least nine additional states have studies
and pilot projects underway. ASR systems are also
used to supply water in other counties, such as in
Australia, Kuwait, the Netherlands, and Israel. In
Florida and South Carolina, operating ASR systems
are storing water in the same aquifer that we use for
groundwater withdrawals in south Georgia, the Upper
Floridan aquifer.
states generally regulate ASR systems through the
underground injection control provisions of the
Safe Drinking Water Act. The Act strictly prohibits
the injection of substances that would endanger
underground sources of drinking water. The federal
underground injection control rules, which mostly
focus on the injection of waste materials, specify
the minimum requirements and allow states to have
stricter policies. Under the federal rules, ASR wells
are regulated as “Class V (5)” or shallow injection
How much water can be stored and recovered using
ASR?
Storage capacity depends in large part on the
characteristics of the receiving aquifer, and the cost
of treating the injected water. Typical systems inject
between 8 and 10 million gallons per day (mgd), and
recover most of that when needed. For example, in
Cocoa, Florida, the ASR system there withdraws 8
mgd during the peak demand season from its storage
300 feet below ground.
Why isn’t all the injected water recovered?
Sometimes it is. However, the “recovery efficiency,”
that is, the ratio of the amount of injected water to the
amount of recovered water that meets the required
water quality standards, is strongly affected by the
quality of the existing groundwater. As soon as the
recovered water drops below the required standards,
it means that native groundwater is being drawn into
the pump, and the withdrawal stops. The recovery
efficiency may be as high as 80 percent, or as low as 20
percent or less.
What challenges do ASR systems face?
ASR systems are subject to the same challenges as
other groundwater injection or withdrawal systems
(e.g. residential wells). Sometimes, the well or the
area directly around it in the aquifer may become
clogged with sand and grit, or bacteria and algae,
that require removal or treatment. On occasion,
unexpected chemical contaminants show up in the
recovered water due to unanticipated minerals in
the aquifer. ASR systems may affect nearby wells
and groundwater levels by increasing or decreasing
hydraulic pressure in the aquifer. Disinfection
byproducts, which are tightly regulated by federal and
state agencies, may be present in the injection water.
And if the injection treatment system fails, there is a
danger of contaminating the aquifer.
How are ASR systems regulated?
Depending on the location, source of injected
water, and use of recovered water, ASR wells
may be regulated differently. The (USEPA) and
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Illustration of an ASR well. From “What is ASR?” http://www.asrforum.com/
wells. As of July 2006, USEPA reports more than
6800 Class V injection wells in Georgia – none are
ASR wells. Because ASR wells do not typically inject
waste materials, some states have chosen to adopt
regulatory provisions specifically for these wells. ASR
wells may also be subject to requirements under state
groundwater protection programs.
The Georgia Environmental Protection Division
requires owners and operators of Class V injection
wells to obtain a permit before construction. GAEPD
rules and the permit include specific requirements
for construction, operation (monitoring, testing,
reporting, and maintenance of mechanical integrity),
and closure. Class V wells must be installed by
a licensed water well contractor, subject to the
requirements of the Water Well Standards Act. In
Georgia, withdrawals from ASR wells would likely
be subject to groundwater withdrawal permitting
requirements. If the ASR system is part of a public
water supply/drinking water system, it would require
approval as part of the system permit.
How are ASR systems developed?
Development of an ASR system requires a series of
carefully planned steps, typically over two to three
years.
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•
First, the feasibility of an ASR system must be
determined, taking into account site selection,
water demand and supply, projected costs of
construction and operation, community and
political concerns, and other relevant matters.
•
Investigations to determine the hydrology,
mineralogy and geochemistry of the receiving
aquifer usually begin by drilling test wells,
recovering samples of the aquifer, and
performing pumping tests on the aquifer.
Groundwater flow models tell hydrologists
the maximum amount of water the aquifer can
receive, the anticipated size of the bubble, and
the expected recovery efficiency.
•
A pilot injection well is then drilled, and
treated water is injected into the aquifer.
Several cycles of injection and recovery
are performed, and scientists evaluate
the amount and quality of the recovered
water and resulting changes in the aquifer.
Any significant water quality or aquifer
performance problems should appear
during this initial testing phase. If the pilot
well meets all technical and regulatory
requirements, approval may be sought for a
fully operational system. It should be noted
that project development may be stopped at
any phase if unresolvable issues arise.
What are some non-technical challenges related to
ASR systems?
Because ASR systems store water, owners and
operators must ensure they have the right to use the
source water for ASR, to manage stored water (most
importantly to exclude others from withdrawing it)
and to withdraw and use recovered water. These
owners/operators must also consider impacts on
existing nearby wells, such as changes in water
levels and pressure during injection and withdrawal.
Many ASR systems are part of public water supply
systems. Therefore, the use of such systems is
among the alternatives available for water storage,
and like other options (such as elevated storage
tanks and reservoirs) they must be cost-effective or
the lowest-cost alternative considering all capital,
operational, and maintenance costs. Social concerns
also play an important role in discussions about ASR
systems. Citizens may express concerns due to the
perceived high costs associated with ASR technology
and/or fears of potential aquifer or drinking water
contamination or water quality reduction.
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What are some benefits of ASR systems?
The benefits of ASR systems vary by project, since
the projects are so location specific. ASR systems
providing monitored underground storage of potable
(drinking) water are not subject to the evaporative
losses, sedimentation, high costs, extensive land
use and environmental impacts associated with
surface water reservoirs. ASR systems can provide
secure long-term storage or emergency supplies.
ASR systems may also be used to maximize the
opportunities for water reuse, allowing injection of
highly treated re-use water during the times when
demand for such water is low. (Lack of storage
capacity is a major deterrent to construction of direct
re-use systems). In areas where coastal salt-water
intrusion is a threat, ASR could be used to restore
groundwater levels or improve water quality, without
increasing the risk of greater salt-water intrusion. In
its 1999 report, The Class V Underground Injection
Control Study, the USEPA stated, “no contamination
incidents associated with the operation of aquifer
recharge or ASR wells have been reported.”
Additionally, the Water Science and Technology
Board of the National Research Council/National
Academies of Science acknowledged, “Collective
experience with MUS [managed underground
storage] systems is substantial. A significant number
of these systems are decades old, and experience
indicates that many of them perform consistently
and well over the longer term.” See Prospects for
Managed Underground Storage of Recoverable Water (The
National Academies, 2008).
Sources:
Ronald Reese, 2002, Inventory and Review of Aquifer
Storage and Recovery in Southern Florida, USGS Water
Resources Investigation Report 2002-4036
David G. Pyne, 1995, Groundwater Recharge Through Wells:
A Gude to Aquifer Storage Recovery