Aquifer Storage and Recovery (ASR) and Shallow Aquifer Recharge (SAR): tools to supplement water supply

ASR and SAR both increase existing groundwater supplies by artificially recharging groundwater.  With ASR, the water is eventually withdrawn for other uses; with SAR, it is not. Both are important tools for helping with water supply, an increasing issue for our state. The availability of water for new uses is very limited throughout Washington, especially during the summer months when water use is high and water is naturally most limited.  ASR and SAR can help increase the availability of water during the summer months by capturing and storing water other times of the year when streamflows are higher and water demands are reduced.

Groundwater is critically important in Washington.  More than 3.5 million people drink groundwater, and it is used to irrigate over 320,000 acres each year.  Groundwater is also used for livestock, aquaculture, mining and industrial purposes.  Total groundwater use in our state is approaching 1.5 billion gallons per day. 

Declining groundwater levels in many areas of our state indicate that water is being withdrawn faster than it can be naturally recharged.  Snowpack, one of nature’s primary ways of storing water for future use, is diminishing.  This may decrease natural recharge and reduce the natural discharge of cool groundwater to streams.  A growing population along with a warming climate will only increase water demand while further decreasing recharge through development.  All of these factors make it critical to find ways to capture and store water when it is available (winter and spring) so it can be used later in the year (summer and fall) when surface water is limited.

All water in the world moves continuously in a closed-loop system.  No new water is being made.  Water from snow and rain becomes surface water (such as rivers and lakes), seeps into the ground (groundwater), which in turn feeds back into surface water.  Water circulates from land to sky and back again.  This is called the water cycle.  Because groundwater is a critical component of this cycle, adding groundwater through artificial recharge can result in significant benefits in certain cases.

Groundwater Recharge Methods

The diagram below illustrates some of the methods used for artificial groundwater recharge.  Water beneath the Earth’s surface occurs in two principal zones, the saturated and unsaturated (or vadose) zones.  ASR recharges water using wells that open to an aquifer below the water table (at far right).  This area is also called the saturated zone, where porous spaces in soil or rock are full of water.  SAR recharges water near the surface into the vadose zone, an unsaturated zone (where porous spaces contain air) above the water table.  The water table is the highest underground level at which the rocks and soil in a particular area are completely wet with water.  The water table fluctuates seasonally from year to year in response to changes in recharge from precipitation and surface-water bodies. 

  Graphic of Groundwater Recharge Methods  
  Groundwater Recharge Methods
Modified from: Brand, C., An ASR Primer, 2008, Southwest Hydrology, Vol. 7, No. 3.
(02/11/2015 the link is no longer available from the source -

Aquifer Storage and Recovery (ASR)

ASR is the process of injecting water into an aquifer, where it is stored for use at a later time.  It is being used throughout the world.  The number of projects in Washington is growing, varying widely in size and scale.  ASR has proven to be a cost-effective way to capture and store water when it is available so it can be used during times when it is limited.

The traditional approach has been to store water in reservoirs.  Groundwater storage can serve the same purposes as surface water reservoirs, without many of the issues and costs related to dams.  It can be used in places where surface storage is not practical or economically possible, including urban, industrial and residential areas.

Some other recognized benefits of ASR:

ASR Site Assessment Decision Tree: Is this a suitable ASR site?

This flow chart is a modified version from Woody, 2007 and illustrates the primary factors that should be considered when evaluating the feasibility of an ASR project:

ASR Application Process

Ecology can accept a single application form covering both a proposed reservoir and a proposed secondary permit or permits for use of water from that reservoir.  Reservoir permits can be used for ASR projects.

Below are the basic steps in applying for a permit for an ASR project (refer to Chapter 173-157 WAC for more details).

  1. Prior to applying, assess potential issues and impacts to the hydrogeologic system and the environment.  If the general setting and conditions cannot be described in sufficient detail for the application, then a more detailed feasibility study must be performed.  The feasibility study should reduce uncertainty with respect to project issues and impacts, as well as better quantify the available storage within the aquifer.
  2. Schedule a pre-application meeting with Ecology to discuss the project plan and likely requirements for monitoring and mitigation.  The pre-application web page has the link to the application form.
  3. Submit an application for an ASR project that contains at a minimum:
    1. Water rights for the source waters for the proposed ASR project.
    2. A general description of the physical design of the hydrogeologic system, prepared by an engineer or geologist registered in the state of Washington.
    3. A general description of the operational design of the hydrogeologic system, prepared by an engineer or geologist registered in the state of Washington.
    4. A project operation plan.
    5. A data monitoring plan.
    6. An environmental assessment and analysis of any potential adverse conditions or potential impacts to the surrounding environment that might result from the project.  A mitigation plan must be included to address any such conditions or impacts.

Water Quality Issues

Ecology’s Water Resources (water quantity) and Water Quality Programs together work on groundwater recharge projects.  They have developed a process to evaluate and develop the appropriate permits.

Water to be stored in an aquifer as part of an ASR project must meet water quality standards for groundwaters of the state of Washington (Chapter 173-200 WAC).  Additionally, injection wells for an ASR project must be registered with Ecology in accordance with Chapter 90.48 RCW (Water Pollution Control Act) and Chapter 173-218 WAC (Underground Injection Control Program).  For more information on the Underground Injection Control program, please see the UIC Web site.

Legal foundation for ASR projects

The state expanded its legal definition of “reservoir” in 2000 to include "any naturally occurring underground geological formation where water is collected and stored for subsequent use as part of an underground artificial storage and recovery project", (RCW 90.03.370).  This legislation allows Ecology to issue reservoir permits to authorize ASR projects.

In early 2003, Ecology adopted the ASR rule, Chapter 173-157 WAC - Underground Artificial Storage and Recovery.  It established standards for review of ASR proposals, and standards for mitigation of any adverse impacts in the following areas:

Shallow Aquifer Recharge (SAR)

SAR shares elements with ASR, but is not intended for storage and subsequent recovery.  Its goal is to supplement the natural pattern of recharging groundwater, which typically comes from rain and snow seeping into the ground.  (More on the water cycle.)  Because of the connection between surface water and groundwater, SAR can, for example, be used to help improve the amount of water in streams and rejuvenate wetlands and springs.  SAR can be an important tool for stabilizing or reversing declining groundwater levels.  It is also used as mitigation for other water withdrawals.

SAR projects release water on or near the surface above the local water table.  Recharge is done, typically on a seasonal basis, by diverting surface water into infiltration sites.  Sites can be natural or man-made features such as fields, gravel pits, ponds, canals, ditches and shallow wells.

Recharged water is usually not actively recovered by wells at the infiltration site but rather, is added to natural groundwater and allowed to flow wherever it goes.  A reservoir permit is usually not needed for SAR projects because water is not stored and actively recovered.  A water right authorizing the diversion of surface water prior to recharge would be needed.


SAR projects can be expensive, but are frequently cost-effective compared to alternatives.  Costs include acquiring or obtaining access to recharge sites, site characterization, construction, monitoring, water quality testing, operation and maintenance, reporting and permitting.  Using existing facilities and infrastructure, such as an existing irrigation canal with a willing landowner, can significantly reduce costs.

Sites must be chosen carefully, and evaluated for potential infiltration rates and volumes as well as anticipated hydrologic and water quality effects resulting from the project.  Suitable sites have permeable material at the surface and a water-table deep enough to allow levels to rise without causing problems such as flooding.

Project objectives also impact costs.  For example, an SAR objective commonly includes increasing flows in a nearby stream at the low flow time, 2-4 months after water is available for recharge.  This could significantly limit suitable sites, affecting project costs.

  Groundwater Recharge Projects in Washington State
(The map is divided up by Ecology regional offices and the counties they serve: Southwest, Northwest, Central and Eastern.)
Map of Groundwater Recharge Projects

Related Links


Barbara Anderson
Department of Ecology
Lacey WA
Voice: (360) 407-6639