Aquifer Storage Recovery (ASR) is the storage of water in a well during times when water is available, and recovery of the water from the same well during times when it is needed. ASR provides a cost-effective solution to many of the world's water management needs, storing water during times of flood or when water quality is good, and recovering it later during emergencies or times of water shortage, or when water quality from the source may be poor. Large water volumes are stored deep underground, reducing or eliminating the need to construct large and expensive surface reservoirs. In many cases, the storage zones are aquifers that have experienced long term declines in water levels due to heavy pumping to meet increasing urban and agricultural water needs. Groundwater levels can then be restored if adequate water is recharged.
The main driving force behind the current rapid implementation of ASR technology around the world is water supply economics. ASR systems can usually meet water management needs at less than half the capital cost of other water supply alternatives. When compared to alternatives requiring construction of water treatment plants and surface reservoirs to meet increasing peak demands, potential cost savings have been known to exceed 90 percent. A second important driving force has been the increased recognition of this technology as being good for the environment, aquatic and terrestrial ecosystems. By reducing or eliminating the need for construction of dams, and by providing reliable water supplies through diversions of flood flows instead of low flows, ASR systems are usually considered to be environmentally friendly.
Storage zones range in depth from as shallow as about 75 m (200 ft) to as deep as 900 m (2,700 ft). Groundwater levels in the storage zones range from as much as 10 m (30 ft) above land surface to more than 300 m (900 ft) below land surface (bls). Natural water quality in the storage zone ranges from fresh, suitable for drinking without treatment, to brackish, including total dissolved solids (TDS) concentrations up to about 5,000 mg/l. Most sites have one or more natural water quality constituents that are unsuitable for direct potable use except following treatment. Such constituents may include iron, manganese, fluoride, hydrogen sulfide, sulfate, chloride, radium (224/226 /228), gross alpha radioactivity, and other elements that are typically displaced by the stored water as the bubble is formed underground. At two sites, one of which is currently in operation, ASR was shown to be feasible and highly cost-effective storing drinking water in an aquifer containing seawater. For most of these sites, it is first necessary to properly develop the storage zone around the well, after which it is possible to recover the same volume as that stored. At a few, more challenging sites water quality, hydraulic or geochemical constraints may limit recovery to somewhat less than the volume stored.
Water is stored deep underground in water-bearing geologic formations, or "aquifers" that may be in sand, clayey sand, sandstone, gravel, limestone, dolomite, glacial drift, basalt and other types of geologic settings. Stored water displaces the water naturally present in the aquifer, creating a very large bubble around the well. The bubble is usually confined by overlying and underlying geologic formations that do not produce water, however at several sites the aquifer is unconfined. Storage volumes in these bubbles range from as small as about 50 Ml (13 million gallons) in individual ASR wells, to as much as 10,000 Ml (2.5 BG) or more in large ASR wellfields.