The method
What Is Aquifer Storage & Recovery?
Aquifer Storage and Recovery (ASR) is the practice of capturing surplus surface water — typically storm and flood flows — treating it to drinking-water standards, and injecting it into an underground aquifer for later recovery during drought. Because the aquifer replaces a surface reservoir, there is no evaporation loss and no dam. Delivered cost is typically $200-800 per acre-foot, against $1800-2500 per acre-foot for seawater desalination. Over 200 ASR systems already operate in the United States, with a track record spanning more than 40 years.
How it works, in four steps
01
Capture the surplus
Take water only when there is genuinely too much of it — storm flows and flood peaks that would otherwise run to the sea. Nothing is diverted during normal or low flow, which is what keeps downstream and ecological rights intact.
02
Treat to drinking-water standard
Water is treated before it goes underground, not after. That protects the aquifer itself from becoming contaminated, and it is the step regulators care most about.
03
Inject into the aquifer
The treated water is injected into the same formation communities already pump from. The aquifer becomes the reservoir — no evaporation loss, no dam, no surface footprint.
04
Recover it in drought
When surface supply fails, the banked water is pumped back out through existing well infrastructure. Storage built in wet years is what carries a region through dry ones.
What does ASR cost per acre-foot?
Cost per acre-foot is the only figure that lets you compare water supply options directly. On that basis ASR is the cheapest new supply available to most regions — the alternatives are not close.
| Option | Cost per acre-foot | Note |
|---|---|---|
| Aquifer Storage & Recovery | $200-800 | No evaporation loss; uses existing wells |
| Seawater desalination | $1800-2500 | ~$100M capex per MGD; $4.00/1,000 gal |
| Long-distance water imports | $800/yr | $5M per mile; political feasibility low |
| Continued groundwater mining | $100 | Cheapest today, 10× long-run multiplier as levels fall |
Groundwater mining looks cheapest per acre-foot precisely because it does not price the depletion it causes. That is the accounting problem this whole analysis exists to correct.
Does ASR actually work? Projects already operating
ASR is not speculative technology. These are US systems already in service, with public capital costs and delivery records.
Orange County GWRS
- Capacity
- 130 MGD
- Investment
- $900M
- People served
- 1.0M
- Demand met
- 39%
- Cost per AF
- $850-1100
- Operating since
- 2008
San Antonio SAWS ASR
- Storage
- 120000-140000 AF
- Investment
- $186M
- Operating since
- 2004
Tucson Water ASR
- Storage credits
- 100,000 AF
- Investment
- $100M
Where is there enough stormwater to make it work?
ASR only helps where surplus flow actually exists to capture. Of the regions analysed, 7 rate as excellent on available stormwater versus the deficit they need to close.
| Region | Stormwater available | ASR potential | Net deficit | Deficit offset |
|---|---|---|---|---|
| Ogallala | 8.91 MAF | 6.24 MAF | 1.65 MAF | 378% |
| Central Valley | 1.02 MAF | 0.72 MAF | 2.77 MAF | 26% |
| Basin & Range | 10.75 MAF | 7.53 MAF | -0.39 MAF | — |
| Edwards | 0.22 MAF | 0.16 MAF | 0.14 MAF | 109% |
| Mississippi Embayment | 3.94 MAF | 2.76 MAF | 0.51 MAF | 542% |
| Columbia Plateau | 2.97 MAF | 2.08 MAF | 0.35 MAF | 594% |
| Great Salt Lake | 1.79 MAF | 1.25 MAF | 0.13 MAF | 985% |
| Denver Basin | 0.36 MAF | 0.25 MAF | 0.05 MAF | 472% |
On the figures
Per-acre-foot costs and operating project figures on this page are drawn from published project records and cost benchmarks. Stormwater availability and deficit offsets are modelled. National programme totals are stated on the investment page and are under revision — treat the per-acre-foot comparison above as the reliable basis for judging ASR against alternatives. Method on the science page.