Aquifer storage recovery
ASR - a technique for storing water in natural aquifers -- is being used throughout the US, with recent schemes using high quality raw water and tertiary treated wastewater. Across the Atlantic, a number of UK water companies are now looking at ASR as an option to meet water supply demand balances. Dave Smith, Terry Foreman, Kevin Bral and David Pyne, all of CH2M Hill, report.
ASR in the US
In Monterrey, California, tertiary treated effluent is being injected into a saltwater intruded coastal sand aquifer for agricultural irrigation. Historic use of the underlying aquifer helped create a thriving vegetable farming community worth millions of dollars growing crops such as artichokes. But dependence on groundwater has resulted in over-abstraction. Widespread saline intrusion now means boreholes in the upper aquifer can no longer be used and quality in the deeper formations is deteriorating.
High quality wastewater is available from Monterrey County, but this potential reuse water requires storage to meet seasonal use. Direct use of this supplementary supply is hampered because storing the treated wastewater in a large surface reservoir is not only expensive, but siting is also politically sensitive.
The solution was to use a large scale 95Ml/d ASR scheme to store the wastewater, but also inject it into the most saline aquifer to assist control of salinisation.
San Antonio, Texas, has relied on the Edwards limestone aquifer since records began. The aquifer has been able to meet peak demands in excess of 950Ml/d. But this level of abstraction has had adverse environmental effects on spring flow.
Consequently the city is being asked to limit abstraction from this aquifer. However, the alternative supply south of the city from the Carrizzo aquifer is not suitable for direct abstraction without treatment.
ASR is being considered as a serious option with a scheme size up to 95Ml/d using the Carrizzo aquifer, but with winter injection of high quality water to displace the native groundwater so that further treatment is not required.
Large-scale use of ASR using untreated raw surface water is also being investigated in Florida with the development of potentially up to 300 ASR wells, each delivering between 18-38Ml/d. If it proceeds, this ASR scheme is designed to store surplus water during the wet season and then re-abstract it during the summer months to supplement the supply for environmental preservation of the Everglades as well as other uses in south Florida such as agricultural irrigation and urban water supply.
ASR in England
ASR in the UK is still in the early stages of development. Finding the right aquifer is not necessarily the prime factor in identifying a suitable location. Factors such as available winter water, spare treatment capacity, and the ability to demonstrate that ASR will have a negligible impact on the surface water environment are equally important.
In the last two years, six UK water companies have undertaken preliminary investigations into ASR technology, locating suitable sites and suitable aquifers. Wessex Water has just completed an 18-month long demonstration trial with the guidance of CH2M Hill's staff, injecting and recovering potable water into a deep confined chalk aquifer using a retrofitted borehole at rates up to 3.5Ml/d.
A feature of this aquifer system is the dual porosity of the rock and the potential for diffusion between the matrix water and fissures water. The field test, supported by numerical modelling confirmed that ASR in the chalk aquifer at this site was feasible although blending of recovered water with a treated water supply is required until the storage zone has been fully conditioned.
Two more UK water companies have completed feasibility studies and are preparing to start demonstration trials in the early part of this year. One will take place in the Sherwood Sandstone aquifer in the north of England for Yorkshire Water and the second will take place in the Lower Greensand aquifer in southern England for Mid Kent Water.
Both schemes will have to overcome the challenges of controlling reduced iron that occurs naturally in the aquifer. If successful, the Sherwood Sandstone scheme would offer significant potential for the development of large scale strategic ASR schemes to not only meet seasonal needs but also maintain distribution system pressure, supply and emergency storage. A successful scheme in the Lower Greensand would help meet local shortfalls during times of peak demand.