What You Need to Know About Aquifer Storage and Recovery (ASR) Wells

If you are in the business of treating, storing and furnishing drinking water, you will want to be familiar with the concept of aquifer storage and recovery (ASR) wells to determine whether your utility should consider their use.  So, what are they and why should a utility consider an ASR well?

The concept is simple; an underground aquifer stores treated drinking water until it can be returned to the distribution system for use. Think of an ASR well as a huge ground storage reservoir. In a normal water system, it is typical to see ground and elevated storage volumes equal to the average day or perhaps peak day demands. This storage provides a means for a water utility to meet peak hourly demand rates while source and treatment facilities run at total daily demand rates. 

The storage can also act as an emergency supply should the ability to produce water be lost for a short period of time (1-2 days). With typical storage capacity, a utility must ensure source and treatment capacity equal to peak day demands so that storage can be replenished daily as needed.

The concept of storage in an ASR well is to provide enough storage so that, theoretically, the utility’s source and storage capacity could be sized for the annual average day demand. When demand is below average, excess production is stored in the ASR well. When demand is above average, the stored water is recovered and pumped back to the distribution system. Thus, the name – aquifer storage and recovery well.

North Liberty ASR well control building

When does it make sense? The principal reason to consider an ASR well is when the utility’s ability to meet peak demands is challenged due to lack of capacity of the existing source and/or treatment facilities.  But why not just build or acquire more capacity? That is a valid option. The possible advantage of an ASR well is that the cost of capacity equivalent to the ASR well may be significantly less than the cost of new source and treatment capacity.

In the capacity range of 1.5 to 3 million gallons per day (MGD) the capital costs of an ASR well in Iowa using the Jordan Aquifer can be expected to be less than $2 per gallon-per-day of capacity.  A treatment plant of similar capacity, assuming that you are providing softening or similar levels of water treatment, could be expected to cost $4 or more per gallon per day.  So capital cost savings may be a reason to consider use of an ASR well.

What about operating costs? They factor into life-cycle costs of a capital investment decision for a utility and the variable unit cost of using an ASR well will likely be higher than the unit cost of treatment alone; you still have to treat water stored in the well and add the cost of the power used to inject the water into the well and pump it out later. There will also be some chemical costs for re-chlorinating the stored water and possibly for readjusting pH or adding back phosphates for stabilization. Finally, there is the cost of labor associated with operating and maintaining the ASR well and chemical feed system(s), and the sampling and testing needed to verify compliance of the recovered water to quality standards and goals.

This means that in considering the use of an ASR well, the savings in capital investment for additional source and treatment capacity will have to be weighed against the added operating costs of the ASR well.

What is involved in building an ASR well in Iowa?  For many locations in Iowa, the use of the Jordan Aquifer may offer the best solution for construction of an ASR well. This is because the aquifer is consistently present with known depth and thickness, is not known to contain materials that are likely to cause quality problems when used for storage, has water movement slow enough such that water injected is not likely to move away before it is reclaimed, and it has storage capacity sufficient for most needs. There are, no doubt, other aquifers that could serve for ASR wells, but the seven ASR wells currently in use in Iowa for municipal utilities have all been developed in the Jordan.

Once the target aquifer is identified and the desired storage capacity, and injection and withdrawal rates are determined, the procedure is very similar to the process for construction of any other water supply well.  The most significant difference is that a permit is required from both the Iowa Department of Natural Resources and from the EPA because of the injection aspects of the well’s operations. 

In addition, there are special design aspects:  

  • providing for proper injection of treated water into the well in a manner that does not include entrainment of air in the injected water
  • accounting for the possibility of material accumulating in the wells during injection periods; this may need to be flushed to waste prior to injecting water back into the distribution system
  • designing chemical feed systems to account for the gradual changes in the necessary feed rates due to changing water quality as the proportions of injected water to native water changes during withdrawal

There is always some mixing of the injected water and the native waters which is why it is best to utilize a receiving aquifer that has reasonably good native water quality.

Still interested? Want to know more?  FOX Engineering has provided design, permitting, and construction administration for three of the seven currently operating ASR wells in Iowa. These wells are in Ankeny, North Liberty, and Grimes.  The remaining four include a second well in Ankeny and three in Des Moines. Ankeny’s was the first ASR well in Iowa and developed by the Des Moines Water Works from an existing city well as a research project. The other three were designed and developed by the Des Moines Water Works for their own use.

Back-up generator at North Liberty ASR site

We would be happy to meet with you to discuss ASR wells in more detail and offer a professional opinion regarding the feasibility of such a well for your situation.  If initial discussions appear promising, we could assist you with the in-depth study and preliminary design necessary for you to make a fully informed decision prior to proceeding with actual implementation.

FOX Engineering is an environmental engineering firm based in Ames, Iowa. We specialize in water and wastewater solutions for our diverse municipal and industrial clients. Our work varies in size and scope and can be found throughout the Midwest and beyond.