Groundwater storage and recovery becoming increasingly important
By Ken Hugo
A recent study conducted by researchers from the University of Victoria, the University of Calgary and other universities has raised the issue of worldwide over-pumping and scarcity of groundwater. “The global volume and distribution of modern groundwater” clarifies earlier rough estimations of the worldwide volume of groundwater and confirms its finite nature.
Conversely, much effort is being made in parts of the world, such as southern Alberta, for flood mitigation. Storage dams and diversions are planned for times of too much water. Alternating droughts with occasional floods illustrates one of the problems with water — it is not always available when needed.
Groundwater storage and recovery
This is one reason we build dams. A lesser-seen but equally valid procedure is to collect and store water from abundant precipitation into aquifers and then pump it out in times of need. This procedure is called aquifer storage and recovery (ASR), and is common in Africa and becoming established in parts of the U.S. ASR is less common in Canada, but there is certainly no reason it should not be undertaken here.
Historical researchers have pointed out that ASR commonly occurs under natural conditions. Forests, wetlands, beaver dams and even gopher holes allowed for greater natural recharge of groundwater. With agricultural and industrial development, these landscape features were removed, leading to dwindling groundwater recharge and sharper surface water events.
The principle of ASR involves collecting water at times of higher precipitation or high river flow into surface water impoundments, and allowing it to percolate into the aquifer. Sometimes, a confining layer such as a clay till is at the surface and water will not percolate at a sufficient rate. Stripping the upper confining layer, if not too thick, is one option. Injecting water into deeper aquifers through wells is another method which provides great flexibility when aquifers are at deeper depths. The use of wells also minimizes site disturbance.
An ideal situation is to use the recharge areas for both flood control and aquifer storage, a practice often undertaken in parts of Africa that experience short rainfall seasons and long hot and dry periods. These regular seasonal events allow for easier planning and rationalization of an ASR project, rather than contingencies for a project to deal with 100-year flood events.
An ASR well field has only a moderate surface disturbance. This is highly advantageous when compared to a surface water impoundment which can have significant surface disturbance and environmental disruption. Not being subject to evaporative losses is another advantage of ASR water storage systems. Other advantages include prevention of land settlement due to over pumping, and lessening of salt water intrusion into coastal aquifers.
Introducing surface water bacteria into aquifers is a concern often raised with ASR. Some water treatment may be required; however, surface water bacteria are accustomed to light and oxygen and have a hard time surviving in the cool, dark, low oxygen environment of the aquifer. Experience has shown that surface water bacteria are not a significant problem with ASR. They are largely predated on by natural aquifer bacteria.
However, injected water does contain oxygen and organic carbon and naturally occurring bacteria may take advantage of these constituents. Some of these bacteria may form encrustations of a polysaccharide film on the well screen, or in the aquifer around the well, which can lead to well bore plugging.
Some elements in the aquifer rock, such as arsenic, can become mobile under changing Eh and pH conditions introduced by the injected water. Issues with arsenic mobilization (and other metals) are a concern in ASR schemes.
Aquifer storage and recovery systems are highly engineered. Some technical issues to consider are:
- Surface water hydrology studies to determine timing and volumes of water available for ASR;
- Geological investigations to determine the extent and capability of the aquifers and the presence or absence of upper or lower permeability confining layers;
- Engineering of well, pumping and delivery systems;
- Testing of the water quality, on both an initial and ongoing basis;
- Treatment of waters prior to injection or upon withdrawal;
- Maintenance of conveyance, treatment and well systems;
- Monitoring of conditions in the aquifer.
Along with these technical issues, both regulatory and community ones should not be overlooked. Although largely out of sight, ASR schemes are not exempt from stakeholder involvement and similar issues arise as those seen in surface water schemes.
The National Ground Water Association has a 22-page free download entitled “Best Suggested Practices of Aquifer Storage and Recovery” which covers these issues. As with all water facilities, a team of specialists is required for successful construction and operation.
Ken Hugo is with Groundwater Information Technologies Ltd. This article appears in ES&E Magazine’s August 2016 issue.