Groundwater banking

What Is Groundwater Banking?

Groundwater banking is a water management strategy that involves intentionally storing surplus surface water underground in aquifers for later retrieval and use. This practice is a crucial component of modern water resource management, falling under the broader category of environmental economics. It aims to enhance water supply reliability, especially in regions prone to drought or facing increasing water scarcity due to climate variability and growing demand⁴⁴. By using natural underground geologic formations as storage reservoirs, groundwater banking allows for the accumulation of water during periods of abundant rainfall or low demand, which can then be extracted when surface water supplies are limited or demand is high. This method helps to balance the fluctuating availability of water resources over time and contributes to long-term water security.

History and Origin

The concept of storing water underground is not new, but organized groundwater banking programs gained significant traction in arid and semi-arid regions facing chronic water shortages. A landmark moment in the formalization of groundwater banking occurred in Arizona. The state's legislature established the Underground Water Storage and Recovery Program in 1986, which allowed entities to store and recover water underground⁴³. This was followed by the Underground Water Storage, Savings, and Replenishment Act in 1994, which streamlined recharge projects and encouraged the use of renewable water supplies over direct groundwater pumping. These legislative efforts paved the way for the creation of the Arizona Water Banking Authority (AWBA) in 1996, an agency specifically tasked with banking surplus Colorado River water underground for future use⁴⁰, ⁴¹, ⁴². This initiative served as an early and influential model for other regions, including California, which later adopted its own comprehensive groundwater management legislation³⁹.

Key Takeaways

• Groundwater banking involves storing excess surface water in underground aquifers for future use.
• It serves as a critical strategy to improve water supply reliability, particularly in drought-prone areas.
• The practice helps manage water resources by mitigating the impacts of fluctuating surface water availability.
• Groundwater banking programs typically utilize accounting systems to track deposits and withdrawals of stored water.
• Benefits include reduced evaporation losses compared to surface reservoirs, flood control, and potential improvements in water quality.

Formula and Calculation

While groundwater banking does not involve a single financial formula like an investment portfolio, it relies heavily on a robust accounting system to track the volume of water deposited into and withdrawn from an aquifer. This system is essential for managing water rights and ensuring the long-term viability of the bank.

The change in stored water volume (( \Delta S )) can be conceptually represented as:

Where:

• ( \Delta S ) = Change in stored water volume over a period
• ( R_{direct} ) = Volume of water added through direct recharge basin infiltration or injection wells
• ( R_{in-lieu} ) = Volume of water "banked" by using surface water instead of pumping groundwater (in-lieu recharge)
• ( I_{external} ) = Volume of water imported or otherwise acquired from external sources for banking
• ( W_{recovery} ) = Volume of water actively withdrawn from storage (recovered)
• ( L_{leakage} ) = Volume of water lost due to natural underground leakage out of the managed basin
• ( L_{evaporation} ) = Volume of water lost to evaporation (primarily during surface spreading for direct recharge)
• ( W_{other} ) = Other unaccounted withdrawals or losses, such as groundwater pumping by other users in the basin

Effective groundwater banking requires meticulous tracking of these inflows and outflows to ensure that the volume of water credited to depositors matches the physically recoverable volume, maintaining the bank's solvency.

Interpreting Groundwater Banking

Interpreting groundwater banking involves understanding its role as a strategic water management tool within a broader hydrological and economic context. The effectiveness of groundwater banking is not just measured by the volume of water stored, but by its contribution to regional water supply security and its ability to mitigate the impacts of climate change. A successful groundwater banking operation means that during wet years, surplus surface water is captured and effectively moved into underground aquifers, preventing it from being lost to runoff or evaporation³⁸. Conversely, in dry periods, the stored water can be reliably extracted to meet demand, preventing over-extraction of native groundwater and reducing the risk of consequences such as land subsidence or deterioration of water quality³⁷. The operational efficiency, the balance between deposits and withdrawals, and the long-term sustainability of the underlying aquifer are key metrics for evaluation³⁶.

Hypothetical Example

Consider an agricultural region, AgriValley, that experiences significant seasonal variations in rainfall, leading to periods of both floods and droughts. Historically, AgriValley relied solely on annual snowmelt and direct river diversions for irrigation, supplemented by heavy groundwater pumping during dry spells, which led to declining groundwater levels.

To address this, AgriValley establishes a groundwater banking program. During a particularly wet winter, when rivers are overflowing, instead of letting the excess water run off, AgriValley diverts a portion of it to several large recharge basins and uses injection wells in designated areas. This action allows 50,000 acre-feet of water to slowly percolate into the underlying aquifer, effectively "depositing" it into the groundwater bank. Farmers are also encouraged to use surface water for irrigation when available, thereby "in-lieu" banking groundwater by reducing their pumping, allowing the aquifer to naturally replenish.

The following summer brings an unexpected drought, and surface water supplies are critically low. AgriValley then "withdraws" 20,000 acre-feet from its groundwater bank by activating recovery wells. This banked water supplements the reduced surface water, ensuring that agricultural production can continue without critically depleting the natural groundwater reserves. The remaining 30,000 acre-feet remain stored for future dry years, demonstrating the bank's role as a resilient water supply buffer.

Practical Applications

Groundwater banking is increasingly applied in regions worldwide to enhance water supply reliability and address the challenges of climate change. One significant application is in California, where numerous groundwater banking programs, such as the Semitropic Water Storage District and the Kern Water Bank, store surplus water during wet periods for use in dry years³³, ³⁴, ³⁵. These programs play a vital role in mitigating the impacts of prolonged droughts and managing the state's complex water rights system³¹, ³².

Beyond direct supply augmentation, groundwater banking contributes to flood control by diverting and storing excess stormwaters that would otherwise cause damage²⁹, ³⁰. It can also improve regional water quality by facilitating the recharge of cleaner surface water into depleted aquifers, potentially diluting contaminants or preventing saltwater intrusion in coastal areas²⁷, ²⁸. Furthermore, the underground storage minimizes evaporation losses, a significant advantage over surface reservoirs, making it a more efficient long-term storage solution in many climates²⁵, ²⁶. This strategic approach to water resource management is critical for both urban and agricultural sectors, providing a crucial buffer against hydrological variability. The Public Policy Institute of California highlights the importance of groundwater recharge as a water management practice that can restore groundwater levels and store water for later use²⁴.

Limitations and Criticisms

Despite its numerous benefits, groundwater banking faces several limitations and criticisms. A primary concern is the potential for water quality degradation if the recharged surface water contains contaminants, or if the act of recharge mobilizes existing legacy pollutants within the aquifer²³. This necessitates careful monitoring and often requires pretreatment of water intended for banking.

Another significant challenge is the potential for land subsidence if the aquifer is over-pumped and the "banked" water is not fully recoverable or if the natural groundwater is depleted faster than it is replenished²¹, ²². This can cause damage to infrastructure. Technical issues related to aquifer response to managed recharge and recovery, such as changes in groundwater flow patterns or less-than-one-to-one recovery rates, can also occur.

Economically, while potentially cost-effective in the long run, the initial capital investment for infrastructure like recharge basins and injection wells, as well as ongoing operation and maintenance costs, can be substantial²⁰. Moreover, the legal and regulatory frameworks surrounding water rights and groundwater ownership can be complex, leading to disputes over the allocation and withdrawal of banked water¹⁸, ¹⁹. For instance, in some areas like Arizona, pumping groundwater remains largely unregulated outside of specific management zones, leading to concerns about unsustainable extraction even with banking efforts¹⁷.

Groundwater Banking vs. Managed Aquifer Recharge

The terms "groundwater banking" and "managed aquifer recharge" (MAR) are closely related but are not interchangeable. Managed aquifer recharge is a broader term that encompasses various techniques for purposefully adding water to aquifers for subsequent recovery or environmental benefit¹⁵, ¹⁶. These techniques include direct infiltration methods, such as spreading basins or stream-bed modification, and injection methods, which use injection wells to directly introduce water into deeper aquifers¹³, ¹⁴.

Groundwater banking is a specific application or type of MAR. It typically refers to MAR projects that involve an active accounting system, where specific entities (e.g., water districts, agricultural users) deposit water during times of surplus and earn "credits" that allow them to withdraw an equivalent volume of water at a later time, much like a financial bank¹². The key distinction lies in the formalized, often transactional, framework of deposits and withdrawals, and the explicit intention of storing water for future recovery and trading of water credits¹¹. MAR, on the other hand, can also be implemented for broader environmental benefits, such as preventing saltwater intrusion, restoring environmental flows, or simply raising groundwater levels for regional benefit without a direct "banking" mechanism or transferable credits⁹, ¹⁰.

FAQs

Q: Why is groundwater banking important?
A: Groundwater banking is important because it helps secure water supply in regions facing variable rainfall and increasing demand, allowing surplus water from wet periods to be stored underground for use during droughts. It provides a resilient buffer against climate variability⁸.

Q: How does water get into the underground "bank"?
A: Water typically enters the underground "bank" through two primary methods: direct recharge, where water is spread in basins or injected via injection wells to percolate into the aquifer, and in-lieu recharge, where surface water is used instead of pumping groundwater, effectively leaving the groundwater in storage⁶, ⁷.

Q: Is groundwater banking always a one-to-one exchange (deposit equals withdrawal)?
A: Not always. While the goal is a one-to-one exchange, various factors such as natural leakage, evaporation during surface spreading, and other nearby groundwater pumping can mean that the recoverable volume is less than the deposited volume. Effective accounting systems aim to minimize these discrepancies⁵.

Q: What are the main benefits of storing water underground compared to surface reservoirs?
A: Key benefits include reduced evaporation losses (as water is stored underground), less environmental impact compared to building new dams, potential water quality improvements as water percolates through natural filtration, and the ability to combine with flood control efforts³, ⁴.

Q: Does groundwater banking affect land?
A: Yes, if not managed properly, excessive groundwater pumping without adequate recharge can lead to land subsidence, where the ground level sinks. Conversely, effective groundwater banking helps to stabilize groundwater levels and can mitigate this issue¹, ².