As California continues to face increasingly unpredictable weather patterns, water agencies across the state are seeking smarter, more resilient ways to manage our most precious resource. Here in Sonoma County, one innovative solution is gaining momentum: aquifer storage and recovery (ASR).
What is aquifer storage and recovery?
Aquifer storage and recovery, commonly known as ASR — or sometimes referred to as groundwater banking — is a forward-thinking water management approach that utilizes excess winter flows. Read more about what aquifers are and how groundwater works here.
Here’s how it works: During rainy seasons, high-quality drinking water from the Russian River is delivered through Sonoma Water’s aqueduct system and injected into deep underground aquifers through specially designed ASR wells. This water can then be safely stored for months or even years and pumped back up when it’s needed most — whether during a summer drought, multi-year dry conditions, or a natural disaster.
Unlike traditional reservoirs, which store water aboveground, ASR taps into natural underground storage — making it an environmentally sound and cost-effective alternative.
Why ASR Matters in Sonoma County
Water reliability is more important than ever. Droughts, climate change, and the threat of natural disasters all place strain on our water systems. ASR offers a smart, sustainable strategy to meet those challenges head-on. Here are just a few of the benefits:
- Improved water security: ASR allows us to save surplus water when it’s plentiful and access it when it’s scarce — especially important during hot, dry summers.
- Cost-effective infrastructure: It reduces the need for major new construction projects like dams or surface reservoirs.
- Efficient use of existing systems: ASR integrates with existing infrastructure, offering phased testing and lower implementation costs.
- Boosts groundwater levels: ASR can help stabilize and even improve groundwater conditions in areas facing long-term declines.
- Supports local ecosystems: By reducing the need to draw from rivers and lakes in dry seasons, ASR helps protect important surface water habitats, like those in the Russian River watershed.
Compared to aboveground storage, ASR has a smaller environmental footprint and makes better use of the infrastructure we already have.
ASR in action: proven success in Sonoma County
Sonoma County has been exploring ASR for over a decade, and early results have been promising.
- In 2013, a regional Groundwater Banking Feasibility Study was completed. This study looked at water availability, aquifer conditions, and technical methods for storing and recovering water. It showed that Russian River water — with its excellent quality and low mineral content — is ideal for long-term underground storage.
- In 2018, Sonoma Water partnered with the City of Sonoma to conduct the region’s first ASR pilot project, injecting and successfully recovering 3.5 million gallons of potable water. The project not only boosted local water reliability, but also helped raise groundwater levels — with no negative impacts to nearby wells.
What’s next for ASR?
Sonoma County isn’t stopping with a single pilot. Building on the groundwork from the 2013 feasibility study, we’re now moving forward with updated planning and regional coordination — supported by funding from the California Department of Water Resources’ Sustainable Groundwater Management (SGMA) Implementation Grant in partnerships with local partners.
This effort has expanded with recently completed and in progress ASR planning and projects across all three groundwater basins in collaboration with Valley of the Moon Water District in the Sonoma Valley Subbasin, the city of Petaluma in the Petaluma Valley Basin, and with Sonoma Water, the town of Windsor and city of Santa Rosa in the Santa Rosa Plain Subbasin. These projects will help incorporate new data and insights into existing models and strategies and help advance toward goals outlined in our local Groundwater Sustainability Plans.
What ASR work is going on in the Sonoma Valley subbasin? With funding from the Urban and Multibenefit Drought Relief Grant Program grant funded by the California Department of Water Resources, the Valley of the Moon Water District (VOMWD) is initiating two ASR projects: the Park Well Replacement and ASR Project and the Verano Well ASR Conversion Project. VOMWD completed successful ASR pilot tests in 2024 which injected, stored and recovered approximately 26 acre feet of treated drinking water at these sites. These two sites will be under construction in July 2025 and be completed in December 2025. These projects will enhance the district’s ability to store water during wet years and recover it during dry years, strengthening long-term water security for the community.
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