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 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 Santa Rosa Plain subbasin? With funding from the Sustainable Groundwater Management Implementation grant funded by the California Department of Water Resources, the Santa Rosa Plain GSA is partnering with the City of Santa Rosa on an ASR pilot project at an existing well on Martha Way on the eastside of Santa Rosa. The Martha Way Well is located near the intersection of Martha Way and Albert Lane, adjacent to the Santa Rosa Southeast Greenway. This location was selected because the existing well was not being actively used, the site has favorable subsurface conditions for recharge, and its proximity to a Sonoma Water Aqueduct. You can learn more about that project HERE.

Another pilot ASR project in the basin is planned for summer 2025 at Sonoma Water’s Sebastopol Road Well. This project is funded through the California Department of Water Resources’ Urban and Multibenefit Drought Relief Program. The test will take place in a confined aquifer of the Wilson Grove Formation. This project will recharge, store, and recover up to 50 acre-feet (16 million gallons) of treated drinking water. The source water is from Sonoma Water’s Russian Riverbank filtration facility. This project is projected to improve groundwater levels, reduce seasonal groundwater level fluctuations, and not impact nearby private wells.

Have you ever wondered what goes on beneath a river? Rivers, lakes and streams – all different types of “surface water” are often interconnected to the underlying groundwater aquifer. This system is known as Interconnected Surface Water (ISW) – and it supports a special type of habitat known as Groundwater Dependent Ecosystems (GDEs). What a mouthful, right? These ecosystems support plant and animal species that live in and along waterways and wetlands where groundwater provides critical water, especially where surface water can be seasonally scarce. Many rare and endangered species in California rely on these types of ecosystems. The Santa Rosa Plain GSA has identified data gaps in our understanding of how surface water conditions and water use activities affect GDE’s.

Using grant funds from the Department of Water Resources (DWR), in 2024 and 2025 the GSA conducted ISW and GDE studies to strategically fill these data gaps and better  understand how we can manage our groundwater resources to avoid significant depletion of surface water and maintain the vitality of groundwater dependent ecosystems.

To evaluate ISW, the GSA conducted seepage runs, compared stream flow measurements with groundwater levels, and piloted temperature sensors as a proxy measurement where groundwater data is unavailable. The ISW analysis indicates varying degrees of surface water interconnection throughout the basin. This study’s findings will help the GSA further refine the ISW sustainable management criteria and help comply with DWR guidance.

The GDE analysis focused on reassessing GDE acreage and health in Santa Rosa Plain. The GSA had initially estimated GDE acreage when developing the Groundwater Sustainability Plan (GSP), by comparing the Sonoma Vegetation Digital Elevation Model (DEM) with 2015 groundwater levels. This analysis was updated using groundwater levels from 2023. The updated GDE analysis estimated that Santa Rosa Plain has 888 more acres of GDE than previously understood, with a total of 5,969 acres of freshwater marsh, oak woodland, riparian forest, wetlands, and vegetation dependent on groundwater. To estimate the health of these areas, the study compared their normalized difference vegetation indexes (NDVI). NDVI is essentially how green vegetation is, which is an indicator of health or stress. Results showed that 75% of GDEs Santa Rosa Plain are moderately healthy or very healthy, but 30% are showing declining trends from 2015-2024.

These studies greatly improved the GSA’s understanding of ISW and GDEs in Santa Rosa Plain. These results alone do not provide enough information to make policy decisions or establish protective metrics, but give further direction to fill remaining data gaps. Through these collaborative efforts and focused scientific studies, we are working towards long-term sustainability and health of our groundwater ecosystem.

To learn more, read the final report here.