In the heart of California’s Tri-Valley region, water agencies face the dual challenge of prolonged drought and emerging contaminants. For the Zone 7 Water Agency, the presence of PFHxS, one of the more toxic per- and polyfluoroalkyl substances (PFAS), in its groundwater wells threatened to sideline its most productive water source at a critical time.
A recent article in Informed Infrastructure titled “Ion-Exchange PFAS Removal Expands in Northern California” highlights how Carollo supported Zone 7 in designing and delivering Northern California’s first ion-exchange (IX) PFAS treatment facility. The article details the urgent challenges posed by rising PFAS concentrations in drinking water, and how fast, flexible engineering solutions helped keep critical water supplies online during historic drought conditions.
California Addresses PFAS in a Time of Drought
In 2022, as California entered its third consecutive year of extreme drought, Zone 7 faced a difficult decision: shut down its highest-producing groundwater well or quickly implement a treatment system to meet new state guidelines for PFHxS, a toxic PFAS compound. Located in Pleasanton, the Stoneridge Well tested above the state’s response levels, putting Zone 7 at risk of noncompliance just as surface water supplies from the State Water Project were reaching historic lows.
Why Ion-Exchange Was the Right Fit to Fight PFAS
Working with Carollo Engineers, Zone 7 selected ion-exchange treatment as the most practical option for PFAS removal. Compared to granular activated carbon and reverse osmosis, IX systems require less space and energy, both critical considerations for a site like Stoneridge, which is surrounded by urban development and had little room for expansion.
From Stoneridge to a Regional PFAS Strategy
The Stoneridge facility began operation in September 2023 after a rapid design and construction process. Its success laid the groundwork for a second facility at Zone 7’s Chain of Lakes wellfield, which launched in March 2025. Carollo also supported this project, continuing its work to standardize IX systems across the agency’s network while navigating local soil conditions and evolving treatment needs.
A third facility is now in development at the Mocho wellfield, where an existing RO system may be used as part of the treatment process to reduce overall project costs.
Meeting National PFAS Regulations with Local Solutions
With the EPA’s 2024 PFAS regulations requiring public water systems to monitor and respond to contamination by 2031, utilities across the country are looking to early adopters like Zone 7 for guidance. The Stoneridge and Chain of Lakes projects show how tailored engineering and proactive planning can deliver effective PFAS treatment under tight time and space constraints.
Read the full article in Informed Infrastructure to explore the project’s technical details and broader implications.