The City of Pomona owns and operates a public water utility that serves more than 30,000 connections with a combined water supply from multiple groundwater basins, a local surface water source, imported water purchased from Three Valleys Municipal Water District, recycled water, and non-potable water. In the city’s 2023 Annual Water Quality Report, it was reported that hexavalent chromium (Chrome 6) concentrations in treated water ranged from non-detect to 7.4 parts per billion (ppb), but the raw groundwater supply exceeded ND-46 ppb. Considered a Contaminant of Emerging Concern, long-term exposure to Chrome 6 may result in liver toxicity, gastrointestinal tumors, and liver cancer. As such, the State Water Resources Control Board (SWRCB) has set a 10-ppb maximum contaminant level (MCL) for Chrome 6, requiring systems to test source waters, conduct ongoing monitoring, and implement a strategy that reduces concentration levels to meet regulatory requirements.
With approximately 73 percent of the city’s overall demand satisfied by the groundwater supply, it made the most sense for Pomona to develop a treatment solution for this source rather than alternative options, such as blending supplies. The existing treatment system for the groundwater supplies included air stripping, granulated activated carbon (GAC), and anion exchange for removal of Volatile Organic Compounds (VOCs), nitrate, and perchlorate. Woodard & Curran was hired to review the advantages and disadvantages of treatment alternatives, including ion exchange (IX), reduction/coagulation/filtration (RCF), and reverse osmosis (RO). The analysis also considered how these best available technologies would treat for nitrate, total dissolved solids (TDS), and per- and polyfluoroalkyl substances (PFAS). Each treatment alternative would also include potential siting for treatment facilities and conceptual level evaluation of pipeline alignments conveying groundwater to the new treatment system.
Groundwater supplies are drawn from the Chino Basin, Spadra Basin, and Six Basins, a group of adjacent basins consisting of Canyon, Upper Claremont Heights, Lower Claremont Heights, Live Oak, Ganesha, and Pomona Basins. The city provided Woodard & Curran water quality data from existing treatment facilities for review to identify what wells were impacted by Chrome 6 and PFAS. While it was determined that the Six Basins and Spadra Basin region wells do not required additional treatment, ongoing monitoring for emerging contaminants is recommended. However, PFAS and Chrome 6 were both detected in wells that supply the city’s Anion Exchange Plant (AEP) and Reservoir 5, located within the Chino and Pomona basins, requiring further evaluation of treatment options.
Since Chrome 6 concentrations broadly impact several wells, a decentralized treatment approach was ruled out by city staff due to overall cost and complexity of implementation. In a collaborative workshop that reviewed findings of the systems analysis with Pomona, the city agreed that the preferred alternative for preliminary design evaluation for Chrome 6 removal two centralized treatment systems that utilize existing infrastructure at the AEP. Furthermore, the city decided that IX, using strong base anion exchange resin, would be the best option compared to other best available technology largely for its smaller footprint, lower treatment complexity, effective treatment of Chrome 6, and city staff’s experience with the treatment process at the AEP. The project team also recommended implementing a new system at Reservoir 5 that leverages IX, as well as replacing the air strippers with GAC pressure vessels with a capacity of 1,700 gallons per minute (gpm) for VOC removal, and additional storage for waste brine and salt. This approach provides the added benefit of removing PFAS if concentrations rise in the groundwater supplying the site.
The final preliminary design report provided presented the client with a comprehensive assessment of treatment and conveyance alternatives, highlighting the advantages and disadvantages. Pomona was actively involved throughout the analysis to ensure Woodard & Curran incorporated strategies that leveraged the knowledge and experience of their operations staff. The Preliminary Design Report sets the stage for Pomona to make informed decisions about how to best treat available water resources to ensure regulatory compliance, as well as public health and safety. The report serves as a basis for detailed design that would allow the city to implement recommendations within two years from the effective date of California’s Chrome 6 regulation.