Upgrades at a Historic Reclamation Facility

24 Aug

For more than six decades, the Whittier Narrows Water Reclamation Plant (WNWRP) has played a vital role in Southern California’s water sustainability efforts. Since opening in 1962 as the nation’s first large-scale water reclamation facility, the plant has served as a benchmark for the beneficial reuse of treated wastewater. Today, Hensel Phelps is helping secure the facility’s future through a comprehensive modernization of its influent and treatment-support infrastructure.

The $48.5 million Whittier Narrows Water Reclamation Plant Influent Pump Station Replacement Project represents a significant investment in the San Gabriel Valley’s water infrastructure. The project includes the replacement of aging systems with modern influent pumping facilities, upgraded electrical and control systems, new buried utilities, seismic enhancements, and improved site access. It also incorporates innovative Cement Deep Soil Mixing (CDSM) shoring techniques designed to protect groundwater resources and nearby active infrastructure during construction.

Beyond its technical achievements, the project demonstrates Hensel Phelps’ collaborative approach to delivering complex water and wastewater infrastructure projects. By combining innovative construction methods with proactive owner engagement, the team is modernizing one of California’s most historically significant reclamation facilities while supporting the region’s long-term water reliability goals.

Modernizing Essential Treatment Infrastructure

Awarded in May 2025 under a design-bid-build contract, the project is scheduled for completion in September 2027. Located in South El Monte, California, the modernization effort includes:

Construction of a new influent pump station
New wet wells and dry wells
Upgraded electrical and control facilities
Hydraulic and process improvements
Replacement and expansion of buried utilities
Seismic upgrades
A new pedestrian bridge to improve site access and safety

The facility currently treats up to 15 million gallons of wastewater per day, producing approximately nine million gallons of recycled water daily for groundwater recharge and non-potable reuse at around 36 sites throughout the San Gabriel Valley. Serving approximately 150,000 residents, the plant remains a critical component of the region’s water management system. These upgrades will help ensure reliable operation for decades to come.

Strengthening Owner Partnerships Through Collaboration

A key element of the project’s success is the close collaboration between Hensel Phelps and the facility owner. On May 12, 2026, the project team hosted approximately 40 members of the owner’s engineering and construction staff for a site visit and project update.

Rather than relying solely on traditional progress reports, the visit gave stakeholders a direct view of construction activities, including the new influent pump station, wet wells, and underground utility installations. Project personnel reviewed upcoming milestones, discussed construction sequencing, and addressed questions related to execution, safety, and operational coordination within the active treatment facility.

The site walk also provided an opportunity to showcase the project’s innovative CDSM shoring system. By discussing the approach in the field and addressing questions in real time, the team reinforced transparency and strengthened stakeholder confidence. This open communication continues to align project goals, expectations, and long-term outcomes among all parties involved.

Building New Infrastructure Within an Active Facility

Constructing major infrastructure improvements within a fully operational treatment plant presents unique challenges. Every phase of work must be carefully coordinated to avoid disrupting ongoing facility operations.

The project team is managing:

Integration of new systems with existing plant operations
Complex underground utility conflicts
Installation of large buried structures and pipelines
Continuous maintenance of wastewater treatment functions during construction

Among the most significant project considerations is groundwater management. Protecting critical plant infrastructure while excavating for new wet wells and the partially buried pump station required a solution that could effectively control groundwater without extensive dewatering operations.

Innovative Groundwater Control with CDSM

Groundwater conditions presented substantial risks during excavation. The water table sits at approximately 196 feet, while the bottom of the new structure reaches 178 feet. Without proper controls, groundwater inflow could exceed 1,000 gallons per minute.

Adding to the challenge, several critical plant assets are located within 100 feet of the excavation area, including the main concrete processing channel, primary effluent sewer line, and flow diversion gate.

Rather than using a conventional beam-and-lagging shoring system—which would have required significant dewatering and increased the potential for settlement near active facilities—the team selected Cement Deep Soil Mixing (CDSM). This approach creates a low-permeability soil-cement cutoff wall around the excavation, substantially reducing groundwater infiltration and minimizing the need for large-scale dewatering.

The system is further strengthened with an integrated bottom plug and tie-down anchors that provide uplift resistance and long-term stability under sustained groundwater pressure.

The decision reflects Hensel Phelps’ philosophy of tailoring solutions to site-specific conditions rather than relying on conventional approaches. By evaluating each project’s unique challenges and engaging owners throughout the decision-making process, the team delivers solutions that balance performance, constructability, and operational protection.

A Commitment to Reliable Water Infrastructure

The Whittier Narrows project exemplifies Hensel Phelps’ broader approach to water infrastructure delivery. Through self-performed work, innovative engineering solutions, and continuous owner collaboration, the team is helping modernize a landmark reclamation facility while ensuring uninterrupted service throughout construction.

As the project progresses, it demonstrates how thoughtful planning, technical expertise, and transparent communication can combine to deliver infrastructure improvements that support the long-term reliability and sustainability of vital water resources.

Project News

ronan McGlade