Recycled Water Tunnels Offer Sustainable Solutions

Tech Insights

Water recycling is the practice of reclaiming wastewater from a variety of sources, treating it, then reusing it to provide alternative supplies for potable or nonpotable uses. Recycled water is being used for a wide variety of applications: ranging from agriculture, irrigation, and industrial to recharging ground and surface water supplies, to augmenting potable drinking water supplies. Advances in wastewater treatment are allowing the safe and sustainable reuse of water to meet specific water supply demands.

Nowadays, the need to speed up Earth’s natural hydrological cycle for water recycling is becoming more critical as water shortages spread around the world. Clients are exploring options and planning for the future as they face challenges with these shortages and seek sustainable solutions. A key challenge to the widespread use of recycled water is the need for new water conveyance, which may include interregional long-distance pipelines through areas of dense urban development or crossings of environmentally sensitive areas. By leveraging our design and construction expertise in water conveyance, geotechnical engineering, and trenchless technologies, Delve Underground continues to serve clients in planning, design, and construction of recycled water projects in a variety of roles—including designer, construction manager, and owner’s engineer. Below are examples of our recent recycled water projects.

Pure Water Southern California Program

Delve Underground has been assisting Metropolitan Water District of Southern California (MWD) with evaluations of trenchless and tunneling options during the feasibility and planning-level studies of MWD’s Pure Water Southern California Program. The program will provide up to 150 million gallons (568 million liters) of recycled water supply to Los Angeles and Orange Counties for use by industry, as groundwater basin replenishment, and potentially for integration into MWD’s existing drinking water system. The project will require 40 to 60 miles (64–97 km) of 7-foot-diameter (2 m) pipeline installation, including 35+ trenchless and tunneled crossings.

For the new conveyance pipeline alignment, Delve Underground has analyzed over 150 potential tunneled and trenchless crossings. Many of the crossings face such challenges as dense development in Los Angeles County and potential seismic and geologic hazards. Using our expertise in trenchless and tunneling means and methods and geotechnical engineering, combined with an understanding of the latest technologies, we assessed costs, risks, and benefits of these options to help MWD narrow the potential feasible alignments for the program. In 2023, we will continue to support the conceptual design for this program in support of the environmental permitting phase.

North Valley Regional Recycled Water Program

The Del Puerto Water District and the cities of Modesto and Turlock worked together on the North Valley Regional Recycled Water Program (NVRRWP) to provide recycled water from the cities for reuse by agriculture and wildlife refuges in California’s Central Valley. This program required construction of over 10 miles (16 km) of large-diameter pipeline to convey recycled water from the cities’ wastewater treatment plants to pump stations for distribution. Delve Underground supported both cities in different roles on various phases of this critical regional project.

Delve Underground was on the Owner Advisor team for the City of Modesto NVRRWP, providing tunnel methods evaluation, geotechnical investigation, preliminary trenchless design, and construction oversight on the design-build project. The 42-inch-diameter (1,065 mm) recycled water pipeline included a large crossing under the San Joaquin River installed by horizontal directional drilling (HDD) and a highway and railroad crossing installed using the auger boring method. Working closely with the city, the design-build contractor, and permitting agencies, Delve Underground provided 65% design of the HDD crossing and was instrumental in expediting approval of the critical levee crossings adjacent to the San Joaquin River.

For the City of Turlock NVRRWP, we provided trenchless design, geotechnical services, and construction observation for three microtunneled crossings, which included 55.5-inch-diameter (1,410 mm) steel casing for the 42-inch-diameter recycled water pipeline.

Pure Water Soquel Program

The Soquel Creek Water District, located in a narrow corridor along the Central Coast of California between the Santa Cruz Mountains and the Monterey Bay, provides potable water to a population of 40,000. The district’s potable water is from groundwater extracted from wells. Since this daily extraction exceeds the aquifer’s natural recharge, it is consequently depressing the fresh groundwater-level surface and allowing seawater encroachment from the Pacific Ocean. The district is implementing the Pure Water Soquel (PWS) Program to remedy this over-drawdown condition by (1) replenishing the basin with clean, safe, near-distilled quality water; (2) creating a barrier against seawater intrusion; and (3) providing a reliable, sustainable, and drought-resistant water supply.

The PWS Program includes two progressive design-build (PDB) contracts for $122.3M to design and construct (1) the Advanced Water Treatment Facilities Project; and (2) the Conveyance Infrastructure Project, which includes 8 miles (13 km) of 14-inch (355 mm) and 16-inch (405 mm) pipeline.

Delve Underground provided geotechnical- and trenchless engineering–related studies and assisted with the preparation of bridging documents as part of our ongoing role as Owner-Advisor for the PWS Program’s two PDB projects. Both projects have completed their design phase, are under construction, and are scheduled to be operational by 2024.

Pure Water San Diego

The Pure Water San Diego Program will use water purification technology to produce nearly half of San Diego’s water supply by the end of 2035. The program offers a cost-effective investment for San Diego’s water needs and will provide a reliable, sustainable water supply.

Delve Underground provided trenchless design services for the Phase 1 North City Pipeline that includes a new UV purification plant to purify recycled water. The purified water will be pumped through a 7+ mile (11+ km) pipeline and will discharge to the bottom of the Miramar Reservoir through a new diffuser pipe. Raw water in the reservoir will be treated again through an existing water treatment plant adjacent to the reservoir for potable water distribution.

The project required six trenchless crossings at locations that include a railroad spur, several congested intersections, an existing box culvert, the Caltrans I-15 freeway, an existing pond, and the Miramar Reservoir, where the pipeline will end with an outlet into the reservoir. The crossings were in alluvial materials, at an alluvial material to Santiago Peak volcanics transition, and in full-face stadium conglomerate. The transition into Miramar Reservoir required an 85-foot-deep (26 m) shaft in stadium conglomerate adjacent to the water treatment plant. The first drive has been completed, and the start of the 60-inch-diameter (1,525 mm) tunnel out into the reservoir is imminent. A wet extraction of the microtunnel boring machine (MTBM) will be required after completion of the reservoir outlet pipeline. Delve Underground provided the crossing designs and a conceptual design for the reservoir tunnel.