In 1995, fast-paced development and competing demands for dwindling ground-water resources in California's central valley forced the cities of Manteca, Escalon, Lathrop, and Tracy to study alternative sources of drinking water. As part of their investigation, they contacted the South San Joaquin Irrigation District (SSJID)—a local government agency established in 1909 to provide a reliable and economical source of irrigation water for local agriculture. Due to water conservation measures taken by the district's agricultural customers, SSJID had water available to allocate to nonagricultural purposes. Through coordination with SSJID, the cities were able to agree on a system for delivery of treated surface water, and the South County Water Supply Program was established.

After years of planning, SSJID is now in the process of implementing the South County Water Supply Program in a series of phases designed to address present and future water demand in the cities. This work involves the construction of a new water treatment plant with state-of-the-art membrane technology, nearly 40 miles of pipeline, and associated pumping stations. By early this summer, work on the first phase will be complete, and SSJID will begin delivery of water to Manteca, Lathrop, and Tracy, with plans to deliver to Escalon by 2012.

The agreement between the district and the cities allows for surplus water to be available for local urban use through facilities to be owned and operated by the district, with oversight from an operations committee composed of the district general manager and the city managers. The agreement further provides for proportional cost sharing by the cities for design, construction, operation, and maintenance of the new facilities. To avoid competition between agricultural and urban interests, the district and cities established how much water will be allotted to each party and how shortfalls will be divided.

“Conservation provided the means for the district to help meet the future water needs of Manteca and Escalon, which are within the district, as well as two cities outside the district,” said SSJID general manager Steve Stroud. “The new coalition has survived many changes in personnel among the participants and has successfully worked through numerous technical and political obstacles in designing, funding, and building this necessary project.”

Getting With the Program

Sacramento-based Water Resources Consultants has provided program management for the South County Water Supply Program. The district also enlisted help from Black & Veatch, an engineering, construction, and consulting company with offices in Concord and Sacramento, Calif. Black & Veatch water specialists worked closely with subconsultants, contractors, and vendors to provide study, design, membrane procurement, and construction management services for the program. The $136.6 million program encompasses watershed protection, raw-water facilities, the water treatment plant near SSJID's Woodward Reservoir in Stanislaus County, and buried transmission lines and pump stations designed to convey water from the treatment plant to the cities.

The design involves the flow of raw water by gravity from Woodward Reservoir through one of two available intakes. Regulatory issues pertaining to potable water and the threat of microbial contamination from body-contact recreation prompted the district to separate the reservoir into upper and lower impoundments in conjunction with the South County Water Supply Program. Body-contact recreation will be confined to the lower impoundment, with supply water drawn from the upper impoundment during the heavy use summer season when irrigation flows keep the reservoir level high.

The existing intake, previously used for irrigation withdrawal and located at the downstream end of the reservoir, will be used in the fall and winter months when body-contact recreation is prohibited. New construction consists of intake screens, a water-quality control wall to inhibit the passage of boaters and swimmers to the upper impoundment, a raw-water pipeline, and a valved connection between the existing intake and the line to the treatment plant.

The district also took specific steps to meet the needs of farmers and protect the source water from livestock-related contamination. This included implementation of drainage diversion measures, installation of wells for cattle watering, and fencing to prevent cattle access to the canal that feeds Woodward Reservoir.

Providing Special Treatment

The South County Water Supply Program water treatment plant incorporates innovative high-rate dissolved air flotation (DAF) and submerged membrane filtration. The plant will have an initial capacity of 40 mgd and an ultimate capacity of 60 mgd. Designed to comply with anticipated and current regulatory requirements for safe drinking water, the facility will provide high-quality treated water within a small surface area. Small-footprint technologies such as high-rate DAF and submerged membrane filtration offered cost savings in California, where labor costs associated with cast-in-place concrete and other construction activities are especially high. The construction of comparatively compact facilities minimized construction costs.

Early evaluation demonstrated that DAF would be effective in addressing potential taste and odor issues resulting from seasonal high algae concentration in Woodward Reservoir. Pilot testing revealed economic advantages, indicating that DAF technology would provide higher quality feedwater to the membrane filters and result in lower operation and maintenance costs compared with other options by allowing longer membrane filtration runs between chemical cleanings.