The cast iron pipe, first installed in the mid-1950s, has been singled out for replacement to reduce the risk of possible catastrophic failure, according to Dan Martin, utilities superintendent for the city of Pleasanton.
No major incidents associated with the old water main have been reported on Vineyard Avenue, but Main Street and Santa Rita Road were both dug up in the winter of 2012 for emergency repairs to leaking cast iron water mains in front of the Pleasanton Hotel and Amador Valley High School, he said.
The city is systematically replacing the system's cast iron mains. Their locations are known, and they are monitored, Martin said. They are replaced when road resurfacing is planned above the pipes or when other issues, such as the one that led to the Vineyard Avenue project, leads to construction.
The cast iron pipe on Vineyard Avenue will actually remain in place to avoid digging a trench for its removal. It has been repurposed to serve as a conduit for 1,550 feet of 8-inch, high-pressure, poly-vinyl chloride (PVC) pipe that will reside inside, said Pleasanton senior civil engineer Adam M. Nelkie.
The new PVC line, which will be pulled through the pipe, will deliver water pressurized to 110 pounds per square inch (psi) to the last enclave of hillside houses south side of Vineyard Avenue that does not have a high-pressure water source.
The 25 residences are located on 300 blocks of Mavis Drive and Ewing Drive and Zwissig Court, a half block long cul de sac just south of Vineyard Avenue. The new water main connects the houses with a city water pump station at the intersection of Vineyard and Bernal avenues. Water for the entire neighborhood originates from the water division's Bonde-1 water tank on a hill overlooking Bernal Avenue at Roselma Place.
The project also involves installation of a new 12-inch PVC water main supplying low pressure (60-75 psi) water to houses and businesses north of Vineyard. The large blue pipe will be connected to the water main installed east of Adams Way in 2008 and a new 16-inch PVC pipe that will cover the final 160 feet before its connection with the Vineyard-Bernal pumping station.
Construction began in early March and is scheduled for completion by mid-June. Road resurfacing on Vineyard Avenue from Bernal to Adams Way is planned for this summer giving the Pleasanton Water Division an incentive to have the underground water mains in place before that phase of the project begins, Martin said.
The two water mains will create two separate distribution loops -- one at 110 psi to improve water delivery to hillside customers in Pleasanton's southeastern neighborhoods, and the other operating at 60-75 psi for the flat-land customers mainly north of Vineyard Ave.
"Lower zone pressure is sufficient for homes north of Vineyard Avenue, but it is only marginally acceptable for higher elevations south of Vineyard Avenue," Martin said.
Gravity as power source
The need for variable pressure zones stems from the use of gravity to move water from 16 storage tanks strategically positioned on hills around the city to customers below, Martin said.
The amount of generated water pressure depends on the difference in elevation from the tanks and the residence or business where the water is used. Water pressures tend to be lower in hilly neighborhoods, such as Kottinger Ranch, than in downtown Pleasanton and neighborhoods on the valley floor.
"Tanks that serve the high zones are not high enough. They just don't generate enough pressure," Martin said.
Variable water pressure zones, supported by 13 electric pumping stations, help address the problem. The high pressure zone south of Vineyard -- referred to as the Bonde Zone -- extends to the city's southern city limits and has a complex eastern border to deliver high-pressure water to the hilltop residences of Vintage Hills and the higher elevation home sites west of Second Street. Its eastern border is at Isabel Avenue.
Additional high-pressure zones have been carved out for Kottinger Ranch, Grey Eagle Estates and Ruby Hill.
On the west side of Interstate 680, the city's water division manages another series of high pressure zones west of Foothill Road along the Pleasanton Ridge.
This two-zone system does not mean that hilltop residents have more water pressure than flatlanders, however. A pressure regulator valve is a standard feature next to the water meters for such Pleasanton homes and many businesses. It reduces pressure to 60-75 psi to conserve water and prevent damage caused by too much pressure to water heaters and other appliances.
More options to improve water quality
Nearly all of Pleasanton's potable water comes from the Zone 7 Water Agency, mostly from the Bay Delta. Martin said the rest originates from municipal wells around the city or Del Valle Reservoir.
By closing the loop for the Bonde high-pressure zone, the new Vineyard water mains will give engineers more control over the quality of the city's water supply, Martin said. Quality is not only defined by the inherent mineral composition of water and its residual chlorine content. It is also affected by how efficiently it circulates through the system and how long it sits in storage tanks, he said.
Fast turnover, measured by the ability to minimize the time between the tank and tap, are important for good water quality, Martin said. Without the parallel water lines on Vineyard Avenue, water division staff can monitor turnover times, but they have limited control over the routes used to transport water from storage to the consumer.
Water from the Vineyard/Bernal pump station currently has to flow east for some distance before connecting with a main that ultimately allows it to flow downhill for use at homes south of Vineyard Avenue, Martin said. The new water mains will close a gap allowing water to flow directly from the pumping station to consumers a few blocks away.
"This will no longer be a dead end," he said. "We will be able to move water in and out from two directions."
Martin said the ability to take such actions may lead to fresher water for Pleasanton, but it will not make its tap water taste better.
"That's a water treatment issue and would take far more than a few hundred feet of water main to address," he said.
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