The project will eliminate the source of much of the salt from Palo Alto and Mountain View's recycled water system, which could potentially harm more than 1,000 redwoods and other large trees watered by the system.
"The bigger trees are part of the heart and nature of North Bayshore," said Mayor Mike Kasperzak. "Losing those trees would be devastating for the atmosphere of the entire area."
The project was quietly approved by a unanimous City Council vote on June 26.
City officials have been concerned since planning began in 2004 for the $20 million recycled water pipeline from Palo Alto's Regional Water Quality Control Plant.
The project was finally done in 2009, and the seven-mile purple pipeline was praised as a major step towards using less potable water. It can supply recycled water to as many as 120 customers in North Bayshore, including Google and Shoreline Golf Links.
A study later found the levels of sodium in the water to be higher than recommended for redwood trees, though grass can easily handle it.
"After investing all this money into (a) recycled water pipeline, we can't use the water because its too salty," Kasperzak said.
In Mountain View, redwoods and pine trees surround much of the massive Google campus, which is noted for using as much recycled water as possible, even to flush toilets.
In Palo Alto, nearly 1,000 trees in Greer Park and the municipal golf course are irrigated with recycled water, and Palo Alto is beginning to require recycled water use for some projects in the Stanford Research Park. In October, Palo Alto officials told concerned residents that the city's trees were not being affected, even though 30 dead or unhealthy trees were set to be replaced at Greer Park.
"Redwoods are some of the most sensitive to this high sodium content in the water," said Ray Morneau, chair of Mountain View Trees, a nonprofit that seeks to preserve the city's tree canopy.
It turned out that Mountain View is a big source of the salty water; the wastewater it sends to Palo Alto for treatment has sodium content of 1,150 parts per million, while the next worst offenders are Los Altos and East Palo Alto at 650 ppm, according to a 2007-08 sampling reported by the city of Palo Alto.
A 1966 sewer line in North Bayshore is to blame — porous enough "in numerous locations over several thousand feet of pipe line" to allow the infusion of salty groundwater into the recycled water system, says a city staff report.
The $3 million project aims to fix that with a resin-impregnated liner to be unfurled in the offending sewer line, then cured in place to seal it.
Effects on Redwood trees
Redwoods that are affected by saltwater look as if they haven't had enough water, with leaves turning brown and falling off, said Nelda Matheny, president of HortScience. It can also shrink the diameter of the tree.
Matheny and Morneau have seen trees killed or ailing because of salt content in recycled water, but in cities south of Mountain View that use water treated in San Jose. They had not heard of trees being significantly affected in Mountain View or Palo Alto.
City employees began monitoring select redwood trees on Charleston Road, Shoreline Boulevard and Garcia Avenue just before recycled water came into use there in 2010, and so far have not seen any changes, said Bruce Hurlburt, parks and open space manager. Sensors in the ground have found higher levels of salinity, he said, but tissue samples from the trees do not.
"Redwood trees are a very salt-sensitive species," he said. With salt in the water "you start to see browning at the tips. We're not seeing that currently."
Recycled water coming from the Palo Alto plant now has salt content of 950 parts per million. That's much lower than the 35,000 ppm found in sea water, but is still enough to cause damage to trees, according to a 2009 investigation by the city of Mountain View, Santa Clara Valley Water District, and South Bay Water Recycling.
"I cannot recommend the continual use of recycled waters with salinities greater than 1 dS/m (640 ppm) to irrigate redwood trees," soil and water specialist J.D. Oster wrote in the report.
Below that "it can be used safely on pretty much everything, including redwoods," Matheny said.
Palo Alto and Mountain View's goal is to reduce the number to 600 ppm.