Palo Alto and other local cities' residents who have been complaining about nasty-tasting water coming from their taps can blame it on water reductions from the Hetch Hetchy supply and blending from other sources, according to City of Palo Alto Utilities spokeswoman Catherine Elvert.

On Nov. 29, several residents began noticing changes in the water's smell and taste, which was described as swampy, earthy or like "pond weed," they said on neighborhood email chats. But despite the nasty taste and smell, Elvert said the water is being continually tested and it remains safe.

Palo Alto receives its water allocations from San Francisco Public Utilities Commission (SFPUC), which manages water from the Hetch Hetchy reservoir near Yosemite. The clear mountain snow melt, which is piped down to the Bay Area, is sometimes mixed with other water sources as needed.

SFPUC began the blending changes on Nov. 28 with water from the Sunol Valley Water Treatment Plant. This is necessary to accommodate routine maintenance on the Hetch Hetchy system according to an SFPUC notice. The change stirred up sediments in a pipeline causing the taste and odor issues. The agency conducted taste and odor tests on water from several locations in the regional system on Monday, and no odor has been detected, the agency said.

The Hetch Hetchy supply was reduced from 145 million gallons per day to 105 million gallons of water, which is coming from surface-reservoir sources, Elvert said. The SFPUC did not initially indicate that there would be any changes to taste or smell, but after receiving a number of complaints, Palo Alto Utilities asked the agency if there was anything unusual in the water.

"There are higher total dissolved solids -- organic matter (sediment, algae) -- but treatment eradicates any potential pathogens. As it settles out, the taste should improve," Elvert said.

Water-quality analysis showed that the blended water has a hardness that is nearly twice what it was on Nov. 22, according to the SFPUC. (Hardness is the amount of dissolved calcium and magnesium in water, according to the U.S. Geological Survey.) Alkalinity also rose.

In addition to the normal flushing the city does, SFPUC is also flushing the system to try to push out the matter. The agency expects that the material in the water should settle out or move through the system.

"It is possible that this change in taste and odor may linger for a few more days until the water moves through the system. We will continue to monitor the water quality conditions very closely and take action as necessary to try to mitigate any taste and odor issues," SFPUC said in its notice.

The water quality is tested every day to ensure that it is safe, Elvert said. The problem is being found throughout Bay Area cities that use the Hetch Hetchy system.

Some highly sensitive customers, such as those with compromised immune systems, can be affected by minor water-quality fluctuations, and they should consult with their physician to determine in general if they should be taking precautionary measures such as adding filtration devices, the city utilities department noted on its website.

Elvert said there are some things that residents can do.

"If a change in taste, odor or color is troubling, one suggestion is to try running the taps (with cold water) for a few minutes to clear out water in pipes or consider using a filter (such as Brita) to alleviate the these issues," Elvert said.

"San Francisco Public Utilities Commission and City of Palo Alto Utilities recognize that changes in water aesthetics can be worrisome, but assure customers that the water is absolutely safe to drink, cook and bathe with."

Palo Alto Utilities also periodically flushes water mains and hydrants with fresh water to prevent the water quality from degrading. The flushing safeguards that the water is safe and follows state and local water-quality standards. The city posts signs at each site when this is being done. Utilities is currently flushing water mains where cul-de-sacs or end-of-the-line piping tend to accumulate vegetation, algae or sediment. The flushing keeps the debris out of the customer's service line, she said.

Anyone with questions can call the city's Water Transmission division at 650-496-6967 or visit www.cityofpaloalto.org/waterquality for more information, including detailed parameters of the water change.