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Algae responsible for potentially strange odors and tastes in water as city of Napa switches to Lake Hennessey as primary water source

Algae responsible for potentially strange odors and tastes in water as city of Napa switches to Lake Hennessey as primary water source

From the Napa News Now: Check out the stories Napa County residents are talking about this week series
Napa water supply at Lake Hennessey

The city of Napa draws the bulk of its local water supply from Lake Hennessey.

City of Napa residents might have noticed earthy, musty or moldy tastes and smells in their water this past week.

The water is still perfectly safe to drink, despite the potential for olfactory or gustatory unpleasantness, officials say.

Algae is the culprit behind the changes in taste and odor, said Joy Eldredge, the city’s deputy utilities director. With warmth and sunlight, algae blooms seasonally upon Lake Hennessey. The algae, when they die, release odor compounds. And, though algae is removed during water treatment, those odors can stick around.

Residual bits of algae are potentially detectable through taste or smell because the city of Napa switched to Lake Hennessy as its primary source of water on Sept. 12. Algae compounds weren’t previously detectable because the city was using water drawn from the Sacramento–San Joaquin River Delta through the State Water Project, Eldredge said.

The city does apply PAK 27 algaecide to the lake, an oxygenator that quickly breaks down the compounds of the algae. Without using the algaecide, the lake would see exponentially more algae and residents would detect a much greater change in taste and odor, according to Eldredge.

“It’s so challenging because it’s two or three parts per trillion that are noted,” Eldredge said. “Even though the algae is removed in our treatment process, if there’s any residual compound in there, it’s detectable.”

Eldredge added that the city made the switch to Hennessey to conserve State Water Project water, and will use the reservoir’s water for the next three to five weeks.

“We’re in a unique situation where we’re actually trying to optimize using every acre-foot, every drop we can get from the State Water Project,” Eldredge said. “We know based on our demands that we need to use some water from Lake Hennessey before the end of the year.”

She also said the city switched to Hennessy water this past week, instead of later in the year, because of a water shifting process that essentially involves the lake flipping over, from top to bottom and bottom to top.

The city typically takes water out of the reservoir at about 15 or 20 feet below the surface, where the water is cool and free from the algae that might be growing on the surface and the sediment at the bottom, according to Eldredge.

But as autumn approaches, water near the surface of the lake cools down and grows denser, causing it to sink to the bottom, she said. The warmer water at the bottom of the lake, in turn, flips to the top.

“When that happens it brings the natural elements that are in that water back up to the top,” Eldredge said. “So it changes the quality of that water. We know that’s coming in November.”

The city will return to using State Water Project water before the water flipping process gets going, according to Eldredge.

This switch to Hennessy water typically happens every year, but changes to taste and odor may be heightened this year because the reservoir’s water levels are abnormally low, according to a city press release.

"We love when the reservoir fills and spills and gives us a natural refresh," Eldredge said. "But we didn't get that natural refresh of the temperature and the water, in general, this year."

Indeed, the city is still in the midst of fending off the 2021 California drought by attempting to reduce water usage by 20% when compared to last year.

The city’s July decision to limit most irrigation to two days each week has made a large difference in water usage so far, bringing Napa’s water use well below 2020 levels and to a target level of 2015 and 2016 usage, according to the city. As part of the decision, irrigation will be further limited to one day each week starting in November. 

The dangers of flood waters extend beyond drowning and structural damage.

You can reach Edward Booth at (707) 256-2213.

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