Smell is cast over south O.C.

What’s that smell?

That’s what the people of Mission Viejo and Rancho Santa Margarita wondered as a rotten egg odor started wafting in thick pockets last week into their yards, garages and living rooms.

Some in the south Orange County suburbs called Public Works to report a sewer backup. But that wasn’t the problem.

Others called the Fire Department, suspecting a gas leak. False alarm.

They were baffled. What was making the evening air hang heavy with that overwhelming stench?

By Halloween, the local water district offered an explanation of the putrid problem: Strong winds and a cold spell conspired to stir up the 1.3-billion-gallon Upper Oso Reservoir, producing an oxygen-depleting, blue-green algae bloom that’s emitting a sulfur-based gas likened to rancid meat or decaying swamp sludge.

And while the Santa Margarita Water District works to neutralize the smell -- churning the waters by taking laps in two outboard motorboats; aerating the lake with various pumps; and replenishing it with fresh, oxygenated water -- residents within a several-mile radius of Upper Oso have had to make some adjustments, starting with keeping their windows closed and, at times, holding their breath.

“It’s a smelly sewer gas smell and it comes in waves,” explained Vince Iorio, 51, who lives about a mile from the offending reservoir. “It will sneak into the house even though you might have good windows, and it collects in pockets and corners. It lifts during the day when the breezes come up, but it’s definitely been a recurring thing.”

While some residents have reported itchy throats and headaches, water- and air-quality officials have conducted testing and said the smell is a temporary annoyance that poses no health risk.

Oxygen levels in the reservoir are still too low to stem the odor, but Dan Ferons, chief engineer for the water district, said there are some positive signs: “Good algae” is returning and producing oxygen through photosynthesis. And Wednesday night was notably less stinky than the night before.

“We’re trying to do everything we can to get this thing turned around,” Ferons said. “We definitely do see improvement, though I’m sure it’s not fast enough for the neighborhood.”

That’s not much consolation to locals such as Iorio, who have been lying low and shunning evening walks and bike rides.

And it sure is hard to fall asleep to the lingering smell of sulfur.

“You’re kind of captive; you’re stuck until the morning when the sun is up,” Iorio said. “I went out to get the paper this morning and [the stench] hit me right in the face.”

Water officials predict it could be a week until the air clears.

So it may be a while before everyone can breathe a collective sigh of relief.

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tony.barboza@latimes.com