Depending on who you ask, it either smells musty, grassy or like sewage.

Algae has invaded Penacook Lake, which serves as Concord's drinking water supply. City officials say it can't hurt you, but they plan to treat the problem later this week.

While they wait, some residents who are turned off by the smell and taste are stocking up on bottled water.

"I'm going through 12 to 14 gallons," said Roberta Page, who lives in The Heights. "If I take a shower (with tap water), I hold my breath. If I drink it, I hold my nose and just try to hit the back of my throat - it's disgusting."

State environmental officials said four different types of algae have taken up residence in the lake during the past week, which is about when the city received a flood of complaints. This is the third time since 1998 that residents have encountered the foul taste at the tap. Two years ago, the same smell and taste required similar treatment.

Penacook Lake, which has supplied the city's water for more than a century, offers very good water quality, according to Jody Connor, state limnology director. The algae problem stems from the city's decision to pump water from the Contoocook River into the lake.

"When they do that, they take a river that has a lot of sewage treatment in it and blend it into a lake which has lower phosphorous,"Connor said. When the higher-phosphorous river water hits the lake, algae growth takes off.

The water passes through the city's treatment plant, where it is cleaned, filtered and disinfected. Flouride is also added before it goes to customers, but none of that kills taste and odor problems that come with algae.

Chip Chesley, the city's general services director, said that could change as soon as next spring. He'll be pitching a $200,000 activated-carbon filter to city council in coming weeks. The idea first came up in 2003, Chesley said, after a similar algal bloom.

Several Concord restaurants contacted yesterday said they already have those installed, which means customers weren't likely to notice a rancid smell in the tap.

Meanwhile, state officials plan to spread copper sulfate in the lake on Thursday, which will kill the algae, but won't harm anything else.

Since the state is treating the lake, the city will save a considerable amount on labor. But officials will have to buy about 1,200 pounds of copper sulfate, at about $1 a pound.

Large burlap bags of the granular chemical will be hung off of the back of the boat, which cruises the lake. The crystals dissolve as the boat moves, without leaving a heavy concentration in one spot. Connor said the algae should be gone within a day after treatment.

For now, the lake will stay a cloudy blue-green color. And Page says she'll keep washing her vegetables with Poland Springs.

(For questions or complaints about Concord's water-quality, call 225-8696.)

By REBECCA T. DICKSON

Monitor staff