The culprit was manganese - a naturally occurring mineral that can cause neurological damage at high levels.

"Nobody told us anything about the safe level of manganese for young kids," said Svanoe, who has a 14- month-old son and has installed a water filter. "Before, we just took our water for granted. But now, having a child, we take things more seriously."

So, what is in Madison's drinking water?

It's a legitimate and important question in a city where residents are calling the water utility by the hundreds in recent years to complain about rusty and sometimes even black water running from their faucets.

And even if your water appears clear, it's unsettling to know that some apparently clear samples of drinking water in the city have tested for manganese at levels as high as 700 parts per billion, well above the recommended federal health standard of 300 parts per billion.

A Wisconsin State Journal analysis of five years' worth of Madison's drinking water test data found dozens of other contaminants in water from the city's 24 wells.

In one aging well, No. 3, which serves the near East Side and East High School, the levels of cancer-causing carbon tetrachloride exceeded federal health standards in October 2000.

While few of the pollutants in Madison drinking water are at concentrations that violate federal health standards, their presence is a worrisome reminder that clean drinking water, even in a place with so much of it, is not to be taken for granted.

4 wells eyed closely

Tom Stunkard, a drinking water specialist with the state Department of Natural Resources, is responsible for monitoring and regulating the Madison Water Utility. He praises the utility's testing program and the overall quality and safety of Madison's drinking water.

Still, there are threats to the city's water supply that Stunkard watches closely.

He has ordered the utility, for example, to do additional testing each year on four wells with suspicious levels of industrial chemicals, known as volatile organic compounds, or VOCs. They include well Nos. 3 and 15 on the city's East Side, No. 18 on the South Side, and No. 6 on the West Side.

Stunkard is especially bothered by well No. 3, which began operating in 1928 and is one of the city's oldest wells. It's in an area of the city, near Fordem Avenue and East Johnson Street, that was the site of numerous now-defunct factories.

In a report last year by a consultant on water utility infrastructure, the quality of water being pumped from the well was described as "poor."

Of particular concern, according to Stunkard, is the presence of carbon tetrachloride in the well. The industrial chemical is a notorious pollutant that has been used as a dry-cleaning agent and as an industrial solvent.

Short-term exposure to levels above the EPA's health standard can cause liver, kidney or lung damage. Long-term exposure can cause liver damage and cancer, according to the EPA.

The legal maximum level of carbon tetrachloride allowed by the EPA is 5 parts per billion and in a test in October 2000, the level in well No. 3 measured 8.3 parts per billion.

David Denig-Chakroff, general manager of the Madison Water Utility, said the well was shut down eight days after the high reading; about 3.2 million gallons of water were pumped during that time.

The well began operating again after the utility completed work there. In 2001, the carbon tetrachloride level was measured at 3.43 parts per billion.

Stunkard said the high reading in October 2000 wasn't considered a violation of standards because a second test came back at levels below the EPA limit. Nor, he added, did the law require that the well be shut down until a second test confirmed the high level.

The ominous test result prompted a flurry of activity to try to pinpoint the source of the pollutant. Numerous test wells and a months-long study of the history of the nearby factories, including those no longer there, turned up nothing - a good example of how frustrating the search for pollutants can be.

More unsettling than anything else, according to Stunkard, is that readers of the utility's annual drinking water quality report did not find out that well No. 3 exceeded EPA health standards, an oversight that didn't come to light until the State Journal's investigation.

The utility reported a maximum level of carbon tetrachloride for 2000 of only 2.9 parts per billion. Al Larson, the utility's chief engineer, said the error was the result of a typo.

Denig-Chakroff said he had no explanation for the mistake. "Obviously," he said, "the error should have been caught when we proofed the document before going to print, and it was not. I do not recall anyone bringing the error to our attention until recently."

Stunkard said any harmful exposure to the chemical as a result of the incident is unlikely. But he said the mistake made by the utility in its annual report is significant because it erodes public confidence.

"It makes you start to wonder what else might be wrong," he said.

Other pollutants come not only from the wells but from the pipes that deliver the water to homes, a growing problem, especially, in a city with an aging water utility infrastructure such as Madison's.

Some pipes on the Isthmus and on the near East Side date to the 1880s. Such old iron pipes can contribute to the problem of water being discolored, turned rusty and brown by the iron sloughing off the inside of the mains.

Recent months have seen hundreds of complaints, for example, about discolored water coming from faucets on the city's East Side.

Other pipes are contaminated by lead, and the water utility has embarked on an ambitious plan to replace those.

Strange happenings

No contaminant has caused more consternation recently in Madison than the mineral manganese.

It was in the late winter of 2004 when Nakoma homeowner Lynn Williamson first called the Madison Water Utility to report strange and unsettling happenings with the family's water.

Thick, black chunks of dirt- like material were showing up in the Williamsons' ice trays and clogging their showerhead. Plumbers had visited the house repeatedly and flushed buckets of black water from the lateral line leading into the home.

The culprit would turn out to be manganese, a naturally occurring mineral that few people knew anything about. The levels in the Williamsons' water were staggering.

The advisory health standard set by the EPA is 300 parts per billion; one sample taken from the home in March 2005 came back with a manganese level of 244,000 parts per billion.

The Madison Department of Public Health has since issued a report saying that current research shows that long-term exposure to high levels of manganese in drinking water can cause neurological problems.

The report also warned of potential health problems from excessive manganese for infants - newborn to 6 months - and for people with chronic liver disease.

Over the next several months, manganese would become an all-too-familiar subject for not only the Williamsons but Madison water and public health officials.

It would show up in hundreds of homes across the city, get flushed in dark torrents from miles of water mains, prompt the mayor to form a task force and the health department to issue alerts.

The mineral showed up at high levels in numerous wells across the city but it also clogs miles of mains. It would prove to be as perplexing a problem as the city's water utility had ever faced.

But, maybe more than anything else, manganese shook a city out of its complacency about the quality of its drinking water.

Utility officials say their initial response to manganese was based on assurances from the EPA that the mineral is generally not a health problem.

That does little to reassure water customers such as Deborah O'Keefe.

O'Keefe has lived in Madison for 30 years and, until her water turned rusty brown last year, she had hardly given a thought to what flowed from her tap. Now, like so many others, she drinks only bottled water.

"You count on your water to be safe," O'Keefe said. "But I haven't counted on that for some time."