High levels of carbon tetrachloride, an industrial carcinogen, have forced the Madison Water Utility to shut down a well that provides drinking water for neighborhoods on the east side of the Isthmus, including East High School.

Tests of water taken from Well No. 3 on Sept. 5 showed levels of carbon tetrachloride at 6.2 parts per billion. The federal Environmental Protection Agency sets a maximum level for the chemical of 5 ppb and a well would be in violation if three consecutive tests were above that health standard.

But David Denig-Chakroff, general manager of the Madison Water Utility, said the previous test, taken in June, showed a level of 4.7 ppb. The well was shut down Thursday.

Denig-Chakroff said the well had been on standby and not in regular use since May because of high levels of manganese and iron. It is unlikely that any water contaminated with high levels of manganese reached homes, he added, because the well had been operated only about four hours since Sept. 4.

The well was also operated infrequently during summer months when the neighborhood's pipes were being flushed of the problem mineral manganese. It was scheduled to be shut down for the winter, according to Denig-Chakroff, but is now being taken out of operation about two weeks earlier than anticipated.

Carbon tetrachloride is an industrial chemical and a notorious pollutant that has been used as a dry-cleaning agent and as a solvent. Short-term exposure to levels beyond the EPA's health standards can cause liver, kidney or lung damage. Long-term exposure can cause liver damage and cancer, according to the EPA.

It's not the first time carbon tetrachloride has plagued the well, which dates from 1928 and is one of the city's oldest. A level of 8.3 ppb was detected in the well in October 2000. While the well didn't test beyond EPA standards three consecutive times - and so was not in violation - the measurement was controversial because the water utility did not report the high reading in its annual water quality report. The oversight, which the utility blamed on a typo, was reported in May in a Wisconsin State Journal series on the city's water quality.

The old well is near the corner of Fordem Avenue and East Johnson Street in an area that is the site of numerous abandoned and razed factories. Tom Stunkard, who oversees regulation of the Madison Water Utility for the state Department of Natural Resources, said the well has long been of concern because of its age and location.

As is probably the case here, industrial pollutants such as carbon tetrachloride can linger in the ground long after their source is gone. Often the contaminant is found in a plume that works itself deeper into the ground over the years, eventually finding its way into the deep sandstone aquifer in which our drinking water is stored.

Because of the age of the well and the perils of finding clean water at the urban location, Denig-Chakroff said solving the contamination problem poses a challenge. He said options include treatment or abandoning the old well and building a new one.

In the meantime, neighborhoods normally served by the well are being served by other, nearby wells. Those wells will be able to supply the area with plenty of water during the winter months, Denig-Chakroff said. But problems could arise during warmer months when more water is needed.

The fate of the well will be much discussed over the winter months, Denig-Chakroff added. The utility is already facing much higher expenditures and has warned of increasing water rates to deal with the problem of manganese as well as to replace aging parts of the water system.

"We're really going to be talking about what we'll do with this well, what we want to do with it," Denig-Chakroff said.