Iron and manganese problems led the utility last year to shut down the well, which was first used in 2005.

Workers are testing five types of filters to see which removes the minerals most cost-effectively, said Al Larson, principal engineer for the utility.

The utility plans to install a permanent filter this fall, at an estimated cost of $2.4 million, or find a nearby site to drill another well; a separate study is assessing potential locations.

Each study, costing about $50,000, will be finished by March, said David Denig-Chakroff, general manager of the utility.

The decision to install a filter or drill a new well will come soon after, Denig-Chakroff said. It's not certain which approach would be cheaper.

"We're still going down two tracks here at the moment," he said.

During the testing, water is being run through the filters and into storm sewers, Denig-Chakroff said. No water from the well, at North Thompson Drive and Mesta Lane, is being pumped into the city's drinking water, he said.

The sample filters are about one-twelfth the size of what the permanent filter will be, Larson said.

In April, the well was put on stand-by, running only a few hours a week to refresh the reservoir. In October, it was fully shut down, as some wells are during the winter when water demand is lower, the officials said.

Tests on the well have shown manganese levels of 185 parts per billion, according to the utility. The federal Environmental Protection Agency recommends levels below 50 ppb to avoid dirtied water and below 300 ppb to prevent health problems.

Iron levels have been found at two times the "aesthetic standard."

The concerns at Well No. 29 are among several problems involving water quality, infrastructure and management that have plagued the utility.

Hundreds of residents complained about manganese discoloring their water and the utility has changed to a new type of high-speed flushing to clean water lines.

Another problem well - No. 3 - on Madison's Near East Side was shut down for the winter after a carcinogen, carbon tetrachloride, was found there.