The viruses weren't found at dangerous levels, Borchardt said.

"It's their presence that concerns me," Borchardt said.

Viruses aren't supposed to be able to penetrate an aquifer as deep and naturally protected as Madison's.

But now viruses are among a growing list of so-called emerging contaminants that are not regulated in drinking water supplies by the federal Environmental Protection Agency but are of concern in both private wells and municipal water supplies.

Other worrisome contaminants rarely tested for and not regulated include pharmaceuticals, such as antidepressants and birth control drugs.

Researchers are uncertain about the health impacts of pharmaceuticals in our water. Some fear exposure to antibiotics could heighten the growing resistance of bacteria to overused antibiotics.

Research has also suggested that declining sperm counts as well as increasing rates of breast, prostate and testicular cancers in humans could be related to higher levels of estrogenic compounds in the environment.

The wells sampled for viruses in Borchardt's study included Nos. 5, 7 and 24. Each is in a different part of the city; No. 5 is in South Madison, No. 7 is on the East Side and No. 24 is on the Isthmus.

Borchardt took 10 samples from each well. None of the samples for well No. 5 came back positive. But four from well No. 7 and three from well No. 24 showed the presence of enteroviruses, a class of viruses that cause everything from colds to meningitis.

They are, according to Borchardt, being considered by the EPA for regulation.

Borchardt said the viruses can be controlled with chlorine and he questions whether the utility is adequately chlorinating drinking water.

Al Larson, chief engineer for the Madison Water Utility, said that since Borchardt's findings, the utility has increased chlorination at some wells and is keeping chlorine at a slightly higher level throughout the system.

Kenneth Bradbury, a hydrogeologist with the Wisconsin Geological and Natural History Survey who has extensively studied Madison's aquifer, said it is significant that Borchardt found viruses at such depths.

The source of the viruses remains a mystery, Bradbury said, but he added that Madison's aging water system may be a factor.

The viruses, he said, could be infiltrating the system through a well that hasn't been lined with protective concrete or through broken pipes.

Borchardt also found viruses in four of 50 private wells throughout Wisconsin.

Now, Borchardt is conducting studies on the health impacts of viruses in drinking water. He's doing research in 14 communities across the state, though Madison is not among them.

More research has shown numerous other contaminants that are not regulated nor tested for in Dane County drinking water.

K.G. Karthikeyan, a UW-Madison researcher in the Biological Systems Engineering Department, tested water for antibiotics at several water treatment plants in Wisconsin.

Although Madison wasn't among the cities on his list, he did find antibiotics both before and after treatment at plants in Green Bay, Oshkosh, Lake Geneva, Barron-Cameron, Hayward and Spooner.

In Lake Geneva and Spooner, Karthikeyan found antibiotics in adjacent groundwater monitoring wells.

Bradbury conducted research in which he found ibuprofen and estrogenic compounds in water from septic systems in a Sun Prairie subdivision.