The Madison Water Utility runs an extensive testing program to make sure contaminant levels do not exceed health standards, according to Al Larson, the utility's chief engineer.

The utility takes more than 20,000 water quality samples per year and tests for 140 contaminants including bacteria, metals, pesticides and petroleum byproducts.

Some substances, such as bacteria, are tested for as frequently as four times a week. Those samples are taken from 30 so-called "compliance points," faucets in places such as schools, fire and police stations and gasoline stations.

Also tested during these more frequent rounds is the level of chlorine in Madison's drinking water. Small amounts of chlorine are added to the water at each well to kill bacteria or viruses.

Most potential pollutants, however, are tested for less frequently.

The federal Environmental Protection Agency requires the utility to test for most contaminants once a year. These include pollutants that fall under the broad categories of inorganic contaminants, such as manganese or lead, and organic contaminants, including pesticides and industrial chemicals.