The study, published this month in the American Journal of Public Health, found that children from untreated systems had a 40% greater chance of getting such illnesses after an inch of rain had fallen in the previous week.

With even heavier rainfall, the chances of children going to the doctor or hospital for such problems grew even higher. The researchers found the incidence of intestinal troubles was 240% higher if rain totaled 4.7 inches or more in the previous week.

The researchers compared the results, taken between 1991 and 2010, to areas in the same region that had private wells or municipal systems that treated their water.

The study is the latest example showing the potential health risks posed by public water systems that don't disinfect water. There are more than 60 Wisconsin municipalities with a population of 85,000 that do not disinfect water, according to the Wisconsin Department of Natural Resources.

Treating municipal water is not required in Wisconsin. The Legislature in 2011 rejected a proposal to require treatment. Democrats have introduced a bill in the current session that would require communities to provide disinfection. However, the measure has little likelihood of passage in the Republican-controlled Assembly and Senate.

The study area composed 24 zip codes and is served by a dominant health system, the Marshfield Clinic, which provides health care services to 97% of the area's 90,000 residents, including 4,800 children.

That gave researchers the opportunity to use relatively uniform medical records to compare health records, according to Christopher K. Uejio, an assistant professor of geography and public health at Florida State University.

Uejio received his PhD at the University of Wisconsin-Madison in environmental studies, where he worked on the study with other researchers from UW-Madison, the Marshfield Clinic Research Foundation and the U.S. Agricultural Research Service.

If predictions hold true that changes in the climate could spur more frequent heavy rain events, Uejio said, the problems for communities with untreated water systems could worsen.

The authors suggest improving the treatment of drinking water by a municipality or at individual homes, schools and businesses. They also said communities should consider better surveillance and the use of boiling water advisories after significant rain.

The most common method of treating water is by adding chlorine. Most of the water in the area comes from private wells (54%), which also often are not treated, followed by municipal systems that treat their water (39%) and finally municipal systems that don't treat their water (6%.)

Uejio said one reason why private wells might not experience the same level of gastrointestinal illnesses in children is that the amount of contamination around wells may be smaller than those around municipal systems.

Sen. Mark Miller (D-Monona), author of the bill requiring systems to disinfect their water, said opposition to mandatory treatment came from municipalities concerned about costs of new equipment.

"There are already costs," Miller said. "It's just a matter of who is paying for it."

For example, he said a business pays the cost when a parent stays home with a sick child.

He predicted eventual passage of mandating treatment of water systems as the public health repercussions become better understood.

In 2012, a study by the Marshfield Clinic Research Foundation and University of California at Davis showed that 14 Wisconsin communities that don't treat their water had human viruses in drinking water in nearly one-quarter of samples taken.

The study was published in Environmental Health Perspectives, a peer-reviewed journal of the National Institute of Environmental Health Sciences.

In 2011, the Legislature broke along party lines and rejected regulations by the state Department of Natural Resources, proposed under the administration of Democratic Gov. Jim Doyle, that would have required all Wisconsin communities to disinfect their drinking systems.

The 14 communities were: Crandon, Cumberland, Barron, Chetek, Ladysmith, Tomahawk, Prairie du Sac, Adams, Spring Green, Rice Lake, Cameron, Baldwin, Lake Hallie and Fall River.

Some community officials say their residents don't want water treated and found additives like chlorine distasteful.

Rep. Erik Severson (R-Star Prairie), an emergency room physician, reviewed results of the 2012 study, was satisfied with the methodology and didn't dispute the findings.

"It goes back to choice for the community," he said in a 2012 interview. "The communities have to make the decision."