About 2 million children die each year from waterborne disease, primarily acquired by drinking water contaminated by pathogens, including bacteria, protozoa and viruses. Disinfection of drinking water is widely recognized as a major advance in protecting public healthand has virtually eradicated cholera and typhoid fever in developed nations.

Due in large part to disinfection and other practices, very few deaths from waterborne disease occur in the U.S. However, not all of our tap water is as safe to drink as it could be.

One primary challenge is not all public water systems disinfect their water supply. Wisconsin is no stranger to water system failures, including the 1993 cryptosporidium outbreak in Milwaukee that resulted in more than 100 deaths and 400,000 cases of illness. This was also the case in 2008, when an outbreak of Salmonella bacteria in non-disinfected municipal groundwater killed one resident and sickened an estimated 1,300 others in Alamosa, Colorado.

More broadly, the tragic lead contamination in Flint, Michigan’s water supply underscores the diligence required by local, state and federal officials to ensure the protection of public health and to provide safe drinking water, free of pathogens and other contaminants.

With respect to infectious pathogens, the federal Safe Drinking Water Act requires disinfection at water systems that obtain water from lakes or rivers. Chlorine addition is the most common method of disinfection, although ozonation and ultraviolet light are also effective.

However, water utilities that rely on groundwater aren’t subject to a disinfection requirement. Until relatively recently, wells constructed to industry standards were thought to be naturally protected from disease-causing organisms. But recent research has shown this assumption is not valid: raw sewage leaks from sanitary sewer systems and under rainy conditions, it can be transported to underlying groundwater. Municipal water systems that serve untreated groundwater may supply water that makes people sick.

It’s difficult to quantify how pathogen-contaminated groundwater affects human health in the U.S. In 2013, researchers reported that untreated groundwater was the likely cause of 30 percent of reported drinking-water disease outbreaks between 1971 and 2008. Together, these epidemics caused over 23,000 illnesses, 390 hospitalizations, and 13 deaths.

Waterborne pathogenic disease in the U.S. primarily exhibits as endemic and mild acute gastrointestinal illness, with typical symptoms including diarrhea, abdominal pain, nausea, and vomiting. These symptoms aren’tunique to waterborne illnesses, though, so it may not occur to people who feel sick that their upset stomach may have been caused by a glass of water. One estimate developed by the U.S. EnvironmentalProtection Agency suggests that about 8.5 percent (16.4 million cases per year) of acute gastrointestinal illness in the population served by community water systems is attributable to drinking water.

Why hasn’t the EPA implemented a groundwater disinfection requirement? Some states, including Pennsylvania, Florida and Texas, go beyond federal standards and require disinfection at all community systems, including those using groundwater. But this isn’t the case in Wisconsin, where almost 60 water utilities serving about 65,000 residents do not provide this treatment. Overall numbers are much larger across the U.S., with some 20 million people relying on water from about 95,000 non-disinfecting systems.

Just a couple years after the Salmonella outbreak in Colorado, the Wisconsin Department of Natural Resources moved to require continuous disinfection at all municipal water systems in Wisconsin out of concern about pathogen transmission in groundwater. The Wisconsin Natural Resources Board unanimously approved the rule in 2010, but it was rescinded by the state Legislature six months later (2011 Wisconsin Act 19).

As a scientist familiar with the risk posed by non-disinfected groundwater, I couldn’t imagine why the relatively simple and inexpensive process of chlorine disinfection is not fully embraced by municipal water systems across the nation. So, I spoke with over 30 water system managers, consulting engineers, public health officials, and regulators, all of whom work to provide safe drinking water to Wisconsin residents.

These conversations revealed two issues that help explain resistance to disinfection.

The first issue is that not everyone believes untreated well water presents a health risk. At present, there is no affordable, reliable and practical test to determine the presence or absence of common pathogens. Contamination in wells is episodic, meaning that a comprehensive testing program would be costly, including frequent sampling and rapid laboratory turn-around times to detect those episodes of contamination.

Current federal rules require testing for coliform bacteria, which doesn’t make people sick, but indicates unsanitary conditions. A positive coliform result means that a well or distribution system is vulnerable to pathogen contamination, but a negative test doesn’t mean that the water is free of disease-causing organisms. Therefore, if put into place, a requirement to disinfect the water would come without direct evidence of contaminated water.

Water supply professionals I spoke with noted that the coliform bacteria test can produce a false sense of security. Consumers reasonably assume that untreated groundwater that meets current standards is safe to drink, when in fact it may not be. Public officials who approve municipal and utility budgets may not fully appreciate the skills, training and equipment necessary to ensure safe drinking water. (Find out if your utility provides disinfection by reading their annual update on water quality and treatment practices through the EPA’s Consumer Confidence Reports.)

The second issue that generates resistance to implementing disinfection is cost. All utilities are required to have equipment on hand for emergency disinfection of the water supply. Converting this equipment from its current condition to continuous operation is typically limited to a few thousand dollars, but the reality of continuous chlorination is not always that simple.

In some regions of Wisconsin, natural groundwater chemistry leads to precipitation of iron and manganese following chlorine addition. The presence of these metals can cause significant inconvenience to utility customers, but removal of excess amounts is very expensive and can overwhelm the financial capacity of smaller water utilities. Chlorination can also cause noticeable changes in the odor and taste of water, which can be made worse when water resides for longer periods within pipes that distribute it into homes. Residence times tend to be longer at smaller municipal systems due to low water use and infrequent flushing of the system, and operators may have difficulty optimizing chlorine addition to minimize odor and taste issues.

These complications are easier to address at larger water utilities, where there is greater water use from a larger number of ratepayers. Larger utilities have more financial capacity to support modernization of their systems. In Wisconsin, smaller municipal water systems in Wisconsin can apply for grants and loans to off-set these costs.

Chlorination may not be universally welcomed, but ensuring that drinking water infrastructure is well-maintained and free of infectious pathogens is a sensible priority. Speaking with people that run utilities in Wisconsin, I was impressed by their commitment to provide high quality, safe drinking water. They pointed out that making improvements will require better communication about public health benefits of disinfection. Another avenue for progress lies in the challenges small cities and towns encounter in operating and maintaining their water utilities, with innovative and thoughtful funding solutions key to sound infrastructure.

More information about pathogens in groundwater and treatment at systems in Wisconsin is available in a report from the Wisconsin Geological and Natural History Survey.

Madeline Gotkowitz, Ph.D., is a professor in University of Wisconsin-Extension’s Department of Environmental Science and a hydrogeologist at the Wisconsin Geological and Natural History Survey. She is president of the city of Madison’s Water Utility Board.