“These are often smaller communities flying under the radar,” said Maura Allaire, an assistant professor of urban planning at the University of California, Irvine, and a lead author of the study. “They’re struggling to maintain their aging infrastructure, and they’re struggling to keep up with the latest water treatment techniques.”

Concerns about the safety of America’s tap water gained national prominence after the 2015 crisis in Flint, Mich., when residents discovered dangerously high levels of lead in their drinking water. Since then, a barrage of reports have revealed that a surprisingly large number of local water systems serving millions of Americans sometimes contain unsafe levels of contaminants like lead, nitrates, arsenic or pathogens that can cause gastrointestinal diseases.

In many cases, it can be unclear whether such contamination is isolated or evidence of a deeper systemic problem at a water utility.

To address that issue in this newest study, Dr. Allaire and co-authors from Columbia University’s Water Center looked for patterns in health-based violations over time at 17,900 local water systems around the United States between 1982 and 2015. She said one question guiding the research was “What kind of factors make some water utilities more susceptible than others?”

One striking finding: Health violations for drinking water surged in rural areas in the 2000s after the E.P.A. enacted regulations focused on disinfectants. Utilities have long used chlorine or other chemicals to disinfect their drinking water supplies. But this process has a troubling side effect. Those chemicals can react with organic matter in the water to create new compounds that may pose their own health risks.