The Flint, Michigan water crisis in 2014 exposed 98,000 people to dangerous levels of lead, disinfection byproducts, E. coli, and Legionella bacteria, causing long-term repercussions like infertility that are still being felt today. What we might not realize is that the dangerous drinking water in Flint, which still needs to be filtered at home, is not an isolated issue, as researchers report in a new article in the Proceedings of the National Academy of Sciences.

In the study, released Monday, researchers from Columbia University and the University of California, Irvine report that between four to 28 percent of the population was affected by water quality violations between 1982 and 2015. That means millions of people were exposed to water that could put their health at risk. In total, health-based violations of water quality standards happen in approximately seven to eight percent of community water systems in any given year, the researchers report. In 2015 alone, 21 million Americans drank water from systems that violated health-based quality standards.

It’s very likely, the researchers note, that this level of exposure is connected to the 16.4 million cases of acute gastroenteritis that happen each year in the United States. Acute gastroenteritis is an intestinal infection that comes with diarrhea, nausea, and vomiting and can be caused by contaminated water.

Contaminated water in Flint, Michigan. Giphy

In the study, the researchers combed through data from 17,900 community water systems collected between 1982 and 2015, looking for violations of the Safe Drinking Water Act. They determined that water health violations happen in 7 to 8 percent of systems every single year and found that repeat violations happen most often in the Southwest, with significant violation “hotspots” in Oklahoma and Texas.

Overall, they found that water health violations are more prevalent in rural areas than urban areas and are especially bad in low-income rural areas. They’re least likely to happen in regions that rely on privately owned water utilities. Aging infrastructure, impaired source water, and strained community finances contribute to this growing challenge of ensuring American water supplies are safe.

The National Guard assessing water in West Virginia after the Elk River chemical spill. Wikimedia Commons

To help America’s water systems improve, we need to rigorously assess how widespread the problem is, the researchers write. Their paper is one of only a few peer-reviewed studies on compliance with the Safe Drinking Water Act, and, the authors write, there’s simply not a “good understanding of quality violations.” While monitoring water systems is federally mandated, most state enforcement agencies actually don’t have a system doing so or a guideline for identifying systems that need extra oversight.

The end result is a country where access to healthy water is determined by wealth and geographic status. “Equity concerns are also gaining recognition as evidence builds regarding lower-income and minority communities receiving poor quality water,” the researchers write.

The goal of the Environmental Protection Agency is to provide at least 91 percent of the American population with reliably safe community water systems. But between 1993 and 2009, the percent of the population that had this access fluctuated between 79 to 94 percent — an inconsistency that means there’s still a ways to go in providing drinking water that’s violation-free.