The EPA keeps a list of about 100 unregulated contaminants that have made their way into water supplies from industrial sites and other sources. Every five years, the agency updates a shorter lineup of chemicals that it thinks should be tracked and studied and requires utilities to do testing.

The current inventory includes two viruses and 28 chemicals, including 1,4-dioxane. The goal is to eventually regulate those that pose the greatest risk to public health.

But critics say that regulators should be moving far more assertively, even as scientists continue researching the short- and long-term health impacts. They blame both the system set up by Congress as well as the agency’s glacial pace.

“For an agency to be unable to adopt a single new standard in 20 years is inexcusable,” said Erik Olson, health and environment program director for the Natural Resources Defense Council. “It’s a combination of a bad law and very bad implementation.”

In the wake of the lead crisis in Flint, Mich., and other problems in communities elsewhere, many people are increasingly wary of what flows from the faucets of their homes and schools – and whether the federal government is doing enough to safeguard drinking water. In April, a Kaiser Family Foundation poll found that more than 60 percent of Americans rate the government’s efforts as just fair or poor.

In 1974, the newly enacted Safe Drinking Water Act gave the EPA broad authority to monitor and regulate the nation’s public drinking-water supplies. The agency adopted existing standards covering nearly two dozen microbial and inorganic chemical contaminants. When regulators took too long to expand that number, Congress made clear in 1986 that it wanted faster action.

A bipartisan majority passed additional legislation requiring the agency to establish drinking-water limits for scores of contaminants – including bacteria such as legionella and chemical compounds from acrylamide to xylene. Lawmakers also directed the agency to set up a system for monitoring still-unregulated contaminants.

The result over the next decade was health-based thresholds for more than 85 substances, including a range of disinfection byproducts and chemicals known to increase the risks of kidney damage, high blood pressure and cancer, among other conditions.

Those efforts prompted complaints from some local water officials about the increased costs and time needed to comply with the wave of new regulations. Utilities faced testing and treatment requirements for a growing list of contaminants – some that appeared only in certain parts of the country and some that scientists were still studying to determine their public health implications.

In 1996, Congress intervened again. This time lawmakers directed the EPA to do detailed cost-benefit analyses on additional contaminants that it sought to restrict. The agency also had to ensure that sufficient science existed to establish the public-health risks of a particular substance before attempting to regulate it.

“It created this Herculean set of tasks that EPA had to go through before they could adopt any new standards,” Olson said.

In the 20 years since, the EPA has come close to successfully regulating only one new chemical contaminant in drinking water. In 2011, reversing a Bush administration decision, the agency announced its intention to set a federal standard for perchlorate. Exposure to the chemicalcan disrupt thyroid function in humans.

Yet the agency still has not put any limits in place. The National Resources Defense Council recently sued, saying that the EPA’s inaction could be exposing children and pregnant women to harm.

Joel Beauvais, who heads the EPA’s Office of Water, acknowledged that the agency’s pace in regulating new chemicals had slowed, in part because of the system mandated by Congress. “It’s a rather intensive process to get one of these drinking-water regulations across the finish line,” he said.

The law demands that the agency move deliberately – and there are reasons for that, he said. A substance may occur in only a very small number of drinking-water systems, for instance, or it may not have been detected at levels of concern. Before the EPA imposes new burdens on thousands of water systems, it must prove that there is a meaningful opportunity to improve public health.

“These are very consequential regulations,” he said. “They are consequential from a health perspective. They are consequential from an economic perspective.”

Beauvais noted that the EPA has updated standards for certain contaminants as well as revised other rules, such as those for treating wastewater, in ways that help contain the number of overall contaminants in drinking water. Officials also have said that they are exploring new approaches and could begin regulating entire groups of substances rather than targeting one at a time.

The agency has issued numerous health advisories – most recently for perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), a potentially toxic compound that has turned up in many water systems – that can prompt state and local officials to take action or at least notify residents about contaminants. Ultimately, though, the advisories are unenforceable.

The American Water Works Association says that the EPA should winnow its list to focus on a handful of chemicals that pose the biggest public-health concerns.

“In a resource-constrained world, it’s hard to make progress spreading your resources broadly,” said Steve Via, the association’s director of federal relations. “The way the current process is running, with large numbers of contaminants on the list you don’t get enough focus to achieve progress. When you don’t achieve progress, folks ask if the process is working.”

Congress on Tuesday passed a sweeping revision of the 1976 Toxic Substances Control Act, which covers thousands of chemicals in products as diverse as sippy cups, paint thinners and permanent-press clothing. The overhaul will give the EPA the power to require health and safety data for untested chemicals and to prevent substances from reaching the market – and, ultimately, drinking-water sources – if they have not been determined to be safe. Implementation will take years, however.

“Prevention is an incredibly important issue for the country over time,” Beauvais said. “If we regulate more on the front end, we’re less likely to have contamination from chemicals with adverse health effects on the back end.”

In North Carolina, environmental officials published a report earlier this year detailing a year’s worth of sampling for 1,4-dioxane within the Cape Fear River basin. It highlighted numerous “hot spots” for the contaminant located immediately downstream of wastewater facilities, suggesting that manufacturers or other industrial operators were sending it into municipal sewers. Current water treatments don’t effectively remove the chemical.

“People are understandably concerned,” said Steve Drew, Greensboro’s director of water resources. “[But] in the absence of enforceable limits, what is a water system to do?”

His department and other downstream communities responded by launching a sort of detective operation. They tested hundreds of miles of sewer lines and met with business owners to track down the possible sources of 1,4-dioxane.

“We got it down to about a half dozen or so businesses – a couple that had very high levels of 1,4-dioxane discharged into our system,” Drew said. “These companies are not even thinking about it because they aren’t regulated on it.”

He said the companies have been “very diligent” in trying to alter supply chains and remove the chemical voluntarily from their manufacturing process. There are early signs that those efforts are slowly beginning to lower 1,4-dioxane levels in the river basin.

But if companies balk, Drew has no way to force them to cooperate.

“Right now,” he said, “it’s completely dependent on good relationships, and ‘please’ and ‘thank you.’ ”