It has been four years since the story of lead-contaminated water in Flint, Michigan, first riveted the country. Yet in recent weeks, news about lead-contaminated water and sluggish government responses are surfacing across the nation from cities including Newark, Chicago, Detroit, Baltimore and Milwaukee.

There is no acceptable level of lead in drinking water, according to the Centers for Disease Control and the U.S. Environmental Protection Agency. So why are so many municipalities, homeowners and schools still finding lead in their systems today?

One reason may be aging infrastructure and the cost to replace old water pipes and lead solder used in household plumbing. Drinking water is delivered via 1 million miles of pipes across the U.S., much of them laid in the early- to mid-20th century with a lifespan of 75 to 100 years, according to a 2017 report from the the American Society of Civil Engineers.



Those pipes are being replaced at an average rate of 0.5 percent a year -- at that pace it would take roughly two centuries to renew the whole system at a cost of around $1 trillion, according to one estimate from the American Water Works Association. Meanwhile, a 2016 CNN report found that more than 5,000 U.S. water systems serving roughly 18 million people violated EPA rules for lead in water.

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The last-mile problem



A September report from the Government Accountability Office outlines one reason it can be difficult to pinpoint when and where lead issues may pop up: Many problems lurk in pipes that link a municipal system to homes, and those links are often at least partly on private property. That makes it hard to locate them, especially given what the GAO notes is a lack of records about the locations of lead service lines.

When the EPA's lead and copper rule was first implemented in 1991, the agency estimated that about 10 million lead lines were in service nationwide. An estimated 6.1 million remain in U.S. communities today, suggesting some progress in removing them, according to a 2016 AWWA study. The cost for the remaining removals? About $30 billion, the analysis estimated. Among the lines left, the largest concentration -- 3.4 million -- is in the Midwest.

In Newark, the nonprofit Natural Resources Defense Council sued earlier this year after the organization said reports showed the city knew of lead in the water beginning in early 2017.

"The city has really been in denial and shrugged their shoulders and said our water is safe, people should drink it, until just a couple of weeks ago -- until we sued them and asked for an injunction on behalf of the citizens in Newark to clean up the water," the NRDC's Erik Olson told CBS News earlier this month.

The group sued Newark for violating the federal drinking water law, saying lead levels are "some of the highest" of "any large city" nationally, a problem the NRDC compared to Flint. Newark officials told CBS it's changing the way it treats water at one plant and is embarking on a lead-pipe replacement program.

But Newark is still looking into how it pays to replace aging lead pipes without putting a big burden on residents, Politico reported. New Jersey freed up $500 million in bonds to help its cities pay for the problem, but many residents still can't afford their share of the bill. Newark can only guarantee replacing lead pipes with copper ones that won't top $1,000 for homeowners. Newark needs more help from the state or EPA to slice costs, Politico said.

Certified testing is best



Lead in water is invisible and odorless. It can enter drinking water when pipes that contain lead corrode. That happens when water is high in acid that induces corrosion in pipes and fixtures.

Testing at the tap using a certified lab is the best way to measure levels in household water, according to the AWWA. And the EPA recommends a state certification officer for consumers who choose to have their own water tested. It can be hard to test for results on your own.

Another problem, according to the GAO, is a lack of information about the private property locations of lines to homes from public systems.

The GAO noted that, of 43 states that responded to an EPA inquiry on lead in 2016, 39 reported that they had encouraged water systems to publicize inventories of such pipes. Few, though, completed those plans. Of the 100 largest U.S. water systems, just 12 had provided public information on the inventory of lead service lines as of January 2018, the GAO report found. Among states, the EPA lists Massachusetts and Washington as the leaders in lead line replacement efforts.

"By sharing information with all states about the approaches that some states and water systems are using to successfully identify and publicize information about lead service lines, including responses to potential challenges, EPA could encourage states to be more transparent to the public and support the agency's objectives for safe drinking water," the GAO concluded.

A need for urgent action



In April, the EPA announced new funding of as much as $5.5 billion in loans to help fund projects through the federal Water Infrastructure Finance and Innovation Act program.

And this summer, the EPA's Office of Inspector General said the agency must take steps to react more quickly during public health emergencies. Flint's tap water became contaminated with lead in 2014 after officials switched from the Detroit system to the Flint River.

One recent EPA update of a "toolkit" for schools and child care centers stresses how to address potential contamination more clearly, according to some watchdog groups.

"Overall, the new toolkit is an improvement," the nonprofit advocacy group Environmental Defense Fund wrote in an October post. "While the protocol itself is largely the same, the new toolkit is more user friendly and written for the non-technical audience, making it more likely that school and child care staff will use it."

-- CBS MoneyWatch's Irina Ivanova contributed to this report.