Samples now have to be taken not only from the first liter drawn from a house with exterior or interior lead plumbing, but also the fifth liter.
Leisl Clark, director of the state Department of Environment, Great Lakes and Energy, said the testing change will provide "more precision and more insight into what's actually happening in the homes."
"We're expecting to see higher lead results in communities across the state," she told The Associated Press before a news conference with Gov. Gretchen Whitmer and state health officials.
Clark said 100 communities will send the state samples in coming weeks, and an additional 300 will follow in the fall.
She had no estimate of how many more systems could exceed the lead limit, which is triggered if concentrations surpass 15 parts per billion in more than 10 percent of customer taps sampled. But she said one municipality, Hampton Township near Bay City, recently went over the threshold because of the fifth-liter sampling and would not have been flagged under the old testing protocol.
"We want to communicate with folks about that. We want them to be aware that the rules changed, and so we're testing with more precision. We're able to protect people in a better way," Clark said.
Whitmer said she knows residents in affected municipalities will have questions and concerns.
"It's important for us to make sure that the public understands that the existing water quality won't have changed, but our testing requirements will have. We will have a lot more information. We will work in partnership with local communities to ensure that every person has access to clean drinking water in Michigan," she said.
A year ago, former Gov. Rick Snyder put in place the nation's strictest regulations for lead in drinking water in the wake of the man-made emergency in Flint, where the toxic metal leached into taps in 2014 and 2015 due to a lack of corrosion-control treatment following a switch in the water source while the city was under state emergency management.
Underground lead service lines connecting water mains to houses and other buildings will be replaced by 2040, unless a utility can show regulators it will take longer under a broader plan to repair and replace its water infrastructure. The "action level" for lead will drop from 15 parts per billion, the federal limit, to 12 in 2025.
The rules also prohibit the partial replacement of lead service pipes except for emergency repairs; require preliminary and final inventories of the lines and other components of a water supply by 2020 and 2025; and ensure samples are taken at the highest-risk sites and with methods designed to more accurately detect lead.
Michigan's environmental and health departments launched a new website Wednesday to show communities' lead and copper data results and give residents information on protecting themselves from lead. They will hold three online town hall-style events July 9-11.
Fifteen of Michigan's 1,400 community water supplies are currently exceeding the lead limit, including the cities of Hamtramck and Benton Harbor, according to the state. They must take steps such as notifying all customers, optimizing corrosion control and accelerating pipe replacements.
Whitmer this week signed a mid-fiscal year spending bill that includes $3 million related to the new regulations. About $1.7 million will cover the cost of water filters for low-income families; $820,000 will go toward educating the public in places with higher lead levels; and $484,000 will fund investigations in homes in those communities.
The Democratic governor is pushing the Republican-led Legislature to allocate $37.5 million more to implement the rules , including for the replacement of lead service pipes. The Senate so far has supported $25 million in the next budget, while the House has not embraced it.