Lead levels in drinking water in the northwest Missouri town — population 5,609 — had spiked.

Over the next two years, one-quarter of the homes tested exceeded the Environmental Protection Agency's action level — 15 parts per billion — at least once.

The culprit, city and state officials believe, was the monochloramine. It likely corroded old lead pipes and caused the surge of lead in the drinking water. Because it hadn't detected high levels of lead in years past, Trenton hadn't been required to test for lead at residents' taps since 2014.

Until the city got test results, "we just thought maybe it was kind of like an isolated spot," said Ron Urton, the city administrator and utility director. "And then once we did the test and saw there [were] other elevated places, that's when we started, I think, kind of figuring out what was going on."

The 62 homes Trenton tested during that period have lead pipes, or service lines, running from the water mains, Urton said. But beyond that, very little is known about where lead pipes remain in the system with about 3,000 water meters.

Trenton has managed to get its lead levels back down again by adding a compound that reduces corrosion. But, experts say, the only permanent solution to stop lead from seeping into America's water is to remove the millions of lead pipes that remain 36 years after environmental regulators banned new ones from being installed.

Therein lies the problem.

Trenton — like many other water systems — doesn't know where all of its lead service lines are.

Corrosion control is essential when delivering city water.