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What's in Marengo's Water? The 2025 Report, Explained

Friday, July 10, 2026

Once a year, every public water system in the country has to give its customers a report card on their drinking water. Marengo's just came out. It's called a Consumer Confidence Report, it covers all of 2025, and if you've ever opened one, you know it's a wall of acronyms and tiny numbers that's easy to set aside. We read the whole thing so you don't have to, and we checked every figure in this story against the city's official report, which you can read for yourself at the end. Here's the plain-English version: what it says about your tap water, and how to read it yourself.

First, where Marengo's water comes from. The city draws all of its water from the ground, through two municipal wells: Well 6, near Prospect and Greenlee, and Well 7, at the east end of Greenlee Street. There's no river or reservoir in the mix. It all comes up from underground, gets treated, and goes to your tap.

Most Tests Came in Under the Limits. One Didn't.

For nearly everything the city is required to test for, Marengo's 2025 numbers came in within the legal limits, and most of them well under:

  • Lead came back non-detect. Every lead sample registered below the level the lab can even measure.
  • Copper was well within limits. The 90th-percentile result was 0.595 ppm against an action level of 1.3, with zero homes over the line.
  • Disinfection byproducts were low. The compounds that form when water is treated (TTHM and HAA5) came in at roughly a fifth to a third of their maximum allowed levels.
  • Barium, fluoride, nitrate, and radium were all comfortably under their limits.

The one exception was manganese.

The Manganese Violation

The report lists one violation for 2025: manganese went over the state's limit in the final quarter of the year, October through December. Samples ran as high as 380 parts per billion against an Illinois limit of 150.

A few things about that violation.

It's an Illinois standard, not a federal one. Manganese is unusual: the federal EPA hasn't set an enforceable national limit for it, but Illinois has, at 150 ppb. That's the standard Marengo exceeded.

Manganese is a naturally occurring mineral, common in Illinois groundwater. The report describes its effects mainly as a nuisance rather than a hazard: it can stain laundry and plumbing fixtures and leave an off taste in coffee, tea, and other drinks.

Who should pay closer attention. For most adults, manganese at these levels isn't considered a health risk. Infants are the group health agencies watch most closely. The EPA, which issues health guidance even where it hasn't set an enforceable national limit, advises that water above 300 ppb not be used to mix formula for babies under six months old, because infants that age absorb manganese differently and formula already carries more of it than breast milk. Marengo's fourth-quarter samples ran as high as 380 ppb, above that level. If you prepare formula with tap water for a baby that age, it's a reasonable thing to raise with your pediatrician.

What the city has done since. When a level crosses a limit like this, the city has to tell its customers. Marengo issued that public notice on February 7, 2026, sampled again the next monitoring period, and the report says it returned to compliance. Manganese is also one of the three things, along with iron and radium, that Marengo's new $7.55 million filtration plant was built to remove. That plant opened on Greenlee Street on June 15 and has been running since. The 2025 report, in other words, covers the year before the plant came online.

How to Read a Water Report (Any Water Report)

The acronyms are the hard part. A few translations make the whole thing readable:

  • MCL (Maximum Contaminant Level): the legal limit, the line a contaminant isn't supposed to cross.
  • MCLG (Maximum Contaminant Level Goal): the level with no known or expected health risk, with a safety margin built in. Limits are set as close to the goal as available technology reasonably allows, so the legal limit is deliberately conservative.
  • ppm and ppb (parts per million / parts per billion): units of concentration. The report puts it in plain terms: one part per billion is about one ounce in 7.35 million gallons. These are very small amounts.
  • Action Level: used for lead and copper. The city tests a sample of home taps, and if too many come back high, it has to act. Marengo had zero sites over the action level for copper.
  • Range vs. average: some limits are judged on a single sample, others on a running average of samples across the year. That's why a report can show a violation even when no single headline number jumps out: it's the average over time that crossed the line.

The quickest way to read the whole document: scan the "Violation" column. If it says no all the way down, the water met every standard that year. One line in Marengo's 2025 report says yes, and it's the manganese line above.

Want to Look It Up Yourself?

Part 2 of 3. This story is the second in a three-part series on Marengo's water. Part one looked at what Marengo pays for it, with a calculator so you could check your own bill. Part three is coming soon. Keep an eye out.

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