Coliform after a flood: what Meramec and Big River well owners should test for

When the water goes down, people want to know if their well is safe. The short answer is that you cannot tell by looking, and the longer answer depends on which river came up. A Meramec flood and a Big River flood are not the same event for a well owner, and our own records are specific about why. Austin and I have this conversation every time the bottoms go under.

How this was built: Figures come from Jones Air & Water's own 57-town water dataset, built from utility consumer confidence reports, EPA and DNR well data, and our own in-home testing, pulled in 2026. Where our notes describe typical regional conditions rather than a measurement at a specific address, I've said so.

Start with the number that should decide this for you

Our Labadie record cites a Missouri survey of 861 private wells that found 56% positive for total coliform and 22% positive for E. coli. That is more than half, and better than one in five, in normal conditions. Our Arnold notes then add the part that matters here: coliform and E. coli risk in shallow or older wells is worse after Meramec and Big River flooding.

So the baseline is already poor, and a flood pushes it the wrong way. Our notes tie that elevated risk to specific well characteristics rather than to geography alone: shallow wells, large diameter wells, older wells, cracked casings, and proximity to septic. If your well is deep, modern and well sealed, you are in better shape than your neighbor with a hand-dug large diameter well fifty feet from a failing lateral field.

Why nobody is going to tell you

This is the part that catches new owners. Private wells are unregulated. Our notes repeat the same line across the region in different words: wells are never tested unless the owner pays for it, nobody tests them but you, no consumer confidence report protects these owners.

Some county health departments do push post-flood testing, and our Hawk Point notes mention exactly that, while our Flint Hill notes describe the county mailing out slow nitrate and bacteria kits. Those are worth using. They are also slow, and they test for a short list.

The Big River is a different problem, and this is the important part

If you are on the Meramec or the Bourbeuse, the post-flood story in our records is mainly bacteria and nitrate, with iron and hydrogen sulfide as the standing nuisances. Our Union notes put it plainly, that Bourbeuse and Meramec river bottom wells are flood prone and bacteria prone from septic and flooding. Our St. Clair and Sullivan notes both say the shallow wells along the Meramec and Bourbeuse should be tested first for coliform, E. coli and nitrate.

The Big River is not that. The Big River has been Superfund listed off the back of more than a century of lead mining in the Old Lead Belt, and our High Ridge record describes the river as the partial source for that town's treated supply. For well owners the relevant detail is in our Cedar Hill notes: the Big River corridor carries mining sediment.

That is what changes the test list. A flood on a river carrying mining sediment is not only a bacteria event, it is a heavy metals event. The numbers in our file are specific. Inside the Southwest Jefferson County Mining Superfund site, EPA tested 1,414 properties and 147 of them, 10.4%, exceeded health limits for lead, cadmium, arsenic or barium. EPA's 2023 remedy for affected wells is free point of use filters. Our Cedar Hill notes also record that children under 7 in that area are advised to get annual blood lead tests.

Towns our records place inside or adjacent to that Superfund area include Byrnes Mill, Cedar Hill, De Soto, Festus and Hillsboro. If that is you, one in ten is not a scare statistic, it is the actual measured hit rate among tested properties, and lead cannot be seen, smelled or tasted.

What I would test for, in this order

First, total coliform and E. coli, every time, no exceptions. This is the one with an immediate health consequence and it is invisible.

Second, nitrate, especially if there is an infant in the house or you are near row crop ground or septic. Our records flag the blue baby risk above 10 mg/L repeatedly across the region.

Third, on the Big River specifically, lead and heavy metals, meaning lead, cadmium, arsenic and barium. This is the item people skip because they are thinking about bacteria, and it is the one with the EPA remedy attached.

Fourth, the nuisance set that a flood often stirs up in alluvial wells: iron, manganese and hydrogen sulfide. These will not hurt you, but they tell you whether sediment got into your well, which is itself a clue about whether the casing held.

One more, if you are in the Missouri or Meramec bottoms near Labadie. Our Labadie record notes that Ameren's coal ash ponds leaked arsenic into floodplain groundwater at roughly 3.5 times the federal drinking water limit, and that Ameren tests its own monitoring wells while nobody tests yours. That deserves an arsenic test on its own merits, flood or no flood.

What actually fixes it

For bacteria after a flood, our records describe the standard sequence as post flood disinfection followed by UV. Our Foley notes pair floodplain coliform risk with post flood disinfection and UV, and our High Ridge notes pair recurring coliform and E. coli with UV sanitation. Shock disinfection handles what is in there now. UV is what keeps handling it, which matters because a well that took on surface water once will likely do it again.

For the heavy metals, if you are inside that Superfund area, start with the EPA program, because a free point of use filter from a federal remedy is a better deal than anything I can sell you. I will say that to your face and I have said it on service calls.

For nitrate, reverse osmosis is the route, and our St. Clair notes say the same, to lead with UV and R/O where bacteria and nitrate are the risks.

What I would not do is reach for equipment before the test comes back. A flood is exactly the moment people buy things out of fear. Test first.

For the county level picture where most of this falls, see Jefferson County water by the numbers, and for the longer view on wells around here, well water vs. city water: what 30 years of testing taught me.

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