You've Tested for Lead and Chlorine. Here's What You're Probably Still Missing.
Ask most people to name a water contaminant and you'll get the same two or three answers every time: lead, chlorine, maybe hard water minerals. That instinct isn't wrong. Those are worth knowing about. But it's an incomplete list, and it's the kind of incomplete that can leave people feeling more confident than they should.
A basic test kit built around the usual suspects can come back completely clean while several quieter contaminants go entirely unmentioned. They're not rare, they're just less famous. Here are four worth knowing about, along with an honest answer to the question everyone eventually asks: does reverse osmosis actually handle this one, or not?
Manganese, the Mineral Behind That Stain You Can't Scrub Out
Manganese is a naturally occurring mineral, usually found alongside iron in groundwater. In small amounts it's harmless, even essential in the diet. At higher concentrations, though, it leaves brownish-black stains on fixtures and laundry and can give water a faint metallic edge.
It's not always part of a basic test panel, so it can go unnoticed for years, especially on well water. If you've ever scrubbed at a stubborn dark stain in the sink that just won't budge, manganese is a quiet, common explanation.
What your RO system does about it: this one's a genuine strength. Reverse osmosis membranes are effective at reducing dissolved metals like manganese, along with related ones like iron, copper, and barium, so a properly maintained system handles this well.
Chromium-6, the "Erin Brockovich Chemical" You Didn't Expect to See Again
Chromium-6 became a household name after the Erin Brockovich case exposed contamination in a California town, but it's not just a one-town story. It occurs naturally in some soil and rock, and it's also a byproduct of industrial processes like metal plating and manufacturing, which means it can turn up in groundwater in places with no obvious industrial history nearby.
It's colorless and odorless, so there's no sensory clue it's there. Even small amounts of exposure have been linked to skin irritation and more serious long-term effects, which is part of why it gets more attention from water quality researchers than from the general public.
What your RO system does about it: another strong point. RO membranes filter down to the micron level, fine enough to effectively remove chromium-6, which makes reverse osmosis one of the more reliable household options against it.
Disinfection Byproducts, a Side Effect of the Process That Keeps You Safe
This one's almost ironic. When utilities disinfect water with chlorine to kill bacteria and pathogens, that chlorine can react with naturally occurring organic matter and form disinfection byproducts, most commonly trihalomethanes and haloacetic acids.
In other words, they're a side effect of the very process built to protect you from something else entirely. Utilities monitor and regulate them, but they rarely come up outside of technical water quality reports, which is exactly why so few homeowners have ever heard the term.
What your RO system does about it: this is where your pre/post filters shine. A membrane alone isn't especially reliable here, since some of these compounds are volatile enough to slip through. But it's the carbon stage in the pre/post filters doing the real work, reducing disinfection byproducts along with chlorine taste and odor.
Nitrates, Not Just a Rural Well Problem
Nitrates usually get filed under "farm runoff" and written off as a well-water issue, but they can also show up in municipal systems near agricultural land or older septic systems. They're colorless, odorless, and tasteless, so there's genuinely no way to notice elevated levels without testing directly for them.
They're especially worth knowing about if there's an infant in the household, since nitrates carry specific risks for very young children that don't apply the same way to older kids and adults.
What your RO system does about it: another one it handles well. Reverse osmosis is one of the few practical household treatments that meaningfully reduces nitrate levels, which makes it a genuinely useful option for well owners in agricultural areas, not just a nice-to-have.
The Fuller Picture: What Else a Good RO System Catches
Beyond these four, a well-built RO system is quietly working on a longer list most homeowners never think about, lead, arsenic, fluoride, sulfates, PFAS, and even trace pharmaceuticals that can find their way into water supplies. None of these get much individual airtime, but together they're a big part of why reverse osmosis has become the go-to option for households that want to stop guessing about water quality altogether.
The Bigger Picture
Lead and chlorine deserve the attention they get, but they're not the only things worth checking for, and not every filter handles every contaminant the same way. Manganese, chromium-6, and nitrates are strong points for reverse osmosis, while disinfection byproducts really come down to whether there's a carbon stage doing its part alongside the membrane. Knowing which is which is the difference between assuming your water is covered and actually knowing it.