Nothing will tell you it is there
Every other problem in this cluster announces itself. Iron stains the laundry. Sulfur smells. Hardness leaves scale on the kettle. Acidic water turns the sink blue-green.
Arsenic does none of that. It is colourless, tasteless and odourless at every concentration that matters. There is no glass test, no smell test, no visible symptom. The only way to know is to send a sample to a lab.
That is the whole argument for this page.
How common is it
Minnesota Department of Health: “About 10 percent of Minnesota’s private wells have arsenic levels higher than 10 µg/L.”
One in ten. That is a Minnesota figure and it reflects Minnesota geology — arsenic “occurs naturally in rocks and soil… and can dissolve into groundwater,” so prevalence tracks bedrock rather than state lines and the number does not transfer elsewhere.
What it does establish is that arsenic in private wells is common where the geology favours it, not a rare event.
The limit, and who it applies to
EPA allows a maximum of 10 micrograms per litre in community water systems.
Your well is not a community water system. Nobody tests it against that number, nobody reports on it, and nothing changes if it is exceeded. The standard is the right benchmark for reading your own result — it just does not come with anyone enforcing it.
Why it matters at low levels
Minnesota Health is specific: “Consuming water with even low levels of arsenic over a long time is associated with diabetes and increased risk of cancers of the bladder, lungs, liver, and other organs.”
Also associated: cardiovascular and respiratory disease, reduced intelligence in children, and skin problems.
Note the framing — low levels over a long time. This is a cumulative exposure problem, not an acute one. There is no moment at which the water tastes wrong and you stop drinking it.
Testing
Minnesota Health’s guidance is modest: “Test your well for arsenic at least one time beyond the initial well water test when the well is first constructed.”
Once, deliberately, is most of the value. Arsenic comes from the rock the water moves through, so unlike bacteria it does not spike after heavy rain or vary with a failed well cap. A clean result on a stable aquifer stays informative.
Treatment
Minnesota Health lists seven methods that reduce arsenic:
- Reverse osmosis
- Distillation
- Adsorptive media
- Anion exchange
- Ozonation and filtration
- Oxidation filtration
- Chlorination and filtration
A removal percentage means nothing until you know which form of arsenic it applies to. Arsenic occurs in more than one chemical form, and the gap between them is not a rounding error.
The form decides the number
Arsenic in groundwater is mostly one of two species: arsenic III (arsenite) or arsenic V (arsenate). New Hampshire’s Department of Environmental Services puts numbers on what that difference costs you:
| Form | Reverse osmosis removal | What to do |
|---|---|---|
| Arsenic V (arsenate) | Greater than 98 percent | Nothing extra |
| Arsenic III (arsenite) | Typically 60 to 65 percent | Add a pre-oxidation step |
NHDES states it plainly: “If As-III is present, DES recommends adding a pre-oxidation cartridge to improve removal of As-III, as it is typically 60 to 65 percent compared to greater than 98 percent removal of As-V.”
A system rated “over 98 percent” is quoting the easy form. On water carrying arsenic III, that same unit passes roughly a third of the arsenic straight through.
Which oxidants convert it, and which do not
This is the part that catches people, because two of the failures are common well-water equipment.
- Effective: chlorine, ozone, permanganate, and solid manganese dioxide (pyrolusite) media.
- Not effective: air, UV light, chlorine dioxide.
Aeration and UV both appear in well treatment trains for other reasons. Neither converts arsenic III, so neither earns you the higher removal figure.
NHDES sets the trigger at raw arsenic III “at or above 10 ppb (0.01 mg/L)”, and puts the line between point-of-use and whole-house treatment at “greater than 250 ppb (0.25 mg/L), or if iron removal is also necessary.”
So the question for a supplier is: which form of arsenic is in my water, how did you determine that, and what is your removal figure for that form?
Mistakes that cost the most money
Accepting a removal figure with no species named. Reverse osmosis removes over 98 percent of arsenic V and typically 60 to 65 percent of arsenic III. A single number is quoting the easier form.
Relying on aeration or UV to oxidise it. NHDES lists air, UV light and chlorine dioxide as not effective for oxidising arsenic III. Both aeration and UV are common in well treatment trains for other reasons.
Treating a neighbour’s result as yours. NHDES notes wells within about 50 feet can differ markedly, because arsenic tracks fractured bedrock.
Treating the whole house. Arsenic is a cumulative ingestion problem. NHDES puts the whole-house line at 250 ppb or where iron removal is also needed.
Testing once and forgetting. Once is most of the value, since arsenic comes from rock rather than seasonal events, but a changed well or pump is a reason to retest.
Three situations, and what changes
You have never tested. Nothing about the water will reveal arsenic. A speciated lab test is the only route to a number, and it decides everything downstream.
A result at or above 10 micrograms per litre. Point-of-use treatment for drinking and cooking, matched to the species present.
Arsenic III confirmed. Add a pre-oxidation step before the removal stage, using chlorine, ozone, permanganate or solid manganese dioxide media.
The objections worth answering
“Our water has always been fine.” Arsenic gives no sensory signal at any concentration that matters, so nothing about long use establishes absence.
“The system is rated over 98 percent.” For which species? On water carrying arsenic III without pre-oxidation, that same unit passes roughly a third of the arsenic through.
“Is a low result worth acting on?” Minnesota Health notes that consuming water with even low arsenic levels over a long time is associated with serious disease. It is a cumulative exposure, not an acute one.
Who this is for
Private-well households deciding whether to test for arsenic, and those with a result choosing between point-of-use options.
Choose to test before anything else if
- You have never tested for arsenic. There is no smell, taste or colour to go on.
- Your only figure came from a neighbour, a map or a regional average.
- A supplier has quoted a removal percentage without naming the arsenic species it applies to.
Choose point-of-use treatment if
- Your result is at or above the 10 micrograms per litre limit EPA sets for community water systems.
- Drinking and cooking water is what needs treating. Arsenic is a long-term ingestion problem, so whole-house treatment buys little that a tap does not.
- The supplier can name which form of arsenic the unit is certified against, and say how they know which form is in your water.
Where point-of-use treatment sits relative to the rest of the system is worked through in the sizing method. The rest of the cluster starts at well water treatment.
→ The 135 filters certified for arsenic, and the two conditions on every claim