Arsenic in well water
If you're here, you probably just got a lab result with a number on it and you're trying to work out how bad it is and what to do.
Here's the direct version.
The standard
The EPA maximum contaminant level for arsenic in drinking water is 10 parts per billion (ppb), also written 0.010 mg/L.
That standard applies to public water systems. Private wells aren't covered by it, Idaho doesn't require private well testing at all, and no agency is checking. The standard is still the number worth measuring yourself against, because it's the one every treatment decision and every real-estate transaction is going to reference.
If your result is above 10 ppb, you're above the level a public utility would be required to treat to. If it's below, you're inside it. If it's near it, seasonal variation matters and one sample is one sample.
We're not going to tell you what arsenic does to people. That's not our lane, and any water treatment company that leads with health effects is doing something other than water treatment. If you want that, the people to read are the EPA, the Agency for Toxic Substances and Disease Registry, and your own physician. What we can tell you is what the standard is, what your number is, and what changes it.
Why it's here
Arsenic in the inland Northwest is overwhelmingly natural, not industrial. It's in the rock. Groundwater moves through it and picks it up, and how much depends on local geology, well depth, and which aquifer you're drawing from.
Which is why two houses on the same road can test differently, and why a neighbor's result tells you very little about yours. Regional guidance from health authorities on both sides of the corridor recommends private well testing for arsenic for exactly this reason.
It has no taste, no smell, and no color at any concentration you'd find in a well. There is no home indication. This is the entire reason a certified lab test exists, and it's why hardware-store test strips are irrelevant here, they don't measure it.
The part most pages skip: there are two forms
This is the fact that determines whether your treatment works.
Arsenic occurs as arsenate (As V) and arsenite (As III).
Most treatment technologies remove As V well and As III poorly. If your water contains As III and the system was specified as though it were all As V, the equipment can be installed correctly, maintained correctly, and still leave arsenic in your water.
The fix is standard and well understood, oxidation before the removal stage converts As III to As V, and then the removal stage does its job. But it has to be specified, which means somebody has to know which form is present, which means the lab work has to be right before anyone selects equipment.
This is the single most common way an arsenic system fails, and it is not a manufacturing problem. It's a specification problem.
What removes arsenic
Broadly, three approaches are used residentially:
Adsorptive media, a bed of specialized media that arsenic binds to. The media has a finite capacity and is replaced on a schedule based on your concentration and your water use. Effective, and the maintenance schedule is part of what you're buying, not an afterthought.
Reverse osmosis, very effective at the tap. Usually installed as a drinking-water stage at the kitchen rather than for the whole house, because whole-house RO is a different scale of equipment and expense.
Anion exchange, works in specific water chemistries and is sensitive to competing ions, particularly sulfate. Right answer sometimes, wrong answer often, and which one depends on the rest of your report.
What does not remove arsenic:
- A water softener. Softeners exchange hardness ions. This is the most common misunderstanding in the category, and plenty of North Idaho houses have a softener and an arsenic result at the same time.
- A standard carbon filter. Carbon handles taste, odor, chlorine, and some organics. Not arsenic.
- Boiling. Boiling concentrates it. It does not remove it.
- A pitcher filter, unless it is specifically certified for arsenic, most are not, and the certification is the thing to check rather than the marketing on the box.
Whole house or just the tap?
A real question with a real answer, and it depends on your number and your household.
Point-of-use, treating the kitchen tap for drinking and cooking, is less expensive and handles the exposure route that matters most for arsenic. For many households at moderate concentrations it's a reasonable answer.
Whole-house treatment matters where the concentration is high enough, or where the household wants every tap treated, or where there are other things in the report that need whole-house treatment anyway, which is common, because arsenic rarely shows up in an otherwise clean report.
We'll tell you which one your report calls for, and if the answer is the cheaper one, that's the answer. We make one recommendation based on the numbers, and it isn't always the big system.
What we'd do
A free consult first, twenty minutes on the phone. If you already have a lab report, have it in front of you. We'll read the number with you, tell you whether the form of arsenic was identified, and tell you what it would take.
If your existing test didn't distinguish As III from As V, most basic panels don't, that's the gap to close before anyone specifies equipment.
The Water Assessment is a paid certified panel, credited in full toward a system if you move forward, and yours to keep if you don't. It covers arsenic alongside the rest of what a well in this region can carry.