Arsenic in U.S. Groundwater Regions

Arsenic occurs naturally in groundwater across the United States, and its presence follows geology rather than state borders. Regions with volcanic, granitic or sedimentary geology — parts of the Southwest, the Northeast, the upper Midwest and the Pacific Northwest — see it more often.

Private wells are the main exposure route, because municipal systems test and treat for it. If you are on a well, arsenic is a test you should run at least once, regardless of region.

Arsenic is odourless, colourless and tasteless. There is no way to judge it by looking at the water, and no home strip that is precise enough for a first decision.

Arsenic also comes in two common forms — arsenate (As(V)) and arsenite (As(III)) — and the ratio matters for treatment. A lab panel that reports both gives you a better basis for choosing media.

Test the raw well water, not water that has been through existing treatment, so you know what you are actually dealing with.

Reverse osmosis: effective for both forms at the point of use, and commonly certified for arsenic under NSF/ANSI 58. It is the standard drinking-water answer for a single tap.

Adsorptive media (iron-based or titanium-based): effective, especially for As(V), and often used at point-of-entry or whole-house scale with a defined bed life.

Anion exchange: effective for As(V); As(III) usually needs oxidation first.

Oxidation plus filtration: converts As(III) to As(V) and then removes it, common in whole-house systems with iron and manganese present.

Standard carbon cartridges do not remove arsenic. Neither do sediment filters, softeners, or boiling — boiling concentrates it.

A pitcher certified for taste and odour has no bearing on arsenic. Check for a listing that names arsenic specifically.

Do not use the water for drinking, cooking or formula until treatment is installed; use bottled or hauled water in the interim.

Retest after installing treatment, and again annually, because media beds exhaust and membranes degrade.

Write down the treatment system's media or membrane model numbers. Arsenic systems have finite bed lives and reordering the wrong part is a common mistake.