Arsenic is the contaminant that punishes buying equipment before testing. It occurs naturally in groundwater across large parts of the country, it has no taste, color or smell, and it exists in two chemical forms that behave completely differently in a filter. A system that removes one may pass the other almost untouched, which is why the laboratory result decides the specification.
Two forms, and why the distinction is everything
Arsenic occurs in groundwater in two oxidation states, commonly written as arsenic three and arsenic five. The five form carries a charge that most treatment media and membranes can capture, and is comparatively straightforward to remove. The three form is uncharged at typical water pH and slips past much of that equipment. The standard answer is to oxidize the three form into the five form before the removal stage, using chlorination or a specialist oxidizing filter, and any specification that does not address this on a well known to carry arsenic three is incomplete.
The technologies that actually work
Three approaches are used in homes. Adsorptive media, typically iron based, hold arsenic on their surface and are simple to operate as a point of entry vessel, with media replaced when exhausted. Reverse osmosis removes it well at the point of use and is the common choice where only drinking and cooking water needs treatment. Anion exchange works but is sensitive to competing ions such as sulfate. Activated carbon on its own is not an arsenic treatment, and neither is a sediment filter, a softener or an ultraviolet lamp, whatever a product page implies.
Water chemistry decides which one performs
Performance depends on the rest of the water, not the arsenic alone. pH strongly affects adsorptive media, which lose capacity as water becomes more alkaline, so some installations need pH adjustment ahead of the vessel. Competing ions matter: sulfate, silica, phosphate and fluoride all compete for the same sites and shorten media life. Iron in the water can help, because arsenic co-precipitates with it, which is why iron based media work. All of this comes from the laboratory report, which is why a full analysis rather than an arsenic figure alone is worth paying for.
Testing, monitoring and the private well context
The EPA notes that private wells are not federally regulated under the Safe Drinking Water Act, and a USGS study of 2,100 private wells found about one in five carried at least one contaminant above a human-health benchmark. Nobody is sampling your well but you. Test for arsenic on a new well, retest periodically since concentrations vary over time, and once treatment is installed, test the treated water on a schedule because adsorptive media exhaust without any visible sign. Our well water testing guide covers what to include in a full analysis.
Questions people ask about water filters for arsenic
Does a standard carbon filter remove arsenic?
No. Activated carbon is not an arsenic treatment. Removal needs adsorptive media, reverse osmosis or anion exchange, matched to the form present.
Why does the form of arsenic matter?
The uncharged form passes through much of the equipment that captures the charged form, so it usually has to be oxidized first for treatment to work.
Can arsenic be tasted or smelled?
No. It has no taste, color or odor at the concentrations that matter, which is why laboratory testing is the only way to know it is there.
Is point of use treatment enough?
For drinking and cooking, generally yes, since ingestion is the exposure route. Point of entry treatment is chosen where a household wants every tap covered.