Arsenic filtration turns on one question that most homeowners have never heard of: which form the arsenic is in. Arsenite, the reduced form common in oxygen-poor well water, is poorly removed by most media. Arsenate, the oxidized form, is removed well by several. Getting a test that speciates the arsenic, or oxidizing everything to arsenate first, is what makes the treatment work.
Two forms, two very different outcomes
Arsenic occurs naturally in bedrock across large parts of the United States, and it dissolves into groundwater with no taste, smell or color at all. In oxygen-poor water it exists mainly as arsenite, which carries no charge at typical groundwater pH and therefore slips past adsorptive media and anion exchange resins. Arsenate carries a negative charge and is captured well by the same media. A laboratory test that reports total arsenic alone does not tell you which situation you are in.
Oxidize first, then remove
The standard fix for arsenite is pre-oxidation: chlorine injection, or an oxidizing medium, converts arsenite to arsenate before the removal stage. On a well already chlorinated for bacteria or iron, this happens as a useful side effect. Where iron is present in the raw water, oxidized iron also co-precipitates arsenic and a well designed iron filter removes a share of it, which is why arsenic and iron treatment are so often designed together rather than separately.
Adsorptive media, anion exchange and reverse osmosis
Iron-based adsorptive media in a tank are the common whole-house route: arsenate binds to the medium, the bed is exhausted rather than regenerated, and it is replaced on a schedule set by testing. Anion exchange removes arsenate too but competes with sulfate, so a sulfate-heavy supply exhausts the resin quickly. Point-of-use reverse osmosis at the kitchen sink rejects arsenate reliably and arsenite much less so, which is why the oxidation step matters even for an under-sink system.
Testing before and after is not optional
Arsenic gives no warning at the tap, so the only proof a system works is a test of the treated water, and the only way to know when a bed is exhausted is to keep testing. 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. Test annually, and after any change to the well. Our well water filtration system guide covers where the arsenic stage sits in the running order.
Questions people ask about arsenic filtration
Why does the form of arsenic matter?
Arsenite carries no charge at typical groundwater pH and slips past most media, while arsenate is captured well. Oxidizing arsenite to arsenate is what makes treatment effective.
Does reverse osmosis remove arsenic?
It removes arsenate reliably and arsenite far less well, so a well with reduced groundwater usually needs an oxidation step ahead of the membrane.
Can I taste or smell arsenic in water?
No. Arsenic is colorless, tasteless and odorless at the levels of concern, which is why laboratory testing is the only way to detect it.
How often should the adsorptive media be replaced?
On a schedule set by testing the treated water rather than by the calendar alone, because capacity depends on your water chemistry and household use.