Arsenic in well water: what the lab result means, why arsenic III or arsenic V decides the treatment, and what a softener cannot do

If the only complaint is white scale on the kettle, this is the wrong page: that is hardness, the hard water guide covers it, and the softener that fixes it does nothing for arsenic.

What does the lab result tell you?

Arsenic gives no sensory sign, so every row below starts with a number from a certified lab: USGS reports that about 7 percent of the wells it sampled exceeded 10 µg/L, about 16 percent in the basin-fill aquifers of the Southwest, with more than 25 states affected and about 2.1 million people on domestic wells potentially exposed.

USGS’s 2016 map, “Arsenic probability in the conterminous United States”, shows where the high-probability aquifers sit. A well in a shaded area with no arsenic line on its last report has simply not been tested for it.

What does the number mean?

The Maximum Contaminant Level is the highest concentration EPA allows in water delivered by a public system. EPA’s NPDWR table lists arsenic at 0.010 mg/L, in force since 23 January 2006, and the WQA fact sheet records that the rule revised the earlier 50 µg/L limit down to 10 µg/L and set the Maximum Contaminant Level Goal at zero.

Labs report the same figure three ways: 0.010 mg/L, 10 µg/L and 10 ppb (parts per billion) are one number, because 1 mg/L equals 1,000 µg/L. A private well is not regulated, so the MCL is a reference line.

Speciation is the split of total arsenic into arsenic III and arsenic V. Per the WQA fact sheet, arsenic III exists mainly as the neutral molecule H3AsO3 at any pH below 8.5, while arsenic V is negatively charged, and the charged form is much easier to remove.

Reduced well water typically carries arsenic III; water that has been aerated or chlorinated carries arsenic V, and WQA notes that chloramine alone will not fully oxidize arsenic III to arsenic V.

Which treatment class fits?

The Minnesota Department of Health’s home water treatment table lists anion exchange, adsorptive media and reverse osmosis as designed to remove arsenic, and oxidizing media only when high iron is also present. The same table lists cation-exchange softening as designed for calcium, copper, iron, magnesium, manganese and radium, which is why a softener does not appear below.

Two chemistry details narrow the choice further. WQA states activated alumina is highly selective for arsenic V and works best with pH lowered below 6.5, and MDH notes that sulfate above 100 parts per billion can also change which treatment removes arsenic. WQA adds that distillation has been shown to bring arsenic under 2 µg/L, at a point-of-use scale.

Which under-sink RO units name arsenic?

Every figure in this table is the manufacturer’s published spec or the Amazon listing text, read on 2026-09-13; nothing was tested. Neither maker’s page states an NSF/ANSI 58 arsenic-reduction certificate number, so the certification column repeats the listing, not a database entry.

The APEC ROES-50 is the one with a production figure on the maker’s page: 50 GPD at 60 psi and a 2,000 ppm inlet ceiling. Skip it if the well has not had a speciation test, and skip it if you need micron ratings or a membrane life, because APEC publishes neither; the ROES-50 parts page covers what it does publish.

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The iSpring RCC7AK adds a remineralization stage, which the remineralization guide explains. Its “up to 99%” arsenic line is marketing wording with no standard next to it, and there is no pre-oxidation stage, so it assumes the incoming arsenic is already arsenic V. Skip it if the well’s arsenic is untested for species or known to be arsenic III, since nothing in the unit oxidizes it first; the RCC7 parts page holds its cartridge intervals.

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Both are point-of-use units for one tap. A whole-house complaint needs point-of-entry media or anion exchange from the table above; the under-sink RO review compares the tanked units on their published specs.

What should you do first?

1. Send one sample to a certified lab for total arsenic in µg/L, with iron, sulfate and pH on the same order.
2. If the result is at or above 10 µg/L, order a speciation test that reports arsenic III and arsenic V separately.
3. Match the species to the table: pre-oxidation first if arsenic III, then point-of-entry media or anion exchange for the house, or point-of-use RO for the drinking tap.
4. Check the certifier’s own listing for the unit under NSF/ANSI 53, 58 or 62 with arsenic named, since neither maker page on this page prints one.
5. Retest six months or more after drilling or any well work, and set the routine arsenic interval with the local health department, as the CDC advises alongside its yearly basic panel.

Frequently asked questions

Sources

1. EPA, National Primary Drinking Water Regulations (government or regulatory source)
2. USGS, Arsenic in Groundwater and Drinking Water (government or regulatory source)
3. USGS, Arsenic probability in the conterminous United States (map) (government or regulatory source)
4. Water Quality Association, Arsenic Fact Sheet (PDF) (standards or certification body)
5. Minnesota Department of Health, Arsenic in Well Water (government or regulatory source)
6. Minnesota Department of Health, Home Water Treatment (PDF) (government or regulatory source)
7. CDC, Private Well Testing (government or regulatory source)
8. APEC Water Systems, ROES-50 product page (manufacturer or vendor document)
9. iSpring Water Systems, RCC7P-AK product page (manufacturer or vendor document)