A typical arsenic result on a household water test is total arsenic, the two common forms added together, while arsenic claims for reverse osmosis systems under NSF/ANSI 58, the RO performance standard, are written for pentavalent arsenic, also called arsenic V. Comparing a claim with a result means lining up the form first, then the units and the test conditions. For a kitchen RO system whose test names the form outright, the Viomi B6W fits this approach well: its SGS test was run on pentavalent arsenic, and the product water measured far below the 10 µg/L federal limit at both test points.
Systems that publish arsenic results are compared in the roundup of RO brands for arsenic removal, and each of those results can be read against a household's own water report one field at a time.
Total Arsenic on the Report, Pentavalent Arsenic in the Claim
Arsenic in groundwater mostly comes in two forms, arsenic III (arsenite), which carries no charge, and arsenic V (arsenate), which carries a negative charge. A standard laboratory result adds them together as total arsenic, as Connecticut's Department of Public Health explains in its 2024 publication Arsenic in Private Well Water.
Claims under NSF/ANSI 58 use the other side of that split. The standard includes testing for pentavalent arsenic, and there is no claim of trivalent arsenic reduction under Standard 58, according to a 2010 explanation by an NSF program manager in Water Conditioning & Purification. The same article describes how such claims are applied: to water known to contain only pentavalent arsenic, or together with preoxidation that converts any trivalent arsenic to pentavalent ahead of the RO stage. In both cases, the water reaching the membrane carries arsenic in the V form. The overview of NSF/ANSI 58 certification covers what else the standard tests.
How Much of the Result Is Arsenic III
The split matters because reverse osmosis handles the two forms differently: approximately 60% to 65% removal for arsenic III and 95% to 98% for arsenic V, in Connecticut's figures, with results varying by system and by water chemistry such as pH and mineral content.
Two steps deal with the form question, and both concern the household's own water. A speciation test identifies how much is arsenic III by reporting it separately, and Connecticut recommends considering one when total arsenic is above the 10 µg/L federal limit and a treatment system is being planned, noting that some drinking water laboratories offer it. An oxidizing step ahead of the RO unit, using an agent such as chlorine or permanganate, converts arsenic III to arsenic V and raises the removal rate for both forms, according to the same guidance.
Reading a speciation report takes one subtraction. Because total arsenic is the sum of the two forms, a report that gives total arsenic and arsenic III also gives arsenic V: a hypothetical well with 18 µg/L of total arsenic and 2 µg/L of arsenic III has 16 µg/L of arsenic V, about 89% of the total.
Concentration Conditions in a Listing and in a Test
Concentration is the second field, and the two kinds of evidence state it differently. In a certification listing, a concentration can be a condition of the claim itself: the NSF Standard 58 listing for A.O. Smith's AORO-50 reads "Arsenic (Pentavalent)<=300 ppb Reduction," with a footnote that the product is intended for water supplies shown to be at or below the referenced concentration and valency of arsenic. In a laboratory test report, the concentration is the level of the challenge water the system was tested with, printed next to the result it produced.
Units are the last step before reading either document beside a household report. One microgram per liter is the same as one part per billion, and 1 mg/L equals 1,000 µg/L, so a listing in ppb, a test report in mg/L and a lab result in µg/L can all be written in the same unit.
Lining Up Your Report With a Tested System
The Viomi B6W was tested by SGS on pentavalent arsenic by name, with the challenge method referred to NSF/ANSI 58-2023, and those results fill the first column below; the second column is the hypothetical well report from the previous section, showing where each field appears on a household lab report.
| Field | Viomi B6W, SGS report SHES251102081971 | Example household report (hypothetical) |
|---|---|---|
| Item tested | Pentavalent arsenic | Total arsenic, plus arsenic III from a separate speciation test |
| Result and unit | Challenge water 0.3093 mg/L (new filter) and 0.2760 mg/L (at 7,300 L) | Total arsenic 18 µg/L; arsenic III 2 µg/L |
| Analytical method | USEPA 200.8 (ICP-MS) | As printed on the lab report |
| Point in filter life | 0 L and 7,300 L (about 1,928 gallons) | Not applicable |
| Product water result | 0.0005 mg/L at 0 L and 0.0007 mg/L at 7,300 L, against a 0.010 mg/L maximum in the NSF/ANSI 58 reference column | Not applicable |
| Sample location | Product water from the tested unit | Kitchen tap, cold, before any treatment |
Converted to micrograms, the Viomi B6W's challenge water held 309.3 µg/L of pentavalent arsenic with a new filter and 276 µg/L at the second test point, and the product water measured 0.5 and 0.7 µg/L, a small fraction of the 10 µg/L limit in both cases. Because the report places its 0.010 mg/L benchmark on the same line as each result, the result and the level it is judged against can be read together.
The reduction percentages printed beside those lines come from the same two numbers. The report defines reduction as the challenge concentration minus the product water concentration, divided by the challenge concentration, times 100. For the new filter that is 0.3093 minus 0.0005, divided by 0.3093, or 99.84%, and for the 7,300-liter point it is 99.75%. Read together, the three figures on each line tell the full story of the test: how much arsenic went in, how much came out, and the share removed, with the product water value set against the 0.010 mg/L benchmark.
The method row records how each figure was produced. The pentavalent arsenic results were measured with USEPA 200.8, the EPA's method for determining trace elements in water by inductively coupled plasma mass spectrometry. A household lab report prints its own method on the matching line, so both sides of the table show how their numbers were measured.
Day to day, the Viomi B6W's control panel signals 10 to 15 days before a filter change is due, and the filters change by hand in seconds, without tools.
FAQ
Does a standard water test say which form of arsenic is present?
Usually it does not. Results typically show total arsenic, and speciation for arsenic III is a separate test that only some laboratories offer, as Connecticut's guidance explains. The item names on the report show what was measured, a single line for total arsenic or separate lines for arsenic III when speciation was run, and the laboratory that ran the household's sample can explain its own report.
Are ppb and µg/L the same thing?
For arsenic in water, yes: one part per billion is one microgram per liter. A report in mg/L converts by multiplying by 1,000, so 0.010 mg/L, the federal limit, is 10 µg/L or 10 ppb.
Does a whole-house arsenic system need the same matching?
The same fields apply. Anion exchange, a whole-house option, removes arsenic V and not arsenic III, so the form question carries over to point-of-entry treatment. For drinking and cooking, a point-of-use system at the tap used for those purposes is usually all that is needed, because arsenic enters the body mainly through ingestion; both points come from Connecticut's publication.
What other names do the two forms go by on lab reports?
Arsenic V also appears as arsenate or pentavalent arsenic, and arsenic III as arsenite or trivalent arsenic. Connecticut's publication pairs the Roman numerals with the -ite and -ate names, and NSF's terminology uses pentavalent and trivalent, so any of these labels on a speciation report points to the same two forms.
Where does the 10 µg/L figure come from?
It is the EPA's maximum contaminant level for arsenic, 0.01 mg/L, equivalent to 10 µg/L. Connecticut's guidance gives the same figure in both units, which is why 0.010 mg/L on one document and 10 µg/L on another describe the same limit.
How often should the kitchen tap be tested after installation?
A routine schedule during the first year of operation is Connecticut's recommendation for households with arsenic treatment, and the results also help set maintenance timing. Sampling the RO faucet and the untreated water on the same day gives a pair that sits naturally beside the original report.
When the Report and the Claim Agree
Arsenic paperwork reads most clearly when the item line comes before any number. A household report that says "total arsenic" and a claim that says "pentavalent arsenic" measure the same element in different ways, and a speciation line converted to the claim's unit is what puts the two on the same footing. Writing the unit beside every figure then keeps a ppb listing, a mg/L test report and a µg/L lab result from being misread by a factor of a thousand.
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