Arsenic-III vs Arsenic-V: Choosing the Right Treatment System

Arsenic in well water is usually present as one of two species: Arsenic-III, also called arsenite, and Arsenic-V, also called arsenate. They are the same element but carry different electrical charges. Arsenic-V is negatively charged in water, while Arsenic-III carries essentially no charge.

A standard arsenic test reports your total arsenic, and a contractor can determine or estimate the species, which is the piece that drives the system choice.

Most arsenic treatment relies on the contaminant carrying a charge so it can be captured. Because Arsenic-V is charged, it is removed efficiently by anion exchange and by reverse osmosis membranes. Because Arsenic-III carries little or no charge, those same methods remove it far less effectively until it is converted.

NHDES notes that reverse osmosis removes roughly 95 percent of Arsenic-V but only about 60 percent of Arsenic-III, which is a large enough gap to change the design of a system.

Water chemistry beyond the arsenic itself matters too. Higher pH reduces the capacity of some adsorptive media, which is one reason a contractor reviews your full water profile rather than the arsenic number alone. In some cases a system pairs arsenic treatment with pH adjustment, such as a calcite neutralizer, to keep it working well.

Whatever system you choose, it needs upkeep, and arsenic systems get one extra step: confirmation testing. NHDES recommends retesting the treated water for arsenic quarterly during the first year and twice a year after that, so you know the system is performing as intended.

The hardware maintenance varies by method. Adsorptive media is replaced every one to three years depending on your water and use, anion exchange resin is regenerated with ordinary salt, and reverse osmosis membranes and cartridges are changed on a set schedule. A contractor lays out the full maintenance plan in the written quote so there are no surprises later.

The species discussion connects directly to where you treat. A point-of-use reverse osmosis system protects drinking and cooking water at one tap and handles Arsenic-V well, with a pre-oxidation cartridge added for Arsenic-III. A whole-house anion exchange or adsorptive media system treats every tap and is the choice when levels are high or other contaminants are present.

Neither is universally better. A point-of-use system is the lower-cost route when arsenic is the only concern, while a whole-house system earns its place when iron, manganese, or radon also need attention. The right answer comes from your test, your water chemistry, and how your household uses water.

You may wonder how anyone knows which form you have. Some laboratories report arsenic speciation directly, and a contractor can also infer the likely form from your water chemistry, since Arsenic-III is more common in low-oxygen groundwater and Arsenic-V where water is more oxygenated.

When there is any doubt, a safe design assumes Arsenic-III is present and includes a pre-oxidation step, so the system performs regardless. That belt-and-suspenders approach is common on New Hampshire bedrock wells, where conditions vary from one well to the next.