Arsenic is one of the few well-water problems where the answer should not start with a brand or a filter style. It should start with a certified laboratory test.
For most homes, the best first filter for arsenic in well water is a point-of-use reverse osmosis system certified for arsenic reduction under NSF/ANSI 58, installed at the kitchen sink or another drinking-water tap. That is the practical choice when the goal is to reduce arsenic in water used for drinking and cooking without treating every gallon in the house.
That said, reverse osmosis is not always enough by itself. Arsenic chemistry matters, especially the difference between arsenic III and arsenic V. In some wells, pretreatment, adsorptive media, anion exchange, or a whole-house system is the better fit.
First: confirm the arsenic level
Private well owners are responsible for testing and maintaining their water. EPA recommends annual testing for basics such as coliform bacteria, nitrate, total dissolved solids, and pH, and testing for other contaminants when local conditions or symptoms suggest a concern.
Arsenic is different from iron or rotten-egg odor because you cannot see, taste, or smell it. Connecticut’s Department of Public Health notes that arsenic in water is not noticeable by taste or odor, so testing is the only way to know whether it is present.
Before shopping for treatment, order a certified lab test for:
- Total arsenic
- Arsenic speciation, if available, to separate arsenic III from arsenic V
- pH
- Iron and manganese
- Hardness
- Sulfate
- Total dissolved solids
- Bacteria and nitrate, if your annual well test is not current
If arsenic is above the drinking-water standard used for public water systems, contact your local health department or state drinking-water program and retest to confirm the result. Use bottled water or another known safe source for drinking, infant formula, ice, and cooking while you decide on treatment.
Best overall choice: certified under-sink reverse osmosis
An under-sink reverse osmosis system is usually the best starting point because it treats the water you actually ingest. RO systems are point-of-use filters, so they are commonly installed at a kitchen sink with a separate faucet.
Look for a system that is:
- Certified to NSF/ANSI 58
- Specifically certified for arsenic reduction, not just “RO” or “TDS reduction”
- Matched to your tested arsenic type and concentration
- Sized for your household’s daily drinking and cooking demand
- Installed with the required prefilters and postfilters
NSF explains that NSF/ANSI 58 covers reverse osmosis drinking-water treatment systems and includes optional contaminant reduction claims such as arsenic reduction. That “optional” detail matters: a system can be certified to NSF/ANSI 58 for some claims without being certified for arsenic. Check the exact contaminant claim in the certifier listing.
When RO is a good fit
Choose under-sink RO when:
- Arsenic is the main health-related contaminant you need to reduce at the tap.
- You need treated water for drinking and cooking, not showers or laundry.
- Your lab result is within the system’s certified treatment range.
- You are willing to replace cartridges and test treated water on schedule.
When RO may not be enough
RO performance depends on water chemistry and maintenance. Mass.gov notes that RO is not effective at removing arsenic III. If your water has meaningful arsenic III, a treatment professional may recommend oxidation before RO or a different treatment design.
RO also should not be treated as a “set it and forget it” device. After installation, test the treated water for arsenic. Then test again at the interval recommended by the lab, manufacturer, or local health department.
Best for arsenic III or higher-risk wells: pretreatment plus adsorptive media
Adsorptive media filters use materials such as activated alumina or iron-based media to bind arsenic as water passes through. These systems can be built as point-of-use cartridges or as larger whole-house systems.
They are often considered when:
- Arsenic III is present and needs oxidation before removal.
- The arsenic level is high enough that a larger treatment system makes sense.
- A household wants treated water at more than one drinking-water location.
- RO wastewater, storage tank size, or flow rate is a concern.
Mass.gov notes that adsorptive media performance can depend on pH and that significant arsenic III removal may require pre-oxidation. This is a place where lab data and a qualified installer matter. The wrong media or missing pretreatment can leave arsenic in the finished water.
Best whole-house option: anion exchange or adsorptive media
Whole-house treatment may be appropriate when arsenic is high, when multiple taps are used for drinking, or when a local health department or treatment professional recommends it. Whole-house systems are usually more expensive and require more design work than an under-sink system.
Common options include:
- Anion exchange: Similar in concept to a water softener, but designed to exchange arsenic species for chloride. It is generally used for whole-house treatment and depends heavily on competing ions and water chemistry.
- Adsorptive media tanks: Often iron-based or alumina-based media selected around pH, arsenic type, and competing contaminants.
- Oxidation plus filtration: Used when arsenic III needs to be converted before removal, or when iron and manganese must be managed to protect the arsenic treatment equipment.
Whole-house treatment should be designed from a complete water test, not a single arsenic number. Competing contaminants can shorten media life or reduce performance.
What certification should you look for?
Use third-party certification as a screening tool. NSF explains that residential treatment standards are voluntary, and that NSF/ANSI 53 covers health-effect contaminant reduction while NSF/ANSI 58 covers reverse osmosis systems.
For arsenic, look for:
- NSF/ANSI 58 with an arsenic reduction claim for RO systems
- NSF/ANSI 53 with an arsenic reduction claim for non-RO filters
- Certification from a recognized certifier such as NSF, WQA, or IAPMO
- A listing that names arsenic specifically
Do not rely on vague claims like “removes heavy metals” or “up to 99% contaminant reduction” unless the product listing shows the standard, certifier, contaminant, cartridge model, and test conditions.
Buyer checklist
Before buying, ask for these details in writing:
- Which arsenic species does the system reduce?
- What influent arsenic range was the system tested for?
- What pH range is allowed?
- Does iron, manganese, hardness, sulfate, or silica interfere?
- Is pretreatment required?
- How often are cartridges or media replaced?
- How will breakthrough be monitored?
- Where should samples be collected for follow-up testing?
- What happens if treated water still tests above the target level?
For arsenic, the cheapest filter is rarely the best value if it cannot prove performance on your water.
Installation and follow-up testing
After installation:
- Flush the system exactly as instructed.
- Test treated water after the system has been conditioned.
- Keep the raw-water and treated-water results together.
- Replace cartridges or media on schedule.
- Retest after service, media replacement, major plumbing work, flooding, or any change in taste, odor, or color.
EPA advises private well owners to use certified laboratories for drinking-water testing and to retest after a contaminant is found. That advice is especially important for arsenic because treatment failure may not produce any visible warning.
Quick recommendation
If your lab test shows arsenic in a private well, start with a certified lab confirmation and speciation if possible. For many homes, choose an under-sink RO system certified to NSF/ANSI 58 for arsenic reduction, then verify with a treated-water test. If arsenic III is present, levels are high, or you want whole-house treatment, work from a full water chemistry panel and consider oxidation plus adsorptive media or anion exchange.
For related treatment problems that often affect system design, see our guides to iron in well water and rotten-egg smell in well water.