Why Arsenic in Well Water Is Different

Arsenic is the rare contaminant that you cannot taste, smell, see, or feel. It does not stain a sink. It does not foam in the toilet. It does not corrode a fixture. The only way you find out you have it is by sending a sample to a certified lab.

That makes arsenic uniquely dangerous compared to the things most well owners worry about. Iron leaves orange stains. Sulfur smells like rotten eggs. Hard water builds visible scale. You notice all of those, then you fix them. Arsenic just sits there, year after year, until either you test for it or somebody in the family develops a problem that traces back to it.

The U.S. EPA Maximum Contaminant Level (MCL) for arsenic in drinking water is 10 parts per billion (ppb), lowered from 50 ppb in 2001. The U.S. Geological Survey estimates that more than two million private well users in the United States are drinking water above that limit, with the highest concentrations clustered in the West (Nevada, Arizona, parts of California), the upper Midwest (Michigan, Wisconsin, Minnesota), New England (especially New Hampshire and Maine), and Texas. For a state-by-state breakdown of where the USGS data shows elevated arsenic, see arsenic in well water by state.

If you have not tested yet and you are on a private well, the place to start is a certified lab analysis, not a home test strip. Arsenic test strips exist but they are not sensitive enough to give you a defensible number near the 10 ppb threshold. Use a real lab. See how to test well water for the full process, our how to test for arsenic in well water guide for the arsenic-specific procedure, or skip to our 53 contaminant lab test kit.

Arsenic Level Severity Scale

Use this scale once you have a lab number in hand. The colors map roughly to the action you need to take.

Is the Whole House Arsenic System Right for Your Home?

Three quick questions. Answer based on your lab water test if you have one. If you do not have a test yet, skip to the test kit and come back.

Why Arsenic Selective Ion Exchange (and Not Adsorptive Media)

If you start price-shopping arsenic filters online you will find systems from $400 to $8,000. They look interchangeable on a product page. They are not. The technology inside the tank is the entire conversation, and there are essentially two camps.

Adsorptive media (the cheap stuff)

Granular iron oxide, activated alumina, titanium dioxide. These media physically trap arsenic on the surface of each grain. They work, sometimes very well at the start, and they are inexpensive to manufacture. Most of the budget arsenic filters on Amazon and the big-box water sites use one of these media types.

The problem shows up at end of life. When the media reaches its arsenic capacity, the next gallon of water through the tank can carry arsenic concentrations higher than the source. This is called arsenic dumping. The media has been holding all that arsenic, and a small chemistry shift in the influent (a change in pH, a slug of competing anions, or simple saturation) releases a concentrated pulse back into the home. If you are not testing the effluent often, you do not see it happen. You just drink it.

Arsenic-selective ion exchange resin (what we use)

The resin in our system is a hybrid anion exchange polymer with hydrated iron oxide monoatomically dispersed throughout the bead. It is WQA Gold Seal certified against NSF/ANSI 61 for potable water. Two properties matter to a homeowner:

• No arsenic dumping at exhaustion. Even when the resin is operated past its useful capacity, the effluent arsenic level cannot exceed the influent. The chemistry simply does not allow it. You may stop getting low arsenic numbers in your treated water once the resin is spent, but you will not get a spike to dangerous levels.
• Long capacity in real well water. Under typical conditions the resin reduces 50 ppb arsenate to under 10 ppb for over 500,000 gallons per cubic foot of media. Our 10 GPM system holds 2 cubic feet, which is over 1 million treated gallons before service for many private wells.
• Tolerant of common well water competing anions. Performance is largely unaffected by chlorides, bicarbonates, and sulfates at typical residential concentrations. That is not true for every adsorptive media.

That combination (no dumping, high capacity, well water tolerance, NSF/ANSI 61 certification) is why this is the resin we install. It costs more per pound than alumina or iron oxide granules. It is also the only chemistry that fails safely.

Comparison: Five Ways to Remove Arsenic

Here is the honest landscape. Every line in this table is a real product category that some company sells for arsenic removal. Each one has a place. Most of them are not the right place for whole-house well water above 10 ppb.

If your only goal is to protect drinking water at one faucet for a borderline (5 to 10 ppb) result, an under-sink RO is a reasonable, cheaper choice. Browse our 75 GPD reverse osmosis system for that case. Above 10 ppb, the conversation changes. Arsenic absorbs through skin during long hot showers and ends up in food cooked with tap water, watered houseplants, and pet bowls. Whole-house treatment is the only way to take all of those exposure routes off the table.

The Two MAW Options

Both systems use exactly the same WQA Gold Seal arsenic-selective ion exchange resin. The difference is the control valve on top of the tank. Pick based on whether the install location has electricity and a drain.

Which one should you pick?

• Pick the 10 GPM metered version if you have a basement or utility space with an outlet within reach and a floor drain or laundry standpipe nearby. The metered valve does the bookkeeping for you. You get a programmable reminder when the system is approaching the gallon count where you should be testing for breakthrough. This is the right choice for the majority of homes.
• Pick the non-backwashing Clack version if the install location is a well house, an off-grid cabin, a detached pump house, or a basement with no drain access. No power means no controller. No drain means no backwash. The trade-off is that you, not the valve, are tracking media life. Plan to send a sample to a lab once a year to confirm the system is still hitting target.

If you are not sure which install location you have, send Aidan a photo of where the system is going. He will tell you in 30 seconds whether you have what you need for the metered version or whether the Clack is the smarter call.

Sizing and Lead/Lag for High Arsenic

The 10 GPM service flow on our standard system covers the demand of 1 to 4 bathroom homes with normal usage patterns. That is the size most well water households need. Two showers running at the same time plus a dishwasher will not exceed the system.

Where the conversation changes is when arsenic levels are very high. The capacity numbers (500,000+ gallons per cubic foot reducing 50 ppb to under 10 ppb) are based on a typical residential influent. If your raw water arsenic is 70 or 100 ppb, the resin gets exhausted faster on a per gallon basis. For those situations we set up a lead/lag configuration:

• Two arsenic systems plumbed in series.
• The lead vessel does the bulk of the arsenic removal.
• The lag vessel acts as a polishing stage and a safety net.
• When the lead resin reaches end of life (confirmed by routine effluent testing between the two tanks), you swap the positions. The old lag becomes the new lead, and you put fresh media in the old lead.

The result is uninterrupted protection, no gambling on whether you replaced the media in time, and maximum utilization of every cubic foot of resin you bought. Lead/lag is the standard configuration for commercial and high-risk residential arsenic installs above roughly 30 ppb.

Call Aidan at 800-460-5810 with your test result. He will walk through whether your situation justifies a single tank or a lead/lag setup before you order anything.

Pre-Treatment: Iron, Hardness, pH, and the As(III) Question

Arsenic resin is the last system in a well water treatment train, not the first. The reason is that almost every other contaminant in well water shortens its life if you let it through. Here is the order Aidan tells callers on the phone, every single time:

1. Sediment pre-filter at the well tank if you have any visible particulate.
2. Iron filter if iron is above 0.3 ppm. Untreated iron will coat the arsenic resin and cut its capacity in half or worse.
3. Acid neutralizer if pH is below 6.5. The resin works at pH 4 to 8, but very low pH usually comes with corrosion problems you need to handle for the rest of the plumbing anyway.
4. Water softener if hardness is above 7 grains per gallon. Hardness competes for resin sites and shortens capacity.
5. Arsenic removal system last, polishing what is now clean, soft, neutralized water down to under 10 ppb arsenic.

For the full sequence with diagrams, see the correct order for well water treatment systems.

The As(III) vs As(V) question (and when it matters)

Arsenic in groundwater shows up in two forms. Arsenate, As(V), is the oxidized form. It is what most well water tests pick up, and it is what arsenic-selective ion exchange resin removes very effectively. Arsenite, As(III), is the reduced form. It is electrically neutral, which means anion exchange resin barely sees it.

If your water is exposed to oxygen (most aerated systems and most homes with iron filters that use air injection), arsenite naturally converts to arsenate as it travels through the treatment train, and the resin removes it just like the rest. If your water is anaerobic and high in iron and sulfide, you may have a meaningful As(III) fraction that needs pre-oxidation with chlorine or an air injection iron filter before the arsenic resin. A good lab test will speciate the arsenic for you. Most do not, by default. Ask the lab if you want both species reported, or send your test to Aidan and he will read it for you.

A Real Customer Scenario

We do not have a Stamped review with an arsenic install photo to share yet (whole-house arsenic systems are a smaller part of our catalog than iron filters and acid neutralizers). What we have is the call log, and the patterns are very consistent.

The order on that call (iron filter, softener, arsenic system) is the same order we recommend for the vast majority of arsenic installs above the EPA MCL. The cost on a complete three system whole-house treatment train for a setup like this lands in the $7,000 to $9,000 range depending on iron level and home size. For a full pricing breakdown of the supporting systems, see the well water treatment system cost guide.

Maintenance, Media Life, and Septic Safety

Is the system safe for a septic system?

Yes, with the same caveat as any backwashing media filter. The 10 GPM metered version backwashes 50 to 100 gallons per cycle on a programmed schedule. A healthy residential septic system in good operating condition handles that without issue. If your septic field is already saturated or struggling, any added discharge is a problem and you should fix the septic side first. The non-backwashing Clack version produces zero wastewater because there is no backwash cycle at all, which makes it the better choice for marginal septic systems or seasonally used cabins.

What does media replacement actually look like?

For a typical 2 to 4 person household with arsenic in the 15 to 25 ppb range and clean pre-treatment in front of the resin, expect 4 to 7 years before the metered valve flags the recommended service interval. At that point the procedure is straightforward: shut off the bypass, depressurize, vacuum out the spent resin, load fresh resin, sanitize the tank, and put it back in service. Most DIY-comfortable owners can do it in an afternoon. We sell the replacement resin direct and walk you through it on the phone.

If you want a deep dive on every well water system you might run alongside the arsenic filter, start with the complete guide to well water filtration systems, or jump to the specific guides on iron filters, acid neutralizers, and iron and manganese in well water.

Keep Reading

• How to Test Your Well Water, the right way to get a defensible arsenic number
• How to Read Well Water Test Results, what every line on the lab report actually means
• The Correct Order for Well Water Treatment Systems, where the arsenic system goes in the train
• Best Iron Filter for Your Well Water, almost always the first system in front of an arsenic install
• Best Acid Neutralizer for Well Water, needed when pH is below 6.5
• Iron and Manganese in Well Water, common companions to arsenic in private wells
• Reverse Osmosis for Well Water, the under-sink polishing option
• Browse all arsenic filters