Arsenic in Drinking Water: The Silent Risk and How to Reduce It

The question I get from well owners more than almost any other is some version of: "Should I be worried about arsenic?" My honest answer is: I don't know — and neither do you — until you test. That's not a deflection. It's the most useful thing I can tell you, because arsenic is the definition of a silent contaminant. It has no taste, no smell, no color. Nothing about the water coming out of your tap tells you it's there. A neighbor two miles away might have undetectable levels; your well might read above the EPA limit. Geology doesn't follow property lines.

What I can do in this article is explain what arsenic is, how it gets into water (especially well water), what the science says about health risks at drinking-water concentrations, and what kind of filtration actually reduces it. Let's start with the basics.

What Arsenic Is and Why It Ends Up in Water

Arsenic is a naturally occurring element found in the earth's crust. It's not a modern chemical — it's ancient geology. Rocks and soils containing arsenic minerals slowly release the element as water percolates through them. That's the primary route into drinking water: natural erosion and leaching, not factory spills or pipeline accidents (though those can contribute too).^[2]

Other contributors include agricultural activity — some older pesticides were arsenic-based and left residues in soil — and certain industrial processes. Smelting, mining runoff, and the use of pressure-treated wood (older formulations used arsenic compounds) have added to local contamination in specific areas. But in most of the United States, the arsenic story in drinking water is primarily geological.

The regions with the highest naturally occurring arsenic in groundwater tend to be areas with specific mineral-rich rock formations: parts of the Southwest (Arizona, Nevada, New Mexico), the Great Plains, New England, and the Upper Midwest. That said, elevated arsenic has been found in groundwater in nearly every state — it's not a regional problem you can ignore just because you're not in a traditionally high-risk area.

Why Well Owners and Geology Are the Key Factors

If you're on a public water utility, the utility is required to test for arsenic and keep levels at or below the EPA's maximum contaminant level (MCL). It's not a perfect guarantee, but there's a monitoring system in place and your annual Consumer Confidence Report (CCR) will show arsenic detection data.

Private well owners have no such safety net. The EPA's drinking water regulations don't apply to private wells — they are the owner's responsibility entirely.^[3] The level in your well depends on what the bedrock and soil around you happen to contain, how deep the well is drilled (deeper wells tend to intersect different geology), and local factors like seasonal variation and nearby land use.

I've talked with well homeowners who tested pristine water for decades, and others who were surprised to find arsenic above the EPA limit on their first-ever test. The only way to know is to test. The EPA recommends well owners test at least annually for a standard contaminant panel — and specifically for arsenic if you're in an at-risk area or have never tested.^[3]

Health Picture: What the EPA Limit Means

The EPA's MCL for arsenic in public drinking water is 10 parts per billion (ppb) — a standard set in 2001 that replaced the previous 50 ppb limit as research on long-term exposure developed.^[1] The World Health Organization's guideline value is also 10 ppb.^[4]

Public health research and agency guidance generally links higher, longer-term arsenic exposure — particularly above the MCL — to a range of potential health effects, including effects on the skin, cardiovascular system, and increased cancer risk with sustained high-level exposure. Children and pregnant women are among those for whom health agencies take arsenic exposure most seriously.

A few important things to keep in mind here, because I want to be straight with you rather than just alarming:

• Risk is generally related to dose and duration. A glass of water that slightly exceeds 10 ppb occasionally is not the same risk as drinking water consistently at much higher levels over years.
• The 10 ppb standard reflects a policy balance: the EPA set it at a level achievable with current technology that meaningfully reduces risk, not necessarily zero risk (the "maximum feasible" level).
• For specific health questions — especially if you're pregnant, nursing, caring for young children, or have known health conditions — talk with your physician or local health department rather than relying on general guidance.

None of that changes my recommendation: if you're on a well, test your water. If you're above 10 ppb, treat it. The peace of mind alone is worth the price of a test kit.

Arsenic III vs. Arsenic V — a Quick Note

Arsenic in water exists in two main chemical forms: trivalent arsenic (As III, also called arsenite) and pentavalent arsenic (As V, also called arsenate). This distinction matters for treatment because they behave differently in filtration.

As V is the easier form to capture — most filtration media handles it well. As III is more soluble and harder to adsorb, and some treatment approaches work better after converting As III to As V through oxidation (a step that some whole-house pre-treatment setups include).^[5]

You don't need to be a chemist to act on this. What it does mean practically is: a water test that specifies which arsenic form is present — and at what level — helps you choose a treatment system configured for your actual situation. A certified lab can provide that detail. This is another reason I always recommend testing first.

Step 1: Test Before You Treat

Every filtration conversation starts here for me. Treating water you haven't tested is guesswork, and guesswork is a poor basis for decisions that affect your family's health.

On a public water utility? Read your annual Consumer Confidence Report — it shows arsenic detection and how it compares to the MCL. Your utility is required to make it available. If arsenic is detected at levels that concern you, or if your utility uses older infrastructure, a point-of-use test can give you a more precise picture of what's actually coming out of your tap after traveling through local pipes.^[6]

On a private well? This is where testing is non-negotiable. Get a certified laboratory test — not a basic at-home dip test — that specifically includes arsenic. If you haven't tested before, a comprehensive well panel (covering arsenic, iron, manganese, nitrates, coliform bacteria, and pH) gives you the full picture of what you're working with. Our well test kit is a good starting point for well owners who want a structured look at their water.

Step 2: Choose Filtration That Reduces Arsenic

Once you have test results in hand, you can match a solution to your actual situation. Not every filter reduces arsenic — a standard pitcher filter, a water softener, or a fridge filter won't do it. You need a system specifically designed to handle heavy metals.

Multi-Stage Catalytic-Carbon Filtration (Our Approach)

Catalytic carbon is a more advanced form of activated carbon that's engineered to chemically reduce a wide range of contaminants — including arsenic, lead, chloramines, VOCs, and fluoride — through adsorption and catalytic reaction, not just mechanical trapping.^[7] This is the core technology behind the Sentry H2O systems we carry, specifically the ToxyGuard media stage.

For households on city water with arsenic as one of several contaminants of concern, the 5-stage Sentry H2O Wellness System is our under-sink drinking system. It runs water through multiple stages — ToxyGuard catalytic carbon for arsenic, heavy metals, chloramines, VOCs, and fluoride; scale-free mineralization; and AlkaGuard remineralization that adds calcium and magnesium and raises pH to roughly 8–9.5. It installs under the sink in about 30 minutes without a plumber, and the filters are replaced about once a year.

For well homes, my honest recommendation is different: if your test shows elevated arsenic (or iron, manganese, bacteria, or other common well contaminants), the right tool is typically a whole-house system that treats every tap — not just the kitchen faucet. The Sentry H2O Well Whole House System is configured for well water with a multi-stage catalytic-carbon tank, scale-control CT resin, and is designed to pair with UV disinfection (VIQUA VH200) for biological protection. Professional installation is recommended. The well tank media typically lasts 5–7 years depending on household size and water quality — longer than city whole-house systems because well water generally skips the chlorination that city water goes through.

What I like about the multi-stage approach for arsenic specifically: it addresses arsenic alongside the other contaminants well water tends to carry, rather than treating one thing in isolation. You're rarely dealing with arsenic alone — you're dealing with a water profile, and a system matched to that profile is more useful than a single-contaminant fix.

Sentry H2O's published independent lab report shows arsenic moving from detectable to non-detect after its under-sink system treated the tested water. That is independent testing, not NSF/WQA certification, and results vary with incoming water. When evaluating any system, look for product-specific third-party testing documentation for heavy metal reduction — not just general marketing claims.

Reverse Osmosis (RO)

RO systems push water through a very fine membrane that blocks arsenic and a broad range of other contaminants. Systems certified to NSF/ANSI 58 for arsenic reduction are among the most thoroughly vetted options for point-of-use treatment.^[8] The tradeoffs are real: most RO systems waste 3 to 5 gallons of water per gallon filtered, strip beneficial minerals (some add them back in a remineralization stage), and treat only a single tap. For households where test results show very high arsenic levels and a point-of-use solution is the right scope, a certified RO system is a legitimate option. For most homes — especially well homes where whole-house coverage matters — I think the multi-stage approach is the better fit.

What Doesn't Work for Arsenic

Standard pitcher filters, water softeners, boiling, and sediment filters don't reduce arsenic. Boiling can actually concentrate it slightly. A softener handles hardness minerals, not heavy metals. The question to ask of any system is: is it independently certified or tested specifically for arsenic reduction? That's your filter, not marketing language.

Step 3: Keep Up with Maintenance

A filtration system is only as good as its upkeep. Every medium has a finite capacity — an exhausted filter doesn't just stop helping, it can stop reducing contaminants you're counting on it for.

For the Sentry H2O Wellness System under-sink drinking filters: annual replacement. For the Well Whole House System tank media: roughly 5–7 years depending on household size (5 years for 4+ people, 7 years for 1–3 people) and your specific water chemistry. Well water with high sediment or mineral loads may need more frequent attention. Mark the dates, keep records, and retest your water after any filter change to confirm you're still in good shape. See our article on when and why to replace your water filter for the full maintenance schedule by system type.

One more thing worth saying: well water quality can change. A drought, a nearby construction project, or agricultural activity can shift what's in your groundwater. That's not a reason to panic — it is a reason to test periodically, not just once and forget it. Annual testing is the standard recommendation for private wells.

Frequently Asked Questions

Can I taste or smell arsenic in my water?

No. Arsenic is colorless, odorless, and tasteless at typical drinking-water concentrations. The only way to know whether it's in your water is to test it — this is especially important for private well owners who don't have utility monitoring as a backstop.

What is the EPA's limit for arsenic in drinking water?

The U.S. EPA's maximum contaminant level (MCL) for arsenic in public water systems is 10 parts per billion (ppb).^[1] Private wells are not regulated by the EPA — owners are responsible for their own testing and treatment.

Is arsenic more of a concern for well water or city water?

Both can be affected, but well owners carry more direct exposure risk because municipal utilities are required to test and treat to the EPA standard. Your geology determines your well's arsenic levels, and only a test tells you where you stand. If you're on city water and are concerned, your Consumer Confidence Report is the place to start.

Does boiling water remove arsenic?

No. Boiling does not remove arsenic and can slightly concentrate it as water volume decreases. Use a certified water test first, then select treatment backed by product-specific arsenic reduction evidence.

What's the difference between arsenic III and arsenic V?

Arsenic V (arsenate) is more easily captured by most filtration media. Arsenic III (arsenite) is harder to adsorb and may require pre-oxidation for effective removal. A certified lab test can tell you which form is in your water, which informs the right treatment approach.

Can a multi-stage catalytic-carbon system reduce arsenic?

Some multi-stage systems can reduce arsenic when designed and tested for the arsenic form and level present. Sentry H2O's published independent test of its under-sink system showed the tested arsenic moving from detectable to non-detect. That is product-specific independent testing, not NSF/WQA certification. Well homeowners should test first and verify product-level arsenic evidence with a qualified water-treatment professional.

Educational guidance only — not health or medical advice. For health concerns related to arsenic or any contaminant in your water, consult your physician or local health department. Regulatory specifics should be verified against current primary sources before any decision. Private well owners should work with a certified laboratory and, where appropriate, a licensed water treatment professional.