Arsenic in Well Water 2026: Causes, Testing & Removal

Arsenic in well water is one of the most underestimated threats to a private well system. It is tasteless, colorless, and odorless, which means you will never know it is there without a proper laboratory test. The Environmental Protection Agency sets a maximum contaminant level of 10 parts per billion (ppb) for arsenic in drinking water, and thousands of private wells across the western and southeastern United States exceed that limit every year. If your well sits in a geologic region known for high background arsenic, or if your water has ever tested above the limit, this 2026 guide walks you through where arsenic comes from, how to test for it correctly, what the levels mean, and which removal systems actually work.

By Robert Harrison, Certified Well Inspector

Robert has 20+ years of experience inspecting and testing residential water wells across the western US.

Published: September 10, 2026

Table of Contents

Why Arsenic in Well Water Is a Serious Health Risk
Where Arsenic Comes From in Well Water
How to Test for Arsenic in Your Well
Arsenic Levels and What They Mean
How to Remove Arsenic From Well Water
How to Choose the Right Arsenic Removal System
Arsenic Removal Costs and Maintenance
Frequently Asked Questions

Why Arsenic in Well Water Is a Serious Health Risk

Among the roughly 30 contaminants the EPA monitors in private wells, arsenic stands apart for two reasons. First, it is found naturally in groundwater at harmful concentrations in more regions than almost any other contaminant. Second, the EPA has designated it a carcinogen with no threshold of zero risk. The maximum contaminant level (MCL) for arsenic is 10 ppb, but that limit was set through a cost-benefit analysis, not through a level where harm disappears. Long-term exposure above 10 ppb is associated with skin lesions and a raised risk of lung, bladder, and kidney cancers, along with cardiovascular disease and developmental problems in children.

Because arsenic accumulates in the body over years of exposure, the danger is rarely dramatic or immediate. A household can drink water that is slightly elevated for a decade before any symptom appears. That is exactly why the EPA recommends that every private well be tested at least once, and annually in areas with known high background levels. In my inspection work, the wells most likely to exceed the limit are not the oldest or the deepest, but the ones drilled in volcanic geology or near former agricultural land where arsenical pesticides were applied before 1990.

The 2026 standard

The EPA has kept the 10 ppb arsenic MCL in place since 1987. No public water system in the United States is allowed to exceed it, and private wells are held to the same benchmark in state risk assessments. If your result comes back above 10 ppb, the only correct next step is treatment or an alternate water source, not retesting and hoping.

Where Arsenic Comes From in Well Water

In the overwhelming majority of private well cases, arsenic is not the product of a nearby factory or a recent spill. It is a natural component of the earth’s crust that dissolves into groundwater over thousands of years. Understanding the two broad source categories helps you predict whether your well is at risk, and whether the problem will persist after treatment.

Natural geologic sources

Arsenic occurs naturally in minerals such as arsenopyrite, realgar, and orpiment, and it leaches into groundwater as water moves slowly through rock and sediment. The highest natural concentrations appear in a few specific settings: volcanic and geothermally active regions across the Pacific Northwest, the West, and the Southwest; glacial deposits overlying volcanic parent material; and confined aquifers with low oxygen where arsenic-bearing minerals dissolve more readily. In these areas, arsenic levels in groundwater are effectively permanent. The contaminant is part of the aquifer itself, not a temporary intrusion, so a treatment system becomes a long-term fixture rather than a short-term fix.

Human-introduced sources

Before 1990, arsenical pesticides and herbicides were applied to crops across much of the country, and some of that legacy still shows up in shallow aquifers. Mining operations, metal smelting, wood preservation, and industrial discharge add to the picture in localized areas. Human-introduced contamination usually appears as a spike above the regional background level, and it tends to be more severe in shallow wells under 100 feet that draw water from near the soil surface. If your neighbors on a shared aquifer have tested high while your result is low, a local source rather than regional geology is worth investigating.

A field observation

When I test a well in a known high-arsenic county, I compare the result against the state’s regional background data before recommending anything. A reading of 12 ppb in an area where the background is 20 ppb is a very different situation from 12 ppb in an area where the background is 2 ppb. The first suggests a system-wide natural level; the second suggests a local source worth digging into.

How to Test for Arsenic in Your Well

The only reliable way to know your arsenic level is a sample analyzed at a state-certified or EPA-accredited laboratory. Home test kits can hint at a problem, but they are not accurate enough at the 10 ppb MCL, and a false negative at the exact threshold is the most expensive mistake you can make with well water. If you are not yet set up for routine testing, our step-by-step walkthrough in How to Test Your Well Water: Complete Guide 2026 covers sample collection, lab selection, and reading the results.

Request a certified analysis

Ask the laboratory to run EPA Method 1625 (ICP-MS) or Method 7062 for total arsenic. Both methods detect arsenic well below 10 ppb with precision suitable for a compliance decision. A standalone arsenic test typically costs between $10 and $30, and many homeowners bundle it into a full contaminant panel for under $100. When you pick up the sample bottle, ask whether the lab can report the result split into trivalent (As-III) and pentavalent (As-V) forms. The split matters for treatment design, because the two forms behave differently in filters, and a system sized for one form can underperform on the other.

Collect the sample correctly

Draw the sample as a first draw, before the pump starts, if your well has a storage tank or the water has sat overnight. First-draw water represents what you actually drink in the morning.
Run the tap for two minutes before collecting, unless the lab specifically asks for a static sample. This clears standing water from the distribution pipes so the sample reflects the aquifer, not your pipes.
Fill the bottle to the line without rinsing it with well water. The bottles are pre-rinsed and sterile for a reason.
Keep the sample cool, label it with the collection date, and deliver or ship it within the lab’s stated window, usually 24 to 48 hours.

Skip the at-home kit for this contaminant

Dip-strip and vial arsenic kits are rated for 50 ppb and above, five times the MCL. A kit that reads pass at 10 to 40 ppb tells you nothing useful. Reserve quick kits for obvious problems like bacteria or heavy iron, and go straight to a certified lab for arsenic, lead, and nitrates, where the decision threshold is low enough that kit error becomes material.

How often should you retest? The baseline recommendation is annually if your well sits in a high-background region, every three to five years if you are outside one, and always after a severe drought, a major change in draw, or new drilling activity nearby. If you have already found arsenic and installed a treatment system, retest the treated water at least once a year to confirm the system is still holding the level below 10 ppb.

Arsenic Levels and What They Mean

When the lab result arrives, the number is only half the story. The correct response depends on which band your result falls into. The table below maps the ranges I see most often in private well work to the response that makes sense at each level.

Arsenic level	Status	Recommended response
0 to 10 ppb	Meets EPA MCL	No treatment required. Continue routine testing on your normal schedule.
10 to 25 ppb	Above MCL, moderate	Install a point-of-use or whole-house treatment system. Retest treated water within 90 days.
25 to 50 ppb	Above MCL, high	Whole-house oxidation and media filtration, or a dedicated RO unit for drinking water. Confirm the arsenic valence split before buying.
50 ppb or higher	Severely elevated	Treat all water with an engineered system, and evaluate water blending or an alternate well before committing to a single-point fix.

Source: EPA maximum contaminant level for arsenic (10 ppb, 40 CFR 141) and standard private well response practice.

Keep in mind that a single sample tells you about one season. Arsenic levels in some aquifers shift with the water table, and a well that reads 8 ppb in winter can drift toward 14 ppb during a deep summer drawdown. If your result lands within a few ppb of the limit, treat it as a compliance risk rather than a clean pass, and retest in a different season before deciding to leave the water untreated.

How to Remove Arsenic From Well Water

There are four treatment approaches that reliably bring arsenic below 10 ppb. The right one depends on your arsenic level, the valence split in your water, your household water use, and whether you need treated water for bathing and laundry or only for drinking. Here is how each method works, followed by a side-by-side comparison.

Reverse osmosis (RO) under-sink systems

A semipermeable RO membrane is the most reliable single-stage arsenic remover in the home. It strips dissolved arsenic from water at the molecular level, and it does not depend on oxidation, water chemistry, or the valence split. RO units remove 90 to 97 percent of arsenic at typical household flow rates, and the treated water from the dedicated faucet is safe to drink. The trade-offs are speed and waste. A standard 75-gallon-per-day unit produces a trickle of pure water and flushes roughly three gallons of reject water for every gallon produced. RO is the right answer for drinking and cooking water, not for the whole house.

Whole-house oxidation with iron or manganese greensand media

Arsenic removal at whole-house scale depends on a two-step process. First, a chemical oxidizer, most commonly chlorine or potassium permanganate, converts the hard-to-catch trivalent arsenic (As-III) into pentavalent arsenic (As-V). Second, a media filter loaded with iron-based or manganese-based media, such as greensand or a catalytic iron media, adsorbs the oxidized arsenic as water passes through. The system removes 90 percent or more of total arsenic when both stages are present and the filter is backwashed on schedule. Without the oxidation stage, the same media can miss As-III entirely, which is the most common reason a whole-house arsenic system underperforms.

Ion exchange and activated alumina

Activated alumina (AA) media is another proven adsorbent, especially effective for moderate arsenic loads in water with the right pH. Ion exchange resins can also capture pentavalent arsenic, though they compete with nitrates and sulfates for the same sites, so a well with high nitrates will exhaust the resin quickly. AA and ion exchange work well as a second polishing stage behind an oxidation and media system, or as a stand-alone fix at modest arsenic levels. They are the least water-intensive of the whole-house options because they do not require a reject stream.

Pro tip: buy the valence split first

Before purchasing any whole-house system, pay the small upgrade fee for the As-III / As-V split on your lab report. If 70 percent or more of your arsenic is trivalent, you must budget for an oxidizer stage. If it is mostly pentavalent, a simpler media-only design may hold the level and save you money on chemicals.

Method	Arsenic removal	Scope	Typical installed cost	Main trade-off
Reverse osmosis (under-sink)	90 to 97 percent	Drinking and cooking water	$400 to $1,200	Slow flow, 3:1 reject water, small output
Oxidation + greensand/iron media	90 percent or higher	Whole house	$3,000 to $8,000	Needs oxidizer chemicals and regular backwashing
Activated alumina media	80 to 95 percent	Whole house or point of use	$2,000 to $5,000	Media replacement every 2 to 5 years; sensitive to pH and nitrates
Ion exchange resin	70 to 90 percent	Point of use to whole house	$1,500 to $4,000	Competes with nitrates and sulfates; resin refresh cycle

Installed cost ranges reflect typical 2026 contractor pricing for a residential well system, including media and installation; regional variation applies.

How to Choose the Right Arsenic Removal System

Work through these five decisions in order, and the choice becomes straightforward.

Confirm the level and the split. A result under 10 ppb needs no system at all. At 10 to 25 ppb, a point-of-use RO unit is often the whole answer. Above 25 ppb, or if the arsenic is mostly trivalent, plan for an oxidation stage regardless of the media you pick.
Decide the scope of treatment. If only drinking water matters, a quality RO unit under the kitchen sink solves the problem for a few hundred dollars and a membrane change every two to three years. If the water is used for bathing, laundry, or livestock and you want one fix at the source, go whole house.
Match the media to your chemistry. High nitrates weaken ion exchange. Low pH favors activated alumina performance. High iron or manganese in the same water can be handled by the same greensand media, which is why arsenic and iron problems are often fixed in a single system.
Budget for the operating cost, not just the purchase. A whole-house arsenic system that needs chlorine or permanganate dosing, backwashing, and periodic media replacement will run you a few hundred dollars a year. A system you cannot maintain will quietly stop working, and the arsenic will come back without any visible sign in the water.
Verify with a post-install test. After the system is installed, run a post-treatment sample through the same lab within 90 days. If the result is below 10 ppb, schedule annual retesting and log the date on the calendar. That annual test is the difference between a system you trust and a system you assume works.

Arsenic Removal Costs and Maintenance

The upfront price is the smaller part of the story. A point-of-use RO unit costs $400 to $1,200 installed and then a membrane every two to three years at roughly $100 each. A whole-house oxidation and media system costs $3,000 to $8,000 installed and then chemicals, backwash water, and a media refresh every two to five years, which typically adds $200 to $600 a year depending on how hard the system works. If you are also dealing with hard water, iron, or sediment, ask the installer whether the same media can be sized to handle all three at once. A combined iron, manganese, and arsenic system is almost always cheaper than three separate units, because the tank, pump, and plumbing are shared.

One cost consideration that surprises homeowners: reject water. An RO unit wastes about three gallons for every gallon of treated water. In a household that drinks four or five gallons a day, that is roughly 40 gallons a month of clean well water sent down the drain. It is a small number, but it is real, and it matters most in wells with limited recovery.

When to call a well contractor instead of buying a filter

If your result is 50 ppb or higher, or if the water table is dropping seasonally and the level is climbing with it, bring in a well professional before you buy hardware. The fix may be a deeper well, blending with a second water source, or a system sized for a much heavier load than a consumer catalog will show you. A cheap filter on a severe problem is the most expensive way to solve nothing.

Frequently Asked Questions

Can you taste or see arsenic in well water?

No. Arsenic is tasteless, colorless, and odorless at every concentration that appears in groundwater. There is no field sign, no discoloration, and no smell that will alert you. A laboratory test is the only detection method that works at the 10 ppb threshold.

How often should I test my well for arsenic?

At least once, as a baseline, for every private well. After that, test annually if you are in a region with known high background arsenic, every three to five years if you are outside one, and always after a drought, new drilling nearby, or a change in your water level. If you have installed a treatment system, retest the treated water once a year.

Does a water softener remove arsenic?

No. A standard ion-exchange water softener exchanges sodium for calcium and magnesium. It does not remove dissolved arsenic, and at the low concentrations involved it has no practical effect on the level. Do not count a softener toward your arsenic treatment plan.

What is the difference between As-III and As-V, and why does it matter?

Arsenic exists in groundwater in two main forms: trivalent (As-III) and pentavalent (As-V). The pentavalent form is far easier for iron and manganese media to capture. The trivalent form passes through much of the same media almost untouched unless it is oxidized first. That is why a whole-house system for arsenic typically includes an oxidation stage, and why the lab split is the single most useful extra data point you can buy.

Is arsenic in well water the same as the arsenic in rice and produce?

The element is the same, but the exposure paths differ. Food, particularly rice and grain products, contributes to total dietary arsenic, and a well at 10 to 20 ppb adds on top of that background. The EPA’s 10 ppb limit for water is set with the rest of the diet in mind, which is why meeting the MCL is the goal rather than driving the number to absolute zero.