Arsenic in Well Water: 2026 Guide

Arsenic is the contaminant that breaks every instinct homeowners have about water safety. It has no taste, no smell, and no color. It does not make the water cloudy or stain the fixtures. A family can drink it for a decade and never suspect a thing — which is exactly why the decision about arsenic is made by a laboratory, not by the senses. If your well is in an arsenic-prone region, the test is not optional. It is the whole game.

This guide covers where arsenic in well water comes from, the 10 parts-per-billion federal standard and what it means for a private well, how to test properly, the two treatment approaches that actually work — point-of-use reverse osmosis and whole-house adsorptive media — and the maintenance discipline that keeps the treatment honest.

Where arsenic hides, and why your region matters

Arsenic in well water is almost always natural, not industrial. It leaches from bedrock — certain granites, shales, and volcanic formations — into the groundwater that feeds drilled wells. That is why arsenic follows geology, not zip codes: it is common in New England bedrock wells, across the upper Midwest, and throughout the Southwest, while neighboring counties on different geology test clean.

The practical implication: your neighbor’s clean test means nothing for your well. Arsenic concentrations can vary dramatically between wells a few hundred feet apart, because they draw from different fractures in the rock. The only test that matters is yours. If you are buying a house in a known arsenic region, make arsenic part of the pre-purchase lab panel — not an afterthought after closing.

The 10 ppb standard: what the number means for a private well

The federal maximum contaminant level (MCL) for arsenic in drinking water is 10 parts per billion — 10 micrograms per liter. That standard is legally enforceable for public water systems: a utility delivering water above 10 ppb is in violation. Private wells are not federally regulated, so no agency will order you to treat — but the number is still the health-based benchmark the science supports, and a private result above it deserves the same response a utility would owe its customers.

Context for the number: the MCL balances health protection against what treatment technology can reliably achieve across thousands of utilities. Some researchers and health agencies argue the health-protective level would be lower, and a few states have considered stricter standards — check whether your state has adopted anything beyond the federal 10 ppb. Either way, the decision rule is simple: test, and if the result is above 10 ppb, treat the drinking water. Below it but detectable, retest periodically and watch the trend.

One more critical fact: arsenic’s health effects are chronic, not acute. This is not a contaminant that sends you to the emergency room; it is one whose risks accumulate over years of exposure. That cuts both ways — there is no emergency, but there is also no safe procrastination. The test-and-treat decision is calm, deliberate, and non-negotiable.

Testing: the only way to know, and how to do it right

Because arsenic is tasteless and odorless, testing is the entire decision process. Order arsenic as part of a certified lab panel — it is typically included in the metals section — and sample at the well tap before any treatment equipment, after running the tap for several minutes. Use a state-certified lab; the result you need is a number in ppb beside the 10 ppb benchmark, not a strip that says “present.”

Two subtleties matter. First, arsenic exists in two common forms in groundwater: arsenate, As(V), and arsenite, As(III). Most lab reports give total arsenic, which is what the standard regulates — but the form matters for treatment, as discussed below. If your total arsenic is elevated, ask the lab whether speciation (identifying the forms) is available; it changes the treatment design. Second, retest every few years even after treatment is installed: test the raw water to watch the source, and test the treated water to verify the equipment is performing.

Treatment option one: point-of-use reverse osmosis

For most households with arsenic above 10 ppb, point-of-use reverse osmosis at the kitchen sink is the standard answer — and it is the answer most water professionals give first. An under-sink RO unit treats the water you drink and cook with, which is where arsenic exposure actually happens. You do not need to strip arsenic from the shower water, the laundry, or the toilets; dermal absorption of arsenic from bathing is minimal, so treating every gallon in the house is usually overkill.

Here is where the chemistry matters: RO membranes remove arsenate, As(V), at roughly 95 percent efficiency, but arsenite, As(III), at only about 50 to 60 percent. If your water’s arsenic is mostly As(III) — common in deeper, oxygen-poor wells — the RO needs a pretreatment step that oxidizes As(III) to As(V), typically a small chlorination or oxidation stage ahead of the membrane. This is why the lab speciation matters: it decides whether a standard RO suffices or the system needs the oxidation step. A competent dealer designs from the lab report, not from a catalog.

Under-sink RO equipment runs roughly $200 to $600, with professional installation typically bringing the total to $350 to $1,100. Annual filter costs run $50 to $150, and the membrane needs replacement every two to three years at roughly $50 to $150. The ongoing discipline is the real cost: a neglected RO fails silently, and the only way to know it is still removing arsenic is periodic testing of the treated water. Test the RO product water annually — no exceptions.

Treatment option two: whole-house adsorptive media

Whole-house (point-of-entry) arsenic treatment uses adsorptive media — typically iron-based media that binds arsenic as water passes through. It treats every gallon in the house, which some households prefer for simplicity: every tap is safe, no separate drinking-water faucet, no under-sink maintenance.

The trade-offs are cost and complexity. Whole-house arsenic systems typically install for $2,400 to $4,500, with media replacement running roughly $700 to $900 a year — an order of magnitude more than the under-sink RO’s operating cost. The media has a finite arsenic capacity and must be replaced on schedule; like carbon, it fails by breakthrough, and the only honest monitoring is periodic treated-water testing. For most households, the whole-house route makes sense when arsenic is very high, when the household wants every tap treated, or when the water chemistry defeats simple RO — your water professional should justify the choice against the lab report.

Whichever route you take, pretreatment matters. Sediment fouls adsorptive media and RO prefilters alike, and iron above low levels fouls both — iron binds to adsorptive media and coats RO membranes. The treatment order from the well remains: sediment filter first, iron treatment if needed, then the arsenic system. Skipping the pretreatment is how $4,000 systems die in two years.

The maintenance discipline: testing treated water, on schedule

Every arsenic treatment system shares one vulnerability: it can stop working without any visible sign. Membranes degrade, media exhausts, oxidation pretreatment drifts — and the water looks, tastes, and smells exactly the same. The maintenance schedule is therefore a testing schedule: test the treated water annually at minimum, and test the raw water every few years to confirm the source has not shifted.

Keep the lab reports in a folder with the house. If you sell, the buyer’s inspector will ask about the arsenic system, and a file of annual treated-water tests showing results below 10 ppb is the strongest answer you can give. It is also your own proof that the money was well spent — and the early warning if it ever stops being enough.

Arsenic, disclosure, and home value: the transaction angle

Arsenic intersects with real estate in ways buyers and sellers both underestimate. In many states, sellers must disclose known water-quality problems — and once you have a lab report showing arsenic above 10 ppb, it is a known problem. That is not a reason to avoid testing; it is a reason to test early and treat, because a documented treatment system with a file of passing treated-water tests turns a disclosure liability into a selling point.

Buyers should make the arsenic test a contingency, not a curiosity. In arsenic-prone regions, add arsenic to the pre-purchase lab panel alongside bacteria and nitrate, and make the sale contingent on results you can live with — either clean results or a seller credit covering treatment installation. Lenders typically require only bacteria and nitrate testing; the lender’s minimum protects the lender, not your family. The few hundred dollars for the expanded panel is the cheapest negotiating leverage in the transaction.

For sellers with a treatment system already installed, assemble the documentation before listing: the original lab report, the equipment specifications, the installation invoice, and every annual treated-water test since. A buyer’s home inspector will flag the arsenic system as a specialty item; a thick folder of passing tests answers the question before it is asked. And if you are selling a home in an arsenic region without ever having tested — test before you list. Discovering arsenic during the buyer’s inspection, under time pressure, is how sellers end up funding rushed treatment at the worst possible price.

2026 cost ranges: testing, treatment, and upkeep

Costs are 2026 US market ranges; get itemized local quotes.

Arsenic testing as part of a certified lab panel: typically included in a $199 to $399 metals-inclusive panel, or $20 to $60 as a single-analyte add-on at many labs. Under-sink RO: $350 to $1,100 installed, plus $50 to $150 a year in filters and a $50 to $150 membrane every two to three years. Whole-house adsorptive media: $2,400 to $4,500 installed, plus roughly $700 to $900 a year in media replacement.

Annualize before you decide. The under-sink RO costs on the order of $100 to $250 a year all-in; the whole-house system costs roughly $1,000 to $1,500 a year all-in. Both protect the household’s drinking water. The price difference buys whole-house coverage and simplicity — worth it for some households, unnecessary for most. Get both quoted against your lab report and decide with the numbers in front of you.

Next steps: test first, treat second, verify always

If you are in an arsenic-prone region and have never tested: order a certified lab panel with arsenic this week. Sample at the well tap, before treatment, after running the tap for several minutes. If the result is above 10 ppb, get quotes from two or three licensed water treatment professionals — each quote should reference your lab report, specify whether the arsenic is As(III) or As(V) and how the design handles it, and include the pretreatment train. Install, then test the treated water to verify, and put the annual treated-water test on the calendar permanently. Testing — not symptoms, not a neighbor’s result, not a salesman’s assurance — drives every decision from here.

Frequently asked questions