Arsenic in Well Water

What Arsenic Is and How It Gets Into Well Water

Most arsenic in private wells is not pollution - it is geology. Groundwater dissolves it straight out of the rock.

Arsenic is a naturally occurring metalloid found throughout the earth's crust. While people associate it with poisoning or industrial waste, the vast majority of arsenic in domestic well water comes from natural geology. As groundwater percolates through bedrock and unconsolidated sediments, it interacts with arsenic-bearing minerals - sulfide minerals like pyrite and iron oxides - and dissolves the arsenic into the aquifer. How much dissolves depends on the water's pH, oxygen level, and how long it sits in the rock fractures.

Human activity can add to the load. Inorganic arsenic is a byproduct of copper and lead smelting, coal-fired power plants, and building- material manufacturing. Historically it was used as a pesticide on cotton and orchards and as a wood preservative, so runoff from legacy agricultural and industrial sites can leach into shallow aquifers on top of the natural risk.

Arsenic III vs. Arsenic V: the distinction that decides your filter

Inorganic arsenic exists in two forms, and they behave completely differently in water. This single fact decides whether a treatment system works or fails.

• Arsenic III (arsenite)is the more toxic form and is notoriously hard to remove. Because it is electrically neutral at typical groundwater pH, it slips right through reverse-osmosis membranes and ion-exchange resins that rely on charge to catch contaminants. It dominates in deep bedrock wells where the water is "reduced" - low in dissolved oxygen.
• Arsenic V (arsenate) is less toxic and far easier to filter. It carries a negative charge, so anion exchange and reverse osmosis capture it effectively. It is more common in shallower, oxygen-rich water.

Because most wells carry a mix of both, you cannot rely on a generic filter. If your well is dominated by Arsenic III, the water must first be oxidized (with chlorine, ozone, or potassium permanganate) to convert it to filterable Arsenic V - which is exactly why a speciation test matters before you buy anything.

Where Arsenic Occurs: The US Hotspots

The rock under your property dictates your risk. Three regions carry the worst of it - and the reasons are geological.

The USGS estimates about 2.1 million Americans drink from private wells with arsenic above the federal limit. The risk is far from uniform across the country - it clusters where specific bedrock and aquifer types release arsenic into the groundwater. If your well sits in one of the three regions below, arsenic belongs on your first test.

In New England, wells are drilled deep into hard fractured bedrock, and water creeping slowly through narrow joints stays in prolonged contact with arsenic-bearing minerals - which is how roughly one in five New Hampshire wells ends up over the limit. In the Southwest, volcanic rock plus an arid climate (aquifers that stagnate without fresh recharge) drives concentration through evaporation and high pH. The Upper Midwest releases arsenic from iron oxides in glacial till, with isolated pockets in Wisconsin and Minnesota exceeding 50 percent.

Your neighbor's results are a hint, not an answer - arsenic can vary well to well within a single neighborhood depending on which fractures each well taps. You can see wells and depths near you on the well map to gauge how deep local wells go and how that lines up with the bedrock, since arsenic correlates strongly with deep bedrock draw zones. State-specific risk is also covered in our state well guides.

Health Effects and the 10 ppb Limit

The danger is not one bad glass - it is years of low-level exposure. Arsenic is a Group 1 carcinogen, and the harm is cumulative.

Long-term ingestion of inorganic arsenic damages DNA, disrupts cell signaling, and behaves as an endocrine disruptor - all precursors to tumor development. Because arsenic is filtered by the kidneys and stored in the bladder before excretion, the bladder is especially vulnerable. Documented long-term effects at the concentrations relevant to wells (roughly 10-50 ppb) include elevated bladder, lung, skin, and kidney cancer rates, plus cardiovascular disease, diabetes, lower birth weights, and lowered IQ in developing children.

Why 10 ppb is a compromise, not a safe number

The US standard started at 50 ppb (set in 1942, adopted by the EPA in 1975). By 1999 the National Academy of Sciences judged that limit grossly inadequate, estimating a lifetime cancer risk as high as 1 in 100. The EPA lowered the Maximum Contaminant Level to 10 ppb (0.010 mg/L) in 2001, with full compliance by 2006.

That 10 ppb figure is a regulatory compromise. When the EPA and WHO evaluated truly safe limits, scientists argued for 3 ppb or lower, but a cost-benefit analysis found that requiring every municipal plant to hit 3 ppb was unfeasible. The EPA openly states that the Maximum Contaminant Level Goal - the level with zero known health risk - for arsenic is exactly zero. For a private well owner, that means a result of 8 ppb is technically "compliant" yet still carries lifetime cancer risk. New Jersey and New Hampshire have set their own state limit at 5 ppb.

How to Test for Arsenic

Arsenic is invisible to taste, smell, and sight. Crystal-clear, great-tasting water can still carry a dangerous dose.

Because private wells are exempt from federal testing mandates, the burden of discovery is entirely on you. The only reliable method is a state-certified laboratory using Inductively Coupled Plasma Mass Spectrometry (ICP-MS), which can detect trace metals down to parts- per-trillion. DIY test strips ($30-$70, results in about 15 minutes) are useful only to flag gross contamination - they rely on color- matching and are not sensitive enough in the critical 0-10 ppb range where health decisions are made.

Reading Your Results

Labs report arsenic in micrograms per liter (ug/L), milligrams per liter (mg/L), or parts per billion (ppb). The units trip people up, so keep this conversion handy:

• 1 ppb = 1 ug/L
• 0.010 mg/L = 10 ug/L = 10 ppb (the EPA limit)
• To convert mg/L to ppb, multiply by 1,000

So a result of 0.015 mg/L is 15 ppb - over the EPA limit and requiring action. A result of 0.05 mg/L is 50 ppb. If your number is at or above 10 ppb, move to the steps below. If it is between 5 and 10 ppb, you are federally compliant but should still weigh treatment, especially with children or pregnancy in the home.

If Your Test Comes Back High: Do This Now

Homeowners often panic and do the one thing that makes it worse. Here is the right sequence.

1. Stop drinking it immediately. Do not drink the water, make infant formula, cook with it, or give it to pets. Switch to bottled water for all consumption until you have a verified fix.
2. Do not boil it. See the warning above - boiling concentrates arsenic.
3. Know what is still safe. Arsenic is not easily absorbed through skin and does not evaporate into the air. Unless your level is extreme (above about 500 ppb), showering, bathing, dishwashing, and laundry remain safe - just keep small children from swallowing bathwater.
4. Run a speciation test before buying treatment. Installing the wrong system because you skipped speciation is the most common - and most expensive - mistake.

Treatment Options Compared

Because arsenic is not a skin hazard, a point-of-use system at the kitchen sink is often enough - and far cheaper than whole-house.

You are choosing between Point-of-Use (POU) systems that treat one faucet and Point-of-Entry (POE) systems that treat the whole house. Since bathing in arsenic water is safe, POU is usually sufficient and much more economical. Go whole-house only if levels exceed about 500 ppb or you want to protect all the plumbing. Two cautions before you choose: standard carbon pitcher filters (Brita, PUR) are essentially useless here - they removed under 26 percent in testing - and anion exchange can "dump" a concentrated slug of arsenic if the resin is not regenerated on schedule.

Prevention and Well Construction

Treatment fixes the symptom. Good construction can block the contaminated water at the source.

In bedrock wells, the casing should be driven and sealed at least 10 feet into competent bedrock to keep shallow and surface water out. If testing shows arsenic bleeding in from one specific deep fracture, a licensed driller can install a secondary well seal - an inflatable packer - to isolate and seal off that weathered fracture, forcing the pump to draw from cleaner zones. This is an advanced hydrogeological procedure that requires a licensed water-well contractor and is never DIY-safe.

Casing modifications and packer installation call for a licensed water-well contractor. You can find a licensed driller near you and cross-check their license with your state agency. For ongoing care, see our well maintenance guide and well water upkeep basics.

Financial Assistance Programs

A $4,000 whole-house filter is a real burden for a rural family. Several federal and state programs exist to help.

The federal USDA Section 504 Home Repair Program (Rural Development) offers very-low-income rural homeowners loans up to $40,000 at a fixed 1 percent interest rate over 20 years to repair their homes, including installing water treatment. Homeowners aged 62 and older who cannot afford repayment may qualify for a lifetime grant up to $10,000 that does not have to be repaid. Loans and grants can be combined for up to $50,000.

State programs add more help:

• Wisconsin - the Well Compensation Grant Program pays 75 percent of eligible costs up to $16,000 to replace, treat, or abandon a contaminated well; in 2022 the state dropped the arsenic qualification threshold from 50 ppb to 10 ppb and raised the income limit to $100,000.
• Maine - the Arsenic Abatement Program provides grants to single-family homeowners with high arsenic, prioritizing households with young children or pregnant women.
• Iowa - county programs (for example, Dubuque County) offer free in-home arsenic sampling and reimbursements up to $2,000 for well reconstruction.

Keep reading

• Well Water Testing: get the right test
• Nitrates in Well Water
• Uranium in Well Water
• Lead in Well Water
• All Well Water Contaminants
• State Well Guides